# Hemoglobin <!-- MICROSIMGEN:BEGIN v1.7 — generated by g08_place_microsims.py; three.js first (§15); do not hand-edit inside --> ## Microsims — three.js ### Hemoglobin (three.js) <div class="microsim-player"> <iframe src="https://wikitube-3d-microsims.netlify.app/Hemoglobin.html" width="100%" height="620" frameborder="0" loading="lazy" sandbox="allow-scripts allow-same-origin" title="Hemoglobin — three.js microsim"></iframe> </div> **Open it full-screen:** [Hemoglobin.html](https://wikitube-3d-microsims.netlify.app/Hemoglobin.html) · library `threejs` · route `microsim/threejs/` ### Related microsims Live sims on neighbouring articles: - [[Allotropes_of_oxygen]] - [[Atomic_orbital]] - [[Hydrogen_bond]] - [[Molecular_orbital]] - [[Ozone_layer]] - [[Silicon_dioxide]] *Sim hosted off-article; the article owns the reference, not the runtime (WIKI_RULES §10.4). Placed by `g08_place_microsims.py`.* <!-- MICROSIMGEN:END --> ## Overview Hemoglobin is the protein that carries oxygen in the blood of nearly all vertebrates. Adult human hemoglobin is a tetramer of two alpha and two beta subunits, each wrapped around one heme group whose central iron(II) binds a single O2 molecule -- four per hemoglobin at saturation. What makes it more than a container is cooperativity. The four sites are not independent: binding at one raises the affinity of the others. That coupling converts what would be a lazy hyperbolic binding curve into a sigmoid, and the sigmoid is the whole trick. A carrier that binds oxygen tightly enough to fill in the lungs would, if it were non-cooperative, refuse to give it up in tissue. Hemoglobin does both because its affinity is not fixed -- it depends on how much oxygen it is already carrying. The reason a dedicated carrier has to exist at all is that oxygen is barely soluble in water. Plasma dissolves only about 0.003 mL of O2 per decilitre for each mmHg of partial pressure, so at the roughly 100 mmHg of arterial blood a litre of plasma holds about 3 mL of oxygen. A litre of whole blood containing about 150 g of hemoglobin, each gram binding about 1.34 mL of O2 when saturated, holds closer to 200 mL. Hemoglobin multiplies the carrying capacity of blood by something like seventyfold, and without that multiplication a resting adult's demand of roughly 250 mL of oxygen per minute would require a cardiac output no heart could produce. The tetramer that does this weighs about 64.5 kDa. The alpha chain is 141 amino acids long and the beta chain 146, and both fold the same way: the globin fold, a compact and almost entirely helical arrangement of eight helices lettered A through H, which is also the fold of myoglobin. The four chains are better described as a dimer of dimers than as four equal parts. An alpha and a beta chain pack together across a large and essentially rigid interface, and it is the other interface, between alpha1 and beta2, that slides when the molecule changes state. Running down the two-fold symmetry axis between the chains is a water-filled central cavity, which turns out to be where the most important small-molecule regulator binds. Each subunit holds its heme in a hydrophobic crevice between the E and F helices, and the geometry of that pocket is what makes the chemistry work. Ferrous iron takes six coordination positions. Four are used by the pyrrole nitrogens of the flat protoporphyrin IX ring. The fifth, on the proximal side, is held by the imidazole nitrogen of the proximal histidine, His F8 -- His87 in the alpha chain and His92 in the beta -- and that single bond is the only covalent tether between the metal and the protein backbone. The sixth position, on the distal side, is where oxygen binds. It is overlooked, but deliberately not occupied, by the distal histidine, His E7, which is His58 in alpha and His63 in beta. That arrangement answers a question the bare chemistry raises: why the iron does not simply rust. Free heme in water is oxidised by oxygen within seconds, because two hemes can sandwich a single O2 molecule into a bridged Fe-O-O-Fe intermediate that collapses to two ferric hemes. The globin blocks that fate three times over. It buries each heme in a separate hydrophobic pocket, so two irons can never reach one another. It places the distal histidine where it can donate a hydrogen bond to the bound dioxygen, which stabilises the oxygenated complex and discourages it from dissociating as superoxide and leaving ferric iron behind. And it uses the same residue sterically: carbon monoxide prefers to bind end-on and perpendicular to the porphyrin plane, whereas oxygen binds bent, so the distal histidine obstructs the geometry CO wants while accommodating the one O2 wants. The discrimination is real but partial. Hemoglobin still binds CO on the order of two hundred times more tightly than O2, against something like twenty thousand times for free heme in solution, which is why carbon monoxide is a poison rather than an instantly fatal one at trace concentrations. The protection against oxidation is likewise good rather than perfect: about three percent of circulating hemoglobin is converted each day to methemoglobin, whose ferric iron cannot bind oxygen at all, and is reduced back by cytochrome b5 reductase. ## The physics Binding is described empirically by the Hill equation, Y = p^n / (P50^n + p^n) where Y is fractional saturation, p is the oxygen partial pressure, P50 is the pressure at half saturation and n is the Hill coefficient. For hemoglobin n is about 2.8 and P50 about 26 mmHg. Note that 2.8 is not 4: the theoretical maximum for four fully coupled