Biochemistry and pathobiochemistry of blood. Respiratory function of erythrocytes. Pathological forms of hemoglobin.
Acid-base state of blood. Non-protein nitrogenous containing and nitrogen not containing organic components of blood. Residual nitrogen. Lipoproteins of blood plasma. Biochemistry of immune processes.
Blood is a liquid tissue. Suspended in the watery plasma are seven types of cells and cell fragments.
· red blood cells (RBCs) or erythrocytes
· platelets or thrombocytes
· five kinds of white blood cells (WBCs) or leukocytes
o Three kinds of granulocytes
o Two kinds of leukocytes without granules in their cytoplasm
§ If one takes a sample of blood, treats it with an agent to prevent clotting, and spins it in a centrifuge,
· the red cells settle to the bottom
· the white cells settle on top of them forming the "buffy coat".
The fraction occupied by the red cells is called the hematocrit. Normally it is approximately 45%. Values much lower than this are a sign of anemia.
Biological functions of the blood
The blood is the most specialized fluid tissue which circulates in vascular system and together with lymph and intercellular space compounds an internal environment of an organism.
The blood executes such functions:
1. Transport of gases – oxygen from lungs is carried to tissues and carbon dioxide from tissues to lungs.
2. Transport of nutrients to all cells of organism (glucose, amino acids, fatty acids, vitamins, ketone bodies, trace substances and others). Substances such as urea, uric acid, bilirubin and creatinine are taken away from the different organs for ultimate excretion.
3. Regulatory or hormonal function – hormones are secreted in to blood and they are transported by blood to their target cells.
4. Thermoregulation function - an exchange of heat between tissues and blood.
5. Osmotic function- sustains osmotic pressure in vessels.
6. Protective function- by the phagocytic action of leucocytes and by the actions of antibodies, the blood provides the most important defense mechanism.
7. Detoxification function - neutralization of toxic substances which is connected with their decomposition by the help of blood enzymes.
Blood performs two major functions:
· transport through the body of
o oxygen and carbon dioxide
o food molecules (glucose, lipids, amino acids)
o ions (e.g., Na+, Ca2+, HCO3−)
o wastes (e.g., urea)
· defense of the body against infections and other foreign materials. All the WBCs participate in these defenses.
The formation of blood cells (cell types and acronyms are defined below)
All the various types of blood cells
· are produced in the bone marrow (some 1011 of them each day in an adult human!).
· arise from a single type of cell called a hematopoietic stem cell — an "adult" multipotent stem cell.
These stem cells
· are very rare (only about one in 10,000 bone marrow cells);
· express a cell-surface protein designated CD34;
· produce, by mitosis, two kinds of progeny:
o more stem cells (A mouse that has had all its blood stem cells killed by a lethal dose of radiation can be saved by the injection of a single living stem cell!).
o cells that begin to differentiate along the paths leading to the various kinds of blood cells.
Which path is taken is regulated by
· the need for more of that type of blood cell which is, in turn, controlled by appropriate cytokines and/or hormones.
Two types of blood cells can be distinguished - white and red blood cells. White blood cells are called leucocytes. Their quantity in adult is 4-9 x 109/L. Red blood cells are called erythrocytes. Their quantity in peripheral blood is 4,5-5 x 1012/L. Besides that, there are also thrombocytes or platelets in blood.
Neutrophils comprise of 60-70 % from all leucocytes. Their main function is to protect organisms from microorganisms and viruses. Neutrophils have segmented nucleus, endoplasmic reticulum (underdeveloped) which does not contain ribosomes, insufficient amount of mitochondria, well-developed Golgi apparatus and hundreds of different vesicles which contain peroxidases and hydrolases. Optimum condition for their activity is acidic pH. There are also small vesicles which contain alkaline phosphatases, lysozymes, lactopherins and proteins of cationic origin.
Glucose is the main source of energy for neutrophils. It is directly utilized or converted into glycogen. 90 % of energy is formed in glycolysis, a small amount of glucose is converted in pentosophosphate pathway. Activation of proteolysis during phagocytosis as well as reduction of phosphatidic acid and phosphoglycerols are also observed. The englobement is accompanied by intensifying of a glycolysis and pentosophosphate pathway. But especially intensity of absorption of oxygen for neutrophils - so-called flashout of respiration grows. Absorbed oxygen is spent for formation of its fissile forms that is carried out with participation enzymes:
Neutrophils are motile phagocyte cells that play a key role in acute inflammation. When bacteria enter tissues, a number of phenomena occur that are collectively known as acute inflammatory response. When neutrophils and other phagocyte cells engulf bacteria, they exhibit a rapid increase in oxygen consumption known as the respiratory burst. This phenomenon reflects the rapid utilization of oxygen (following a lag of 15-60 seconds) and production from it of large amounts of reactive derivates, such as O2-, H2O2, OH. and OCl- (hypochlorite ion). Some of these products are potent microbicidal agents. The electron transport chain system responsible for the respiratory burst contains several components, including a flavoprotein NADPH:O2-oxidoreductase (often called NADPH-oxidase) and a b-type cytochrome.
This is a never-ending task, even in healthy people: Our throat, nasal passages, and colon harbor vast numbers of bacteria. Most of these are commensals, and do us no harm. But that is because neutrophils keep them in check.
HOCl, the active ingredient of household liquid bleach, is a powerful oxidant and is highly microbicidial. When applied to normal tissues, its potential for causing damage is diminished because it reacts with primary or secondary amines present in neutrophils and tissues to produce various nitrogen-chlorine (N-Cl) derivates; these chloramines are also oxidants, although less powerful than HOCl, and act as microbicidial agents (eg, in sterilizing wounds) without causing tissue damage. Responsible for the green color of pus.
Neutrophils contain a number of proteinases (elastase, collagenase, gelatinase, cathepsin G, plasminogen activator) that can hydrolyze elastin, various types of collagens, and other proteins present in the extracellular matrix. Such enzymatic action, if allowed to proceed unopposed, can result in serious damage to tissues. Most of these proteinases are lysosomal enzymes and exist mainly as inactive precursors in normal neutrophils. Small amounts of these enzymes are released into normal tissues, with the amounts increasing markedly during inflammation. The activities of elastase and other proteinases are normally kept in check by a number of antiproteinases (a1-Antiproteinase, a2-Macroglobulin, Secretory leukoproteinase inhibitor, a1-Antichymotrypsin, Plasminogen activator inhibitor-1, Tissue inhibitor of metalloproteinase) present in plasma and the extracellular fluid.
Basophiles make up 1-5% of all blood leukocytes. They are actively formed in the bone marrow during allergy. Basophiles take part in the allergic reactions, in the blood coagulation and intravascular lipolysis. They have the protein synthesis mechanism, which works due to the biological oxidation energy . They synthesize the mediators of allergic reactions – histamine and serotonin, which during allergy cause local inflammation. Heparin, which is formed in the basophiles, prevents the blood coagulation and activates intravascular lipoprotein lipase, which splits triacylglycerin.
