Evolution has honed the Hb tetramer in different species into a molecular variant ideally suited for its tasks. Human Hb, adapted for life largely near sea level, must behave differently than that of high-altitude dwelling species or species inhabiting hypoxic locales. Many different variants of the same basic molecular design have thus evolved. Because of the exigencies of molecular evolution, we find in the genome of all animals, including humans, attempts by nature to propagate a variety of different globin genes. The crystallographic studies of Perutz et al. defined the oxygenated and deoxygenated structures of Hb at Ångström-unit resolution and provided an exquisitely detailed picture of how the globin chains and individual amino acid residues respond to the loading and unloading of oxygen. All of these, however, share the properties of highly reversible oxygen binding and high solubility in cytoplasm. We know more about the function of Hb than about virtually any other protein, and the knowledge of this mechanism provides a beautiful and intellectually satisfying culmination to decades of study by many investigators.

The oxygen dissociation curve of Hb, shown in Fig. 1, describes the percent saturation of Hb with oxygen at different oxy gen tensions. The sigmoidal shape of this curve is a result of interaction among the subunits of Hb. Communication within the tetramer is called heme–heme interaction or cooperativity. This implies that the four heme groups do not undergo simultaneous oxygenation or deoxygenation but rather that the state of each heme unit with regard to the presence or absence of bound oxygen influences the binding of oxygen to other heme groups. Myoglobin, a heme-containing protein with virtually the same tertiary structure as globin, exists in muscle as a monomer. The oxygen equilibrium curve of myoglobin is a rectangular hyperbola; in physiologic terms, it rapidly becomes fully saturated at low oxygen tensions and remains saturated as the oxygen tension plateaus. The difference in the oxygen equilibrium curves of myoglobin and Hb lies in the tetrameric nature of the Hb molecule and the cooperativity permitted by the association of simi lar but unlike subunits. Compared with Hb, myoglobin has a very low P50 (i.e., oxygen partial pressure at which the molecule is one half saturated). It therefore has an extremely high oxygen affinity and would not be useful for delivering oxygen to tissues. The oxy gen in myoglobin is passed on to the mitochondria, where oxidative metabolism occurs. The sigmoidal shape of the oxygen dissociation curve of Hb indicates that the totally deoxygenated Hb tetramer is slow to become oxygenated, but as oxygenation proceeds, the reaction of heme with oxygen accelerates. Perutz has drawn an analogy in which the “appetite” of heme for oxygen grows with the “eating,” and conversely, loss of oxygen by heme lowers the oxygen affinity of the remaining heme groups. The Hill coefficient, n, that can be calculated from plots of oxygen equilibrium curves is a description of heme–heme interaction or cooperativity that explains in part the oxygen-binding properties of Hb and myoglobin. The Hill coefficient for myoglobin is 1, indicating no cooperativity; n is approximately 3 for the normal human HbA molecule.

Fig1. OXYGEN DISSOCIATION CURVE OF HEMOGLOBIN. The percent saturation of hemoglobin (Hb) with oxygen at different oxygen tensions is depicted by the red sigmoidal curve. The P50 (i.e., oxygen tension at which the Hb molecule is one-half saturated) is approximately 27 mmHg in normal erythrocytes (dotted lines). Heterotopic modifiers of Hb function can shift the curve leftward by increasing or rightward by decreasing its oxy gen affinity. BPG, Bisphosphoglycerate Pco2 , partial pressure of carbon dioxide; Po2 , partial pressure of oxygen. (Reproduced with permission from Benz EJ, Jr. Synthesis, structure, and function of hemoglobin. In: Kelly WN, DeVita VT, eds. Textbook of Internal Medicine. vol 1. Philadelphia, PA: JB Lippincott; 1989:236.)

The oxygen affinity of Hb within the erythrocyte does not depend solely on the intrinsic properties of the tetramer. The position of the Hb oxygen dissociation curve, and therefore the P50, can be influenced by a number of heterotropic modifiers, including temperature, pH, and small organic phosphate molecules in the cell. The effects of these modifiers on P50 are shown in Fig1.