sites would be 4, and the measured value falls short because cooperativity is strong but not perfect. Myoglobin, the single-site oxygen store in muscle, is the control experiment. With n = 1 and P50 about 2.8 mmHg it gives a hyperbola that saturates almost immediately and holds on until the partial pressure is very low. The physiological numbers show what the sigmoid buys. At alveolar pO2 near 100 mmHg hemoglobin is about 98 percent saturated; at resting tissue pO2 near 40 mmHg it is about 77 percent. The difference, roughly 21 percent of capacity, is the oxygen actually delivered per pass. Myoglobin over the same interval delivers about 4 percent. Two further mechanisms tune the curve. The allosteric T (tense, low affinity) and R (relaxed, high affinity) quaternary states shift with binding, which is the structural basis of cooperativity. The Bohr effect shifts the curve right as pH falls: actively metabolising tissue produces CO2 and acid, which raises P50 and releases oxygen exactly where it is needed. Temperature and 2,3-bisphosphoglycerate act in the same direction. The T and R states are worth drawing in more detail, because the movement between them is what the sigmoid is made of. Deoxygenated hemoglobin is held in the T structure by a network of salt bridges involving the C-terminal residues of all four chains, and by a single molecule of 2,3-bisphosphoglycerate wedged in the central cavity. In the R structure those constraints are broken, the central cavity has narrowed, and the oxygen affinity of the sites is one to two orders of magnitude higher. Max Perutz, working from the crystal structures he and his colleagues began solving in 1960, identified the lever that converts one into the other. In deoxyhemoglobin the ferrous iron is high-spin and slightly too large for the hole at the centre of the porphyrin, so it sits about 0.4 angstrom out of the ring plane, domed toward the proximal histidine. Binding oxygen switches the iron to a low-spin state that fits the hole, and it moves into the plane, dragging His F8 and the whole F helix behind it. A few tenths of an angstrom at one heme are then amplified at the alpha1-beta2 interface: the T-state salt bridges break and one alpha-beta dimer rotates roughly 15 degrees against the other. That rotation is a property of the whole molecule, which is how oxygen bound at one heme raises the affinity of hemes it never touches. Two classical descriptions of that coupling are in general use, and it is worth naming them as models rather than as established mechanism. The concerted or MWC model of Monod, Wyman and Changeux (1965) supposes that the tetramer exists in equilibrium between only two conformations, that all four subunits must occupy the same one so that molecular symmetry is preserved, and that ligand binds more tightly to R. On this account oxygen does not induce the change at all; it selects R molecules already present in the ensemble and pulls the equilibrium after it. Three parameters suffice: a dissociation constant for each state and an allosteric constant giving their ratio in the absence of ligand. The sequential or KNF model of Koshland, Nemethy and Filmer (1966) instead lets subunits change one at a time by induced fit, each altered subunit changing the affinity of its neighbours. Symmetry need not be conserved, and negative cooperativity, which MWC cannot produce, falls out naturally. Real hemoglobin is not fully described by either. Later work adds tertiary conformational change occurring within a quaternary state, and tertiary two-state treatments fit the equilibrium and kinetic data better than the quaternary two-state picture alone. Eaton and colleagues put the position honestly in the title of a 1999 review, "Is cooperative oxygen binding by hemoglobin really understood?" The structural cause is known in outline and still argued over in detail, and a learner is better served by holding MWC and KNF as two useful idealisations than as rival accounts of what the molecule is really doing. The Hill coefficient deserves the same care. A. V. Hill introduced the expression in 1910 as a curve fit rather than a mechanism, and n is an empirical index of steepness, not a physical count of anything. In particular it is not the number of binding sites: hemoglobin has four sites and an n of about 2.8, and no reading of the equation makes those two numbers the same quantity. Nor is n a fixed property of the molecule, since the slope of a Hill plot varies with saturation and with conditions. The quoted 2.8 is the maximum slope, reached near half saturation, where cooperativity is doing the most work; at the extremes of the curve, where the molecule is nearly empty or nearly full, the local slope falls back toward 1. ## Physiological modulation Almost everything that changes how much oxygen hemoglobin actually delivers does so by moving a single number: P50, the half-saturation pressure, about 26 mmHg for adult hemoglobin in whole blood. Raising P50 slides the curve right and unloads more oxygen at a given tissue pressure; lowering it slides the curve left and holds oxygen more tightly. The modulators do not change the number of sites, and they barely change the shape of the sigmoid; they move it along the pressure axis. In a working body they nearly all move it the same way at the same time. The Bohr effect, described by Christian Bohr, Karl Hasselbalch and August Krogh in 1904, is the fall in oxygen affinity that accompanies a fall in pH. The mechanism is preferential proton binding to the T state, chiefly at the C-terminal His146 