The number of basophils also increases during infection. Basophils leave the blood and accumulate at the site of infection or other inflammation. There they discharge the contents of their granules, releasing a variety of mediators such as:
They make up 3-6% of all leukocytes. Eosinophiles as well as neutrophiles defend the cells from microorganisms, they contain myeloperoxidase, lysosomal hydrolases. About the relations of eosinophiles with testifies the growth of their amount during the sensitization of organism, i.e. during bronchial asthma, helminthiasis. They are able to pile and splits histamine, “to dissolve” thrombus with the participation of plasminogen and bradykinin-kininase.
They are formed in the bone marrow. They make up 4-8% of all leukocytes. According to the function they are called macrophages. Tissue macrophages derive from blood monocytes. Depending on their position they are called: in the liver – reticuloendotheliocytes, in the lungs - alveolar macrophages, in the intermediate substance of connective tissue – histocytes etc. Monocytes are characterized by a wide set of lysosomal enzymes with the optimum activity in the acidic condition. The major functions of monocytes and macrophages are endocytosis and phagocytosis.
The amount – 20-25%, are formed in the lymphoid tissue or thymus, play important role in the formation of humoral and cellular immunity. Lymphocytes have powerful system of synthesis of antibody proteins, energy is majorily pertained due to glycolysis, rarely – by aerobic way.
Although bone marrow is the ultimate source of lymphocytes, the lymphocytes that will become T cells migrate from the bone marrow to the thymus where they mature. Both B cells and T cells also take up residence in lymph nodes, the spleen and other tissues where they
The amount – less than 1%, they play the main role in the process of hemostasis. They are formed as a result of disintegration of megakaryocytes in the bone marrow. Their –life-time is 7-9 days. In spite of the fact that thrombocytes have no nucleus, they are able to perform practically all functions of the cell, besides DNA synthesis.
When blood vessels are cut or damaged, the loss of blood from the system must be stopped before shock and possible death occur. This is accomplished by solidification of the blood, a process called coagulation or clotting.
Human blood contains 25 trillion of erythrocytes. Their main function – transportation of O2 and CO2 – they perform due to the fact that they contain 34% of hemoglobin, and per dry cells mass – 95%. The total amount of hemoglobin in the blood equals 130-160 g/l. In the process of erythropoesis the preceding cells decrease their size. Their nuclei at the end of the process are ruined and pushed out of the cells. 90% of glucose in the erythrocytes is decomposed in the process of glycolysis and 10% - by pentose-phosphate way. There are noted congenital defects of enzymes of these metabolic ways of erythrocytes. During this are usually observed hemolytic anemia and other structural and functional erythrocytes’ affections.
Thus RBCs are terminally differentiated; that is, they can never divide. They live about 120 days and then are ingested by phagocytic cells in the liver and spleen. Most of the iron in their hemoglobin is reclaimed for reuse. The remainder of the heme portion of the molecule is degraded intobile pigments and excreted by the liver. Some 3 million RBCs die and are scavenged by the liver each second.
· Under the conditions of lower temperature, higher pH, and increased oxygen pressure in the capillaries of the lungs, the reaction proceeds to the right. The purple-red deoxygenated hemoglobin of the venous blood becomes the bright-red oxyhemoglobin of the arterial blood.
· It probably enters (and leaves) the cell by diffusing through transmembrane channels in the plasma membrane. (One of the proteins that forms the channel is theD antigen that is the most important factor in the Rh system of blood groups.)
Only about 5% of the CO2 generated in the tissues dissolves directly in the plasma. (A good thing, too: if all the CO2 we make were carried this way, the pH of the blood would drop from its normal 7.4 to an instantly-fatal 4.5!)
Figure 1 is a model of human deoxyhemoglobin. It was created in RasMol version 2.6 by Roger Sayle using the pdb coordinates from the pdb file 4hhb. The 3D coordinates were determed from x-ray crystallography by Fermi, G., Perutz, M. F., Shaanan, B., Fourme, R.: The crystal structure of human deoxyhaemoglobin at 1.74 A resolution. J Mol Biol 175 pp. 159 (1984)
is the protein that carries oxygen from the lungs to the tissues and carries
carbon dioxide from the tissues back to the lungs. In order to function most
efficiently, hemoglobin needs to bind to oxygen tightly in the oxygen-rich
atmosphere of the lungs and be able to release oxygen rapidly in the relatively
oxygen-poor environment of the tissues. It does this in a most elegant and
intricately coordinated way. The story of hemoglobin is the prototype example
of the relationship between structure and function of a protein molecule.
hemoglobin molecule consists of four polypeptide chains: two alpha chains, each
with 141 amino acids and two beta chains, each with 146 amino acids. The
protein portion of each of these chains is called "globin". The a and
b globin chains are very similar in structure. In this case, a and b refer to
the two types of globin. Students often confuse this with the concept of a
helix and b sheet secondary structures. But, in fact, both the a and b globin
chains contain primarily a helix secondary structure with no b sheets.
Figure 2 is a close up view of one of the heme groups of the human a chain from dexoyhemoglobin. In this view, the iron is coordinated by a histidine side chain from amino acid 87 (shown in green.)
Each a or b globin chain folds into 8 a helical segments (A-H) which, in turn, fold to form globular tertiary structures that look roughly like sub-microscopic kidney beans. The folded helices form a pocket that holds the working part of each chain, the heme.
A heme group is a flat ring molecule containing carbon, nitrogen and hydrogen atoms, with a single Fe2+ ion at the center. Without the iron, the ring is called a porphyrin. In a heme molecule, the iron is held within the flat plane by four nitrogen ligands from the porphyrin ring. The iron ion makes a fifth bond to a histidine side chain from one of the helices that form the heme pocket. This fifth coordination bond is to histidine 87 in the human a chain and histidine 92 in the human b chain. Both histidine residues are part of the F helix in each globin chain. t
The ability of hemoglobin to release oxygen, is affected by pH, CO2 and by the differences in the oxygen-rich environment of the lungs and the oxygen-poor environment of the tissues. The pH in the tissues is considerably lower (more acidic) than in the lungs. Protons are generated from the reaction between carbon dioxide and water to form bicarbonate:
This increased acidity serves a twofold purpose. First, protons lower the affinity of hemoglobin for oxygen, allowing easier release into the tissues. As all four oxygens are released, hemoglobin binds to two protons. This helps to maintain equilibrium towards the right side of the equation. This is known as the Bohr effect, and is vital in the removal of carbon dioxide as waste because CO2 is insoluble in the bloodstream. The bicarbonate ion is much more soluble, and can thereby be transported back to the lungs after being bound to hemoglobin. If hemoglobin couldn’t absorb the excess protons, the equilibrium would shift to the left, and carbon dioxide couldn’t be removed.
the lungs, this effect works in the reverse direction. In the presence of the
high oxygen concentration in the lungs, the proton affinity decreases. As
protons are shed, the reaction is driven to the left, and CO2 forms as an insoluble gas to be
expelled from the lungs. The proton poor hemoglobin now has a greater affinity
for oxygen, and the cycle continues.