Hb is the prototype of an allosteric protein; its structure and function are influenced by other molecules. The major intracellular modulator of Hb–oxygen affinity in human erythrocytes is 2,3-BPG, an intermediate product of glycolysis that is present within the erythrocyte at concentrations equimolar to Hb. The synthesis of 2,3-BPG is enzymatically regulated, and its levels can change depending on the conditions extant. 2,3-BPG is able to bind stereospecifically within the central cavity of the Hb tetramer. Hb prepared in the absence of 2,3-BPG has a very high oxygen affinity, but as 2,3-BPG is added to a Hb solution, the oxygen affinity progressively decreases. 2,3-BPG is a polyanion that binds strongly to the deoxygenated form of Hb but poorly to its oxygenated or other liganded forms. Specific amino acids are involved in the binding of 2,3-BPG; these β-chain residues include the N-terminal valines, H21 histidine (position 143), and EF6 lysine (position 82). In oxyhemoglobin, the H helices of the β-chains are insufficiently spread to permit firm binding of 2,3-BPG; this, along with other conformational changes, favors the binding of this anion to the deoxygenated rather than the oxygenated form of Hb. The binding of 2,3-BPG stabilizes the tense (T) structure of the deoxygenated form at the expense of the relaxed (R) structure of the oxyhemoglobin tetramer.

The transition from the deoxy (T) to the oxy (R) form of Hb is accompanied by rotation of the αβ dimers along the α1–β2 con tact region (Fig. 2). The T structure is stabilized by salt bridges, which are broken as the molecule switches into the R structure. Some abnormal Hbs with an intrinsically high oxygen affinity, or low P50 , occur as a result of an amino acid substitution that leads to the loss of bonds that stabilize the tetramer in the T conformation. Hydrogen ions, chloride ions, and carbon dioxide all decrease the affinity of Hb for oxygen by strengthening the salt bridges that lock the molecule into its T conformation. The corollary of the lowering of Hb–oxygen affinity by protons is the combination of Hb with protons on deoxygenation. This is known as the Bohr effect and is responsible for car bon dioxide transport in blood, another critical function of the Hb molecule. Deoxyhemoglobin binds the hydrogen ion liberated by the reaction of carbon dioxide with water, increasing the concentration of bicarbonate. Within the lungs, hydrogen ions are lost as Hb binds oxygen; therefore, carbon dioxide leaves the solution and is excreted from the body through the lungs. Deoxyhemoglobin can also directly bind carbon dioxide; however, this process involves the minority of carbon dioxide exchanged by the RBCs.

Fig2. SUBUNIT MOTION IN THE HEMOGLOBIN TETRAMER. The relative motion of hemoglobin subunits on oxygenation and deoxygenation is shown. The α1 β1 dimer (black) is moving relative to the α2 β2 dimer (shaded). The oxyhemoglobin tetramer (R state) is more compact than the deoxyhemoglobin configuration (T state). (Reproduced with permission from Dickerson RE, Geis I. Hemoglobin: Structure, Function, and Evolution Pathology. Menlo Park, CA: Benjamin-Cummings; 1983.)

RBCs containing high levels of Hb F have high oxygen affinity because it binds to 2,3-BPG poorly. Physiologically, this predicts that the Hb of fetuses should be oxygenated at the expense of the maternal HbA. The high oxygen affinity of HbF is accounted for by a single change in its primary structure, the presence of a serine residue at helical position H21 in place of the histidine found in the β-globin chain. This weakens the binding of 2,3-BPG and leads to the stabilization of the molecule in its R state.