Haemoglobin or hemoglobin (frequently abbreviated as Hb or Hgb) is the iron-containing oxygen-transport metalloprotein in the red blood cells of the blood in vertebratesand other animals; in mammals the protein makes up about 97% of the red cell’s dry content, and around 35% of the total content including water. Hemoglobin transports oxygen from the lungs or gills to the rest of the body, such as to the muscles, where it releases the oxygen load. Hemoglobin also has a variety of other gas-transport and effect-modulation duties, which vary from species to species, and which in invertebrates may be quite diverse.
The name hemoglobin is the concatenation of heme and globin, reflecting the fact that each subunit of hemoglobin is a globular protein with an embedded heme (or haem) group; each heme group contains an iron atom, and this is responsible for the binding of oxygen. The most common type of hemoglobin in mammals contains four such subunits, each with one heme group.
Mutations in the genes for the hemoglobin protein in humans result in a group of hereditary diseases termed the hemoglobinopathies, the most common members of which are sickle-cell disease and thalassemia. Historically in human medicine, the hemoglobinopathy of sickle-cell disease was the first disease to be understood in its mechanism of dysfunction, completely down to the molecular level. However, not all of such mutations produce disease states, and are formally recognized as hemoglobin variants (not diseases).
Hemoglobin (Hb) is synthesized in a complex series of steps. The heme portion is sythesized in both the the mitochondria and cytosol of the immature red blood cell, while the globin protein portions of the molecule are sythesized by ribosomes in the cytosol . Production of Hb continues in the cell throughout its early development from theproerythroblast to the reticulocyte in the bone marrow. At this point, the nucleus is lost in mammals, but not in birds and many other species. Even after the loss of the nucleus in mammals, however, residual ribosomal RNA allows further synthesis of Hb until the reticulocyte loses its RNA soon after entering the vasculature (this hemoglobin-synthetic RNA in fact gives the reticulocyte its reticulated appearance and name).
The empirical chemical formula of the most common human hemoglobin is C2952H4664N812O832S8Fe4, but as noted above, hemoglobins vary widely across species, and even (through common mutations) slightly among subgroups of humans.
In humans, the hemoglobin molecule is an assembly of four globular protein subunits. Each subunit is composed of a protein chain tightly associated with a non-proteinheme group. Each protein chain arranges into a set of alpha-helix structural segments connected together in a globin fold arrangement, so called because this arrangement is the same folding motif used in other heme/globin proteins such as myoglobin. This folding pattern contains a pocket which strongly binds the heme group.
A heme group consists of an iron (Fe) atom held in a heterocyclic ring, known as a porphyrin. The iron atom, which is the site of oxygen binding, bonds with the fournitrogens in the center of the ring, which all lie in one plane. The iron is also bound strongly to the globular protein via the imidazole ring of a histidine residue below the porphyrin ring. A sixth position can reversibly bind oxygen, completing the octahedral group of six ligands. Oxygen binds in an "end-on bent" geometry where one oxygen atom binds Fe and the other protrudes at an angle. When oxygen is not bound, a very weakly bonded water molecule fills the site, forming a distorted octahedron.
The iron atom may either be in the Fe2+ or Fe3+ state, but ferrihemoglobin (methemoglobin) (Fe3+) cannot bind oxygen. In binding, oxygen temporarily oxidizes Fe to (Fe3+), so iron must exist in the +2 oxidation state in order to bind oxygen. The body reactivates hemoglobin found in the inactive (Fe3+) state by reducing the iron center.
In adult humans, the most common hemoglobin type is a tetramer (which contains 4 subunit proteins) called hemoglobin A, consisting of two α and two β subunits non-covalently bound, each made of 141 and 146 amino acid residues, respectively. This is denoted as α2β2. The subunits are structurally similar and about the same size. Each subunit has a molecular weight of about 17,000 daltons, for a total molecular weight of the tetramer of about 68,000 daltons. Hemoglobin A is the most intensively studied of the hemoglobin molecules.
Oxyhemoglobin is formed during respiration when oxygen binds to the heme component of the protein hemoglobin in red blood cells. This process occurs in the pulmonary capillaries adjacent to the alveoli of the lungs. The oxygen then travels through the blood stream to be dropped off at cells where it is utilized in aerobic glycolysis and in the production of ATP by the process of oxidative phosphorylation. It doesn't however help to counteract a decrease in blood pH. Ventilation, or breathing, may reverse this condition by removal of carbon dioxide, thus causing a shift up in pH.
Deoxyhemoglobin is the form of hemoglobin without the bound oxygen. The absorption spectra of oxyhemoglobin and deoxyhemoglobin differ. The oxyhemoglobine has significantly lower absorption of the 660 nm wavelength than deoxyhemoglobin, while at 940 nm its absorption is slightly higher. This difference is used for measurement of the amount of oxygen in patient's blood by an instrument called pulse oximeter.
Assigning oxygenated hemoglobin's oxidation state is difficult because oxyhemoglobin is diamagnetic (no net unpaired electrons), but the low-energy electron configurations in both oxygen and iron are paramagnetic. Triplet oxygen, the lowest energy oxygen species, has two unpaired electrons in antibonding π* molecular orbitals. Iron(II) tends to be in a high-spin configuration where unpaired electrons exist in eg antibonding orbitals. Iron(III) has an odd number of electrons and necessarily has unpaired electrons. All of these molecules are paramagnetic (have unpaired electrons), not diamagnetic, so an unintuitive distribution of electrons must exist to induce diamagnetism.
X-ray photoelectron spectroscopy suggests that iron has an oxidation state of approximately 3.2 and infrared stretching frequencies of the O-O bond suggests a bond length fitting with superoxide. The correct oxidation state of iron is thus the +3 state with oxygen in the -1 state. The diamagnetism in this configuration arises from the unpaired electron on superoxide aligning antiferromagnetically in the opposite direction from the unpaired electron on iron. The second choice being correct is not surprising because singlet oxygen and large separations of charge are both unfavorably high-energy states. Iron's shift to a higher oxidation state decreases the atom's size and allows it into the plane of the porphyrin ring, pulling on the coordinated histidine residue and initiating the allosteric changes seen in the globulins. The assignment of oxidation state, however, is only a formalism so all three models may contribute to some small degree.
Early postulates by bioinorganic chemists claimed that possibility (1) (above) was correct and that iron should exist in oxidation state II (indeed iron oxidation state III as methemoglobin, when not accompanied by superoxide .O2- to "hold" the oxidation electron, is incapable of binding O2). The iron chemistry in this model was elegant, but the presence of singlet oxygen was never explained. It was argued that the binding of an oxygen molecule placed high-spin iron(II) in an octahedral field of strong-field ligands; this change in field would increase the crystal field splitting energy, causing iron's electrons to pair into the diamagnetic low-spin configuration.
Binding and release of ligands induces a conformational (structural) change in hemoglobin. Here, the binding and release of oxygen illustrates the structural differences between oxy- and deoxyhemoglobin, respectively. Only one of the four heme groups is shown.
As discussed above, when oxygen binds to the iron center it causes contraction of the iron atom, and causes it to move back into the center of the porphyrin ring plane (see moving diagram). At the same time, the porphyrin ring plane itself is pushed away from the oxygen and toward the imidizole side chain of the histidine residue interacting at the other pole of the iron. The interaction here forces the ring plane sideways toward the outside of the tetramer, and also induces a strain on the protein helix containing the histidine, as it moves nearer the iron. This causes a tug on this peptide strand which tends to open up heme units in the remainder of the molecule, so that there is more room for oxygen to bind at their heme sites.
In the tetrameric form of normal adult hemoglobin, the binding of oxygen is thus a cooperative process. The binding affinity of hemoglobin for oxygen is increased by the oxygen saturation of the molecule, with the first oxygens bound influencing the shape of the binding sites for the next oxygens, in a way favorable for binding. This positive cooperative binding is achieved through steric conformational changes of the hemoglobin protein complex as discussed above, i.e. when one subunit protein in hemoglobin becomes oxygenated, this induces a conformational or structural change in the whole complex, causing the other subunits to gain an increased affinity for oxygen. As a consequence, the oxygen binding curve of hemoglobin is sigmoidal, or S-shaped, as opposed to the normal hyperbolic curve associated with noncooperative binding.
Hemoglobin's oxygen-binding capacity is decreased in the presence of carbon monoxide because both gases compete for the same binding sites on hemoglobin, carbon monoxide binding preferentially in place of oxygen. Carbon dioxide occupies a different binding site on the hemoglobin. Through the enzyme carbonic anhydrase, carbon dioxide reacts with water to give carbonic acid, which decomposes into bicarbonate and protons:
Hence blood with high carbon dioxide levels is also lower in pH (more acidic). Hemoglobin can bind protons and carbon dioxide which causes a conformational change in the protein and facilitates the release of oxygen. Protons bind at various places along the protein, and carbon dioxide binds at the alpha-amino group forming carbamate. Conversely, when the carbon dioxide levels in the blood decrease (i.e., in the lung capillaries), carbon dioxide and protons are released from hemoglobin, increasing the oxygen affinity of the protein. This control of hemoglobin's affinity for oxygen by the binding and release of carbon dioxide and acid, is known as the Bohr effect.
The binding of oxygen is affected by molecules such as carbon monoxide (CO) (for example from tobacco smoking, cars and furnaces). CO competes with oxygen at the heme binding site. Hemoglobin binding affinity for CO is 200 times greater than its affinity for oxygen, meaning that small amounts of CO dramatically reduces hemoglobin's ability to transport oxygen. When hemoglobin combines with CO, it forms a very bright red compound called carboxyhemoglobin. When inspired air contains CO levels as low as 0.02%, headache and nausea occur; if the CO concentration is increased to 0.1%, unconsciousness will follow. In heavy smokers, up to 20% of the oxygen-active sites can be blocked by CO.
In similar fashion, hemoglobin also has competitive binding affinity for cyanide (CN-), sulfur monoxide (SO), nitrogen dioxide (NO2), and sulfide (S2-), including hydrogen sulfide (H2S). All of these bind to iron in heme without changing its oxidation state, but they nevertheless inhibit oxygen-binding, causing grave toxicity.
The iron atom in the heme group must be in the Fe2+ oxidation state to support oxygen and other gases' binding and transport. Oxidation to Fe3+ state converts hemoglobin into hemiglobin or methemoglobin (pronounced "MET-hemoglobin"), which cannot bind oxygen. Hemoglobin in normal red blood cells is protected by a reduction system to keep this from happening. Nitrogen dioxide and nitrous oxide are capable of converting a small fraction of hemoglobin to methemoglobin, however this is not usually of medical importance (nitrogen dioxide is poisonous by other mechanisms, and nitrous oxide is routinely used in surgical anesthesia in most people without undue methemoglobin buildup).
In people acclimated to high altitudes, the concentration of 2,3-bisphosphoglycerate (2,3-BPG) in the blood is increased, which allows these individuals to deliver a larger amount of oxygen to tissues under conditions of lower oxygen tension. This phenomenon, where molecule Y affects the binding of molecule X to a transport molecule Z, is called a heterotropic allosteric effect.
A variant hemoglobin, called fetal hemoglobin (HbF, α2γ2), is found in the developing fetus, and binds oxygen with greater affinity than adult hemoglobin. This means that the oxygen binding curve for fetal hemoglobin is left-shifted (i.e., a higher percentage of hemoglobin has oxygen bound to it at lower oxygen tension), in comparison to that of adult hemoglobin. As a result, fetal blood in the placenta is able to take oxygen from maternal blood.
Hemoglobin also carries nitric oxide in the globin part of the molecule. This improves oxygen delivery in the periphery and contributes to the control of respiration. NO binds reversibly to a specific cystein residue in globin; the binding depends on the state (R or T) of the hemoglobin. The resulting S-nitrosylated hemoglobin influences various NO-related activities such as the control of vascular resistance, blood pressure and respiration. NO is released not in the cytoplasm of erythrocytes but is transported by an anion exchanger called AE1 out of them.
When red cells reach the end of their life due to aging or defects, they are broken down, the hemoglobin molecule is broken up and the iron gets recycled. When the porphyrin ring is broken up, the fragments are normally secreted in the bile by the liver. This process also produces one molecule of carbon monoxide for every molecule of heme degraded ; this is one of the few natural sources of carbon monoxide production in the human body, and is responsible for the normal blood levels of carbon monoxide even in people breathing pure air. The other major final product of heme degradation is bilirubin. Increased levels of this chemical are detected in the blood if red cells are being destroyed more rapidly than usual. Improperly degraded hemoglobin protein or hemoglobin that has been released from the blood cells too rapidly can clog small blood vessels, especially the delicate blood filtering vessels of the kidneys, causing kidney damage
Decrease of hemoglobin, with or without an absolute decrease of red blood cells, leads to symptoms of anemia. Anemia has many different causes, although iron deficiencyand its resultant iron deficiency anemia are the most common causes in the Western world. As absence of iron decreases heme synthesis, red blood cells in iron deficiency anemia are hypochromic (lacking the red hemoglobin pigment) and microcytic (smaller than normal). Other anemias are rarer. In hemolysis (accelerated breakdown of red blood cells), associated jaundice is caused by the hemoglobin metabolite bilirubin, and the circulating hemoglobin can cause renal failure.