Interactions of Hb with NO have been a recent focus of investigation. NO, generated from l-arginine by NO synthases, activates soluble guanylate cyclase to produce the second messenger cyclic guanosine monophosphate. As a potent vasodilator, NO is an important regulator of vascular tone. The reaction of free NO with erythrocytes is diffusion limited. Normally, the primary NO–Hb adduct is nitrosyl (heme) Hb (HbFe[II]NO). Within the erythrocyte, β93 cysteine is reduced and seems incapable of NO storage and delivery by S-nitrosohemoglobin, as originally proposed. NO was thought to form S-nitrosylhemoglobin in the lungs, where Hb is in its R or oxygenated state, and liberate NO in the microcirculation, where the transition of the R to T conformation induced by deoxygenation released NO from Hb. However, studies suggest that NO binding to heme groups is physiologically a rapidly reversible process. This view supports a model of Hb delivery of NO distinct from its dissociation from the β93 cysteine residues. Small nitrosothiol molecules could also be involved in NO transfer. The thiol groups of Hb can exchange NO with small nitrosothiols derived from free cysteine and glutathione. Accordingly, the thiol groups of Hb could bind and transfer NO or exchange NO with small shuttle molecules, increasing the per fusion of hypoxic tissues. It has been suggested that cytoskeletal and other erythrocyte proteins slow NO influx into the cell and, coupled with NO heme binding, preserve NO bioactivity. NO Hb interactions, whether through S-nitrosohemoglobin formation at the β93 cysteine or the formation of nitroso intermediates, are likely to be physiologically important. Hb liberated from intra vascularly hemolyzed RBCs rapidly inactivates NO. As the RBC lyses, arginase is also released and destroys the substrate for NO synthases, l-arginine. Together, this leads to a reduction in bio logically active NO. With hemolysis, as in sickle cell disease or thalassemia, reduced NO bioavailability is associated with disease complications such as pulmonary hypertension, leg ulcers, priapism, and perhaps increased risk of stroke. Lactic dehydrogenase also released from the RBC in hemolytic anemia is an excellent marker of these complications.

In summary, the primary amino acid structure of α-globin and non-α-globin chains dictates the inevitable quaternary structure, within which resides the ability of Hb to serve as a respiratory protein. Cooperativity ensures rapid binding of oxygen in the lungs and unloading in tissues. Similarly, carbon dioxide is transported from tis sues to lungs. The function of Hb may be influenced by mutation and by heterotropic effectors such as protons and 2,3-BPG. The molecule itself changes shape as it provides oxygen for metabolism; it is a lung in miniature, breathing as it allows the body to respire.

اخر الاخبار

اخبار العتبة العباسية المقدسة

ضمن خدماته لشهر رمضان.. قسم الشؤون الطبية يواصل تقديم الاستشارات الصحية للزائرين

ضمن أنشطتها الرمضانية.. الهيأة العليا لإحياء التراث تقيم دورة علمية لطلبة العلوم الدينية في النجف الأشرف

قسم الشؤون الفكرية يصدر العدد 61 من مجلّة عطاء الشباب

المجمع العلمي يقيم ختمة رمضانية مركزية في مزار الصحابي سلمان المحمّدي (رضوان الله عليه) ببغداد

المزيد

الآخبار الصحية

مفاجأة.. تحسين القدرة على التحمل لا يعتمد العضلات فقط

المرتفعات والأكسجين.. دراسة "مبشرة" لمرضى السكري

علماء يبتكرون "آلية" تتوقع موعد ظهور أعراض الزهايمر

ماذا يحدث لجسمك عند التوقف عن تناول الخضراوات والفواكه؟

المزيد

الآخبار التكنلوجية

رصد "القرش النائم" في أحد أقسى الأماكن على وجه الأرض

أسماك أعماق البحر تكشف نظاما بصريا يتجاوز "المألوف"

زوكربيرغ أمام القضاء في قضية إدمان المراهقين

"علي بابا" تكشف عن نموذج جديد للذكاء الاصطناعي

المزيد

مواضيع ذات صلة

Hemoglobin Structure

Phagocytes Ingest Target Cells

Cellular and Molecular Mechanisms in Development: Morphogens and Cell-to-Cell Signaling

Thrombopoietin Signaling

Mechanisms of Endomitosis in Megakaryocytes

Megakaryocytopoiesis

Ontogeny of Erythropoiesis

Cellular Dynamics in Erythropoiesis: Steady State and Stress Erythropoiesis

Erythropoiesis and Iron Regulation

Hematopoietic Microenvironment

Intrinsic Control of Erythropoiesis:Erythropoietin

Nuclear Organization