Some mutations in the globin chain are associated with the hemoglobinopathies, such as sickle-cell disease and thalassemia. Other mutations, as discussed at the beginning of the article, are benign and are referred to merely as hemoglobin variants.
There is a group of genetic disorders, known as the porphyrias that are characterized by errors in metabolic pathways of heme synthesis. King George III of the United Kingdom was probably the most famous porphyria sufferer.
To a small extent, hemoglobin A slowly combines with glucose at a certain location in the molecule. The resulting molecule is often referred to as Hb A1c. As theconcentration of glucose in the blood increases, the percentage of Hb A that turns into Hb A1c increases. In diabetics whose glucose usually runs high, the percent Hb A1c also runs high. Because of the slow rate of Hb A combination with glucose, the Hb A1c percentage is representative of glucose level in the blood averaged over a longer time (the half-life of red blood cells, which is typically 50-55 days).
Hemoglobin levels are amongst the most commonly performed blood tests, usually as part of a full blood count or complete blood count. Results are reported in g/L, g/dLor mol/L. For conversion, 1 g/dL is 0.621 mmol/L. If the total hemoglobin concentration in the blood falls below a set point, this is called anemia. Normal values for hemoglobin levels are:
Anemias are further subclassified by the size of the red blood cells, which are the cells which contain hemoglobin in vertebrates. They can be classified as microcytic (small sized red blood cells), normocytic (normal sized red blood cells), or macrocytic (large sized red blood cells). The hemaglobin is the typical test used for blood donation. A comparison with the hematocrit can be made by multiplying the hemaglobin by three. For example, if the hemaglobin is measured at 17, that compares with a hematocrit of .51
Glucose levels in blood can vary widely each hour, so one or only a few samples from a patient analyzed for glucose may not be representative of glucose control in the long run. For this reason a blood sample may be analyzed for Hb A1c level, which is more representative of glucose control averaged over a longer time period (determined by the half-life of the individual's red blood cells, which is typically 50-55 days). People whose Hb A1c runs 6.0% or less show good longer-term glucose control. Hb A1c values which are more than 7.0% are elevated. This test is especially useful for diabetics.
Acid–base homeostasis is the part of human homeostasis concerning the proper balance between acids and bases, also called body pH. The body is very sensitive to its pH level, so strong mechanisms exist to maintain it. Outside the acceptable range of pH, proteins are denatured and digested, enzymes lose their ability to function, and death may occur.
A buffer solution is an aqueous solution consisting of a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. Its pH changes very little when a small amount of strong acid or base is added to it. Buffer solutions are used as a means of keeping pH at a nearly constant value in a wide variety of chemical applications.
Many life forms thrive only in a relatively small pH range so they utilize a buffer solution to maintain a constant pH. One example of a buffer solution found in nature is blood. The body's acid–base balance is normally tightly regulated, keeping the arterial blood pH between 7.38 and 7.42. Several buffering agents that reversibly bind hydrogen ions and impede any change in pH exist. Extracellular buffers include bicarbonate and ammonia, whereas proteins and phosphate act as intracellular buffers. The bicarbonate buffering system is especially key, as carbon dioxide (CO2) can be shifted through carbonic acid (H2CO3) to hydrogen ions and bicarbonate (HCO3-):
Acid–base imbalances that overcome the buffer system can be compensated in the short term by changing the rate of ventilation. This alters the concentration of carbon dioxide in the blood, shifting the above reaction according to Le Chatelier's principle, which in turn alters the pH.
The kidneys are slower to compensate, but renal physiology has several powerful mechanisms to control pH by the excretion of excess acid or base. In response to acidosis, tubular cells reabsorb more bicarbonate from the tubular fluid, collecting duct cells secrete more hydrogen and generate more bicarbonate, and ammoniagenesis leads to increased formation of the NH3 buffer. In responses to alkalosis, the kidneys may excrete more bicarbonate by decreasing hydrogen ion secretion from the tubular epithelial cells, and lowering rates of glutamine metabolism and ammonium excretion.
This huge buffer capacity has another not immediately obvious implication for how we think about the severity of an acid-base disorder. You would think that the magnitude of an acid-base disturbance could be quantified merely by looking at the change in [H+] - BUT this is not so.
Because of the large buffering capacity, the actual change in [H+] is so small it can be ignored in any quantitative assessment, and instead, the magnitude of a disorder has to be estimated indirectly from the decrease in the total concentration of the anions involved in the buffering. The buffer anions, represented as A-, decrease because they combine stoichiometrically with H+ to produce HA. A decrease in A- by 1 mmol/l represents a 1,000,000 nano-mol/l amount of H+ that is hidden from view and this is several orders of magnitude higher than the visible few nanomoles/l change in [H+] that is visible.) - As noted above in the comments about the Swan & Pitts experiment, 13,999,994 out of 14,000,000 nano-moles/l of H+ were hidden on buffers and just to count the 36 that were on view would give a false impression of the magnitude of the disorder.
The major buffer system in the ECF is the CO2-bicarbonate buffer system. This is responsible for about 80% of extracellular buffering. It is the most important ECF buffer for metabolic acids but it cannot buffer respiratory acid-base disorders.
The pK’a value is dependent on the temperature, [H+] and the ionic concentration of the solution. It has a value of 6.099 at a temperature of 37C and a plasma pH of 7.4. At a temperature of 30C and pH of 7.0, it has a value of 6.148. For practical purposes, a value of 6.1 is generally assumed and corrections for temperature, pH of plasma and ionic strength are not used except in precise experimental work.
The concentration of H2O is so large (55.5M) compared to the other components, the small loss of water due to this reaction changes its concentration by only an extremely small amount. This means that [H2O] is effectively constant. This allows further simplification as the two constants (Ka and [H2O] ) can be combined into a new constant K’a.
On chemical grounds, a substance with a pKa of 6.1 should not be a good buffer at a pH of 7.4 if it were a simple buffer. The system is more complex as it is ‘open at both ends’ (meaning both [HCO3] and pCO2 can be adjusted) and this greatly increases the buffering effectiveness of this system. The excretion of CO2 via the lungs is particularly important because of the rapidity of the response. The adjustment of pCO2 by change in alveolar ventilation has been referred to as physiological buffering.
The bicarbonate buffer system is an effective buffer system despite having a low pKa because the body also controls pCO2
At the prevailing pH values in most biological systems, monohydrogen phosphate (HPO4-2) and dihydrogen phosphate (H2PO4-) are the two species present. The pKa2 is 6.8 and this makes the closed phosphate buffer system a good buffer intracellularly and in urine. The pH of glomerular ultrafiltrate is 7.4 and this means that phosphate will initially be predominantly in the monohydrogen form and so can combine with more H+ in the renal tubules. This makes the phosphate buffer more effective in buffering against a drop in pH than a rise in pH.
Note: The ‘true’ pKa2 value is actually 7.2 if measured at zero ionic strength but at the typical ionic strength found in the body its apparent value is 6.8. The other factor which makes phosphate a more effective buffer intracellularly and in urine is that its concentration is much higher here than in extracellular fluid.
Protein buffers in blood include haemoglobin (150g/l) and plasma proteins (70g/l). Buffering is by the imidazole group of the histidine residues which has a pKa of about 6.8. This is suitable for effective buffering at physiological pH. Haemoglobin is quantitatively about 6 times more important then the plasma proteins as it is present in about twice the concentration and contains about three times the number of histidine residues per molecule. For example if blood pH changed from 7.5 to 6.5, haemoglobin would buffer 27.5 mmol/l of H+ and total plasma protein buffering would account for only 4.2 mmol/l of H+.
Deoxyhaemoglobin is a more effective buffer than oxyhaemoglobin and this change in buffer capacity contributes about 30% of the Haldane effect. The major factor accounting for the Haldane effect in CO2 transport is the much greater ability of deoxyhaemoglobin to form carbamino compounds.
This buffer functions in exactly the same way as the phosphate buffer. Additional H+ is consumed by HCO3- and additional OH- is consumed by H2CO3. The value of Ka for this equilibrium is 7.9 Ч 10-7, and the pKa is 6.1 at body temperature. In blood plasma, the concentration of hydrogen carbonate ion is about twenty times the concentration of carbonic acid. The pH of arterial blood plasma is 7.40. If the pH falls below this normal value, a condition called acidosis is produced. If the pH rises above the normal value, the condition is called alkalosis.
The concentrations of hydrogen carbonate ions and of carbonic acid are controlled by two independent physiological systems. Carbonic acid concentration is controlled by respiration, that is through the lungs. Carbonic acid is in equilibrium with dissolved carbon dioxide gas.
The much higher concentration of hydrogen carbonate ion over that of carbonic acid in blood plasma allows the buffer to respond effectively to the most common materials that are released into the blood. Normal metabolism releases mainly acidic materials: carboxylic acids such as lactic acid (HLac). These acids react with hydrogen carbonate ion and form carbonic acid.
The condition called respiratory acidosis occurs when blood pH falls as a result of decreased respiration. When respiration is restricted, the concentration of dissolved carbon dioxide in the blood increases, making the blood too acidic. Such a condition can be produced by asthma, pneumonia, emphysema, or inhaling smoke.
Metabolic acidosis is the decrease in blood pH that results when excessive amounts of acidic substances are released into the blood. This can happen through prolonged physical exertion, by diabetes, or restricted food intake. The normal body response to this condition is increases breathing to reduce the amount of dissolved carbon dioxide in the blood. This is why we breathe more heavily after climbing several flights of stairs.
Respiratory alkalosis results from excessive breathing that produces an increase in blood pH. Hyperventilation causes too much dissolved carbon dioxide to be removed from the blood, which decreases the carbonic acid concentration, which raises the blood pH. Often, the body of a hyperventilating person will react by fainting, which slows the breathing.
Metabolic alkalosis is an increase in blood pH resulting from the release of alkaline materials into the blood. This can result from the ingestion of alkaline materials, and through overuse of diuretics. Again, the body usually responds to this condition by slowing breathing, possibly through fainting.
The carbonic acid-hydrogen carbonate ion buffer works throughout the body to maintain the pH of blood plasma close to 7.40. The body maintains the buffer by eliminating either the acid (carbonic acid) or the base (hydrogen carbonate ions). Changes in carbonic acid concentration can be effected within seconds through increased or decreased respiration. Changes in hydrogen carbonate ion concentration, however, require hours through the relatively slow elimination through the kidneys
Albumins – multidispersed fraction of blood plasma which are characterized by the high electrophoretic mobility and mild dissolubility in water and saline solutions. Molecular weight of albumins is about 60000. Due to high hydrophilic properties albumins bind a significant amount of water, and the volume of their molecule under hydratation is doubled. Hydrative layer formed around the serum albumins provides to 70-80 % of oncotic pressure of blood plasma proteins, that can be applied in clinical practice at albumins transfusion to patients with tissue edemas. The decreasing of albumins concentration in blood plasma, for example under disturbance of their synthesis in hepatocytes at liverfailure, can cause the water transition from a vessels into the tissues and development of oncotic edemas.
Albumins execute also important physiological function as transporters of a lot of metabolites and diverse low molecular weight structures. The molecules of albumins have several sites with centers of linkage for molecules of organic ligands, which are affixed by the electrostatic and hydrophobic bonds. Serum albumins can affix and convey fatty acids, cholesterol, cholic pigments (bilirubin and that similar), vitamins, hormones, some amino acids, toxins and medicines.
Globulins - heterogeneous fraction of blood proteins which execute transport (a1-globulins – transport of lipids, thyroxin, corticosteroid hormones; a2-globulins - transport of lipids, copper ions; b-globulins - transport of lipids, iron) and protective (participation of b-globulins in immune reactions as antitoxins; g-globulins as immunoglobulins) functions. They also support the blood oncotic pressure and acid-alkaline balance, provide amino acids for the organism requirements. The molecular weight of globulins is approximately 150000-300000.
The globulin level in blood plasma is 20-40 g/l. A ratio between concentrations of albumins and globulins (so called “protein coefficient”) in blood plasma is often determined in clinical practice. In healthy people this coefficient is 1,5-2,0.
Fibrinogen – important protein of blood plasma, precursor of fibrin, the structural element of blood clots. Fibrinogen participates in blood clotting and thus prevents the loss of blood from the vascular system of vertebrates. The approximate molecular weight of fibrinogen is 340000. It is the complex protein, it contains the carbohydrate as prosthetic group. The content of firinogen in blood is 3-4 g/l.
Immunoglobulins (Ig A, Ig G, Ig E, Ig M) - proteins of g-globulin fraction of blood plasma executing the functions of antibodies which are the main effectors of humoral immunity. They appear in the blood serum and certain cells of a vertebrate in response to the introduction of a protein or some other macromolecule foreign to that species.
Immunoglobulin molecules have bindind sites that are specific for and complementary to the structural features of the antigen that induced their formation. Antibodies are highly specific for the foreign proteins that evoke their formation.
The acute phase response develops in a wide range of acute and chronic inflammatory conditions like bacterial, viral, or fungal infections; rheumatic and other inflammatory diseases; malignancy; and tissue injury or necrosis. These conditions cause release of interleukin-6 and other cytokines that trigger the synthesis of CRP and fibrinogen by the liver. During the acute phase response, levels of CRP rapidly increase within 2 hours of acute insult, reaching a peak at 48 hours. With resolution of the acute phase response, CRP declines with a relatively short half-life of 18 hours. Measuring CRP level is a screen for infectious and inflammatory diseases. Rapid, marked increases in CRP occur with inflammation, infection, trauma and tissue necrosis, malignancies, and autoimmune disorders. Because there are a large number of disparate conditions that can increase CRP production, an elevated CRP level does not diagnose a specific disease. An elevated CRP level can provide support for the presence of an inflammatory disease, such as rheumatoid arthritis, polymyalgia rheumatica or giant-cell arteritis.
The physiological role of CRP is to bind to phosphocholine expressed on the surface of dead or dying cells (and some types of bacteria) in order to activate the complement system. CRP binds to phosphocholine on microbes and damaged cells and enhances phagocytosis by macrophages. Thus, CRP participates in the clearance of necrotic and apoptotic cells.
CRP is a member of the class of acute-phase reactants, as its levels rise dramatically during inflammatory processes occurring in the body. This increment is due to a rise in the plasma concentration of IL-6, which is produced predominantly by macrophages as well asadipocytes. CRP binds to phosphocholine on microbes. It is thought to assist in complement binding to foreign and damaged cells and enhances phagocytosis by macrophages (opsonin mediated phagocytosis), which express a receptor for CRP. It is also believed to play another important role in innate immunity, as an early defense system against infections.
CRP rises up to 50,000-fold in acute inflammation, such as infection. It rises above normal limits within 6 hours, and peaks at 48 hours. Its half-life is constant, and therefore its level is mainly determined by the rate of production (and hence the severity of the precipitating cause).
C-reactive protein (g-fraction). This protein received the title owing to its capacity to react with C-polysaccharide of a pneumococcus forming precipitates. According to its chemical nature C-reactive protein is glycoprotein.
In blood plasma of healthy people the C-reactive protein is absent but it occurs at pathological states accompanied by an inflammation and necrosis of tissues. The availability of C-reactive protein is characteristic for the acute period of diseases – “protein of an acute phase”. The determination of C-reactive protein has diagnostic value in an acute phase of rheumatic disease, at a myocardial infarction, pneumococcal, streptococcal, staphylococcal infections.
Measuring and charting CRP values can prove useful in determining disease progress or the effectiveness of treatments. Blood, usually collected in a serum-separating tube, is analysed in amedical laboratory or at the point of care. Various analytical methods are available for CRP determination, such as ELISA, immunoturbidimetry, rapid immunodiffusion, and visual agglutination.
Normal concentration in healthy human serum is usually lower than 10 mg/L, slightly increasing with aging. Higher levels are found in late pregnant women, mild inflammation and viral infections (10–40 mg/L), active inflammation, bacterial infection (40–200 mg/L), severe bacterial infections and burns (>200 mg/L).
CRP is a more sensitive and accurate reflection of the acute phase response than the ESR (Erythrocyte Sedimentation Rate). The half-life of CRP is constant. Therefore, CRP level is mainly determined by the rate of production (and hence the severity of the precipitating cause). In the first 24 h, ESR may be normal and CRP elevated. CRP returns to normal more quickly than ESR in response to therapy.
Arterial damage results from white blood cell invasion and inflammation within the wall. CRP is a general marker for inflammation and infection, so it can be used as a very rough proxy for heart disease risk. Since many things can cause elevated CRP, this is not a very specific prognostic indicator. Nevertheless, a level above 2.4 mg/L has been associated with a doubled risk of a coronary event compared to levels below 1 mg/L; however, the study group in this case consisted of patients who had been diagnosed with unstable angina pectoris; whether elevated CRP has any predictive value of acute coronary events in the general population of all age ranges remains unclear.
Crioglobulin - the protein of the g-globulin fraction. Like to the C-reactive protein crioglobulin absent in blood plasma of the healthy people and occurs at leukoses, rheumatic disease, liver cirrhosis, nephroses. The characteristic physico-chemical feature of crioglobulin is its dissolubility at standard body temperature (37 oC) and capacity to form the sediment at cooling of a blood plasma up to 4 oC.
The biological role of a2-macroglobulin consists in regulation of the tissue proteolysis systems which are very important in such physiological and pathological processes as blood clotting, fibrinolysis, processes of immunodefence, functionality of a complement system, inflammation, regulation of vascular tone (kinine and renin-angiothensine system).
a1-antitrypsin (a1-globulin) – glycoprotein with a molecular weight 55 kDa. Its concentration in blood plasma is 2-3 г/л. The main biological property of this inhibitor is its capacity to form complexes with proteinases oppressing proteolitic activity of such enzymes as trypsin, chemotrypsin, plasmin, trombin. The content of a1-antitrypsin is markedly increased in inflammatory processes. The inhibitory activity of a1-antitrypsin is very important in pancreas necrosis and acute pancreatitis because in these conditions the proteinase level in blood and tissues is sharply increased. The congenital deficiency of a1-antitrypsin results in the lung emphysema.
Fibronectin – glycoprotein of blood plasma that is synthesized and secreted in intercellular space by different cells. Fibronectin present on a surface of cells, on the basalmembranes, in connective tissue and in blood. Fibronectin has properties of a «sticking» protein and contacts with the carbohydrate groups of gangliosides on a surface of plasma membranes executing the integrative function in intercellular interplay. Fibronectin also plays important role in the formation of the pericellular matrix.
Haptoglobin - protein of a2-globulin fraction of blood plasma. Haptoglobin has capacity to bind a free haemoglobin forming a complex that refer to b-globulins electrophoretic fraction. Normal concentration in blood plasma - 0,10-0,35 g/l.
Haptoglobin-hemoglobin complexes are absorbed by the cells of reticulo-endothelial system, in particular in a liver, and oxidized to cholic pigments. Such haptoglobinfunction promotes the preservation of iron ions in an organism under conditions of a physiological and pathological erythrocytolysis.
Transferrin - glycoprotein belonging to the b-globulin fraction. It binds in a blood plasma iron ions (Fe3+). The protein has on the surface two centers of linkage of iron.Transferrin is a transport form of iron delivering its to places of accumulation and usage.
Ceruloplasmin - glycoprotein of the a2-globulin fraction. It can bind the copper ions in blood plasma. Up to 3 % of all copper contents in an organism and more than 90 % copper contents in plasma is included in ceruloplasmin. Ceruloplasmin has properties of ferroxidase oxidizing the iron ions. The decrease of ceruloplasmin in organism (Wilson disease) results in exit of copper ions from vessels and its accumulation in the connective tissue that shows by pathological changes in a liver, main brain, cornea.
Albumins, a1-globulins, fibrinogen are fully synthesized in hepatocytes. Immunoglobulins are produced by plasmocytes (immune cells). In liver cryoglobulins and some other g-globulins are produced too. a2-globulins and b-globulins are partly synthesized in liver and partly in reticuloendothelial cells.
Hypoproteinemia - decrease of the total contents of proteins in blood plasma. This state occurs in old people as well as in pathological states accompanying with the oppressing of protein synthesis (liver diseases) and activation of decomposition of tissue proteins (starvation, hard infectious diseases, state after hard trauma and operations, cancer). Hypoproteinemia (hypoalbuminemia) also occurs in kidney diseases, when the increased excretion of proteins via the urine takes place.
Absolute hyperproteinemia – accumulation of the proteins in blood. It occurs in infection and inflammatory diseases (hyperproduction of immunoglobulins), rheumatic diseases (hyperproduction of C-reactive protein), some malignant tumors (myeloma) and others.
Relative hyperproteinemia – the increase of the protein concentration but not the absolute amount of proteins. It occurs when organism loses water (diarrhea, vomiting, fever, intensive physical activity etc.).
Electrophoresis is the separation of proteins on the basis of their electric charge. It depends ultimately on their base-acid properties, which are largely determined by the number and types of ionizable R groups in their polipeptide chains. Since proteins differ in amino acid composition and sequence, each protein has distinctive acid-base properties. There are a number of different forms of electroforesis useful for analyzing and separating mixtures of proteins
If a precursor of an antibody-secreting cell becomes cancerous, it divides uncontrollably to generate a clone of plasma cellssecreting a single kind of antibody molecule. The image (courtesy of Beckman Instruments, Inc.) shows — from left to right — the electrophoretic separation of:
§ Gamma globulins can be harvested from donated blood (usually pooled from several thousand donors) and injected into persons exposed to certain diseases such as chicken pox and hepatitis. Because such preparations of immune globulin contain antibodies against most common infectious diseases, the patient gains temporary protection against the disease.
The state of protein nutrition can be determined by measuring the dietary intake and output of nitrogenous compounds from the body. Although nucleic acids also contain nitrogen, protein is the major dietary source of nitrogen and measurement of total nitrogen intake gives a good estimate of protein intake (mg N Ч 6.25 = mg protein, as nitrogen is 16% of most proteins). The output of nitrogen from the body is mainly in urea and smaller quantities of other compounds in urine and undigested protein in feces, and significant amounts may also be lost in sweat and shed skin.
The difference between intake and output of nitrogenous compounds is known as nitrogen balance. Three states can be defined: In a healthy adult, nitrogen balance is inequilibrium when intake equals output, and there is no change in the total body content of protein. In a growing child, a pregnant woman, or in recovery from protein loss, the excretion of nitrogenous compounds is less than the dietary intake and there is net retention of nitrogen in the body as protein, ie, positive nitrogen balance. In response to trauma or infection or if the intake of protein is inadequate to meet requirements there is net loss of protein nitrogen from the body, ie, negative nitrogen balance. The continual catabolism of tissue proteins creates the requirement for dietary protein even in an adult who is not growing, though some of the amino acids released can be reutilized.
Nitrogen balance studies show that the average daily requirement is 0.6 g of protein per kilogram of body weight (the factor 0.75 should be used to allow for individual variation), or approximately 50 g/d. Average intakes of protein in developed countries are about 80–100 g/d, ie, 14–15% of energy intake. Because growing children are increasing the protein in the body, they have a proportionately greater requirement than adults and should be in positive nitrogen balance. Even so, the need is relatively small compared with the requirement for protein turnover. In some countries, protein intake may be inadequate to meet these requirements, resulting in stunting of growth.
Organic and inorganic compounds of residual nitrogen are as follows: urea (50 % of the residual nitrogen), amino acids (25 %), creatine and creatinine (7,5 %), salts of ammonia and indicane (0,5 %), other compounds (about 13 %).
Urea is formed in liver during the degradation of amino acids, pyrimidine nucleotides and other nitrogen containing compounds. Amino acids are formed as result of protein decomposition or owing to the conversion of fatty acids or carbohydrates to amino acids. The pool of amino acids in blood is also supported by the process of their absorption in intestine. Creatine is produced in kidneys and liver from amino acids glycine and arginine, creatinine is formed in muscles as result of creatine phosphate splitting. In result of ammonia neutralization the ammonia salts can be formed. Indicane is the product of indol neutralization in the liver.
The removal of the amino group of an amino acid by transamination or oxidative deamination produces an α-keto acid that contains the carbon skeleton from the amino acid (nonnitrogen residues). These α-keto acids can be used for the biosynthesis of non-essential amino acids or undergoes a different degradation process. For alanine and serine, the degradation requires a single step. For most carbon arrangements, however, multistep reaction sequences are required. There are only seven degradation sequences for 20 amino acids. The seven degradation products are pyruvate, acetyl CoA, acetoacetyl CoA, α-ketoglutarate, succinyl CoA, fumarate, and oxaloacetate. The last four products are intermediates in the citric acid cycle. Some amino acids have more than one pathway for degradation.
The major point of entry into the tricarboxylate cycle is via acetyl-CoA; 10 amino acids enter by this route. Of these, six (alanine, glycine, serine, threonine, tryptophan and cysteine) are degraded to acetyl-CoA via pyruvate, five (phenylalanine, tyrosine, leucine, lysine, and tryptophan) are degraded via acetoacetyl-CoA, and three (isoleucine, leucine and tryptophan) yield acetyl-CoA directly. Leucine and tryptophan yield both acetoacetyl-CoA and acetyl-CoA as end products.
Amino acids that are degraded to citric acid cycle intermediates can serve as glucose precursors and are called glucogenic. A glucogenic amino acid is an amino acid whose carbon-containing degradation product(s) can be used to produce glucose via gluconeogenesis.
Amino acids that are degraded to acetyl CoA or acetoacetyl CoA can contribute to the formation of fatty acids or ketone bodies and are called ketogenic. Aketogenic amino acid is an amino acid whose carbon-containing degradation product(s) can be used to produce ketone bodies.
Only two amino acids are purely ketogenic: leucine and lysine. Nine amino acids are both glucogenic and ketogenic: those degraded to pyruvate (alanine, glycine, cysteine, serine, threonine, tryptophan), as well as tyrosine, phenylalanine, and isoleucine (which have two degradation products). The remaining nine amino acids are purely glucogenic (arginine, asparagine, aspartate, glutamine, glutamate, valine, histidine, methionine, proline)
The regulation of protein metabolism. Protein metabolism is regulated by different hormones. All hormones according to their action on protein synthesis or splitting are divided on two groups: anabolic and catabolic. Anabolic hormones promote to the protein synthesis. Catabolic hormones enhance the decomposition of proteins.