of the beta chains and at the N-terminal amino groups of the chains, and anything that stabilises T raises P50. The empirical slope is about -0.48 in d log10 P50 per pH unit. In the lung the same coupling runs backwards: as carbon dioxide leaves the blood the pH rises, affinity rises with it, and loading is assisted by exactly the coupling that assisted unloading in the tissue. Carbon dioxide acts twice over. Indirectly it generates protons, through carbonic anhydrase and the bicarbonate equilibrium, and so feeds the Bohr effect. Directly it forms carbamino adducts with the N-terminal amino groups of the globin chains, which also stabilise T; roughly a tenth of the carbon dioxide carried by blood travels this way, as carbaminohemoglobin. The relationship is reciprocal, and the reverse statement has its own name. Deoxygenated hemoglobin binds more carbon dioxide and more protons than oxygenated hemoglobin does, which is the Haldane effect, and it is what allows venous blood to take up carbon dioxide in the same capillary transit in which it gives up oxygen. 2,3-bisphosphoglycerate, identified as a hemoglobin effector in 1967 independently by Benesch and Benesch and by Chanutin and Curnish, is the most quantitatively important of the modulators and the least intuitive. It is a small, strongly anionic product of a side branch of glycolysis peculiar to the red cell, present at about 5 mmol per litre, which is roughly equimolar with the hemoglobin tetramer itself. Exactly one molecule binds in the central cavity of deoxyhemoglobin, cross-linking the two beta chains through Val1, His2, Lys82 and His143. The R-state cavity is too narrow to hold it, so oxygenation expels it, which makes the binding of oxygen and the binding of 2,3-BPG directly antagonistic. The easiest way to see how much work it does is to take it away: hemoglobin stripped of organic phosphates has a P50 of only a few mmHg and would be very nearly useless as a transporter, filling in the lung and keeping what it filled with. Red-cell concentrations rise over hours to days at altitude, in chronic hypoxaemia and in anaemia, shifting the curve right; they fall in stored blood, shifting it left. Temperature works by plain thermodynamics. Oxygen binding is exothermic, so warming drives the equilibrium toward the unbound state and moves the curve right, with an empirical coefficient of about +0.024 in d log10 P50 per degree Celsius. Exercising muscle is warm, acidic and rich in carbon dioxide at once, and all three effects push in the unloading direction together, which is why the same hemoglobin that surrenders about a fifth of its cargo at rest can surrender far more of it under load without any change to the protein. Fetal hemoglobin, two alpha chains and two gamma chains, has a curve left of the maternal one, with a P50 near 19 mmHg. That offset is what makes placental transfer work: at any given oxygen pressure in the intervillous space the fetal molecule is the more saturated of the two, so oxygen keeps moving down a gradient from mother to fetus instead of stalling at a shared equilibrium. The usual shorthand, that fetal hemoglobin binds oxygen more tightly, is misleading about the cause. Stripped of organic phosphates, fetal and adult hemoglobin have very similar intrinsic affinities. The difference is that the gamma chain carries serine at position 143 where the beta chain carries histidine, which removes two positive charges from the 2,3-BPG pocket. Fetal hemoglobin therefore binds 2,3-BPG weakly, is less stabilised in the T state, and behaves as though its red cells held very little of the effector. The left shift is an effector effect rather than an intrinsic one, which is the same lesson as before: P50 is the variable, and the chemistry of the heme is very nearly a constant. ## Controls -> what each maps to | Control | Maps to | Range / values | Physical meaning | |---|---|---|---| | Oxygen partial pressure | p | 0 - 120 mmHg | Moves the operating point along the curve, from tissue to lung | | Blood pH | Bohr effect | 7.2 - 7.6 | Shifts P50; lower pH raises P50 and releases oxygen | | Hill coefficient n | n | 1 - 4 | Strength of cooperativity; n = 1 destroys the sigmoid | | Show the myoglobin curve | -- | on / off | The non-cooperative comparison, n = 1 and P50 = 2.8 mmHg | | Spin the model | -- | on / off | Rotation only; disabled under prefers-reduced-motion | ## Learning objective After playing, a learner can explain why cooperative binding produces a sigmoid rather than a hyperbola, read oxygen delivery off the curve as the difference between lung and tissue saturation, and describe how the Bohr effect matches oxygen release to metabolic demand. A learner who has also worked through the sections above should be able to go further: to justify the sigmoid from the T-to-R mechanism rather than merely asserting it, to predict the direction in which a change of pH, carbon dioxide, 2,3-BPG, temperature or a switch to fetal hemoglobin moves P50 and therefore moves delivery, and to say why a Hill coefficient of 2.8 is a description of how steeply a curve rises rather than a statement about how many sites the molecule has. ## Limits and connections The three-dimensional model is a schematic tetramer, not crystallographic coordinates: subunit shapes are stylised, and the T-to-R rotation and the iron's movement into the heme plane are real mechanisms drawn at exaggerated scale. The Hill equation is itself a fitting function rather than a mechanism -- it does not describe the four sequential binding steps, and models such as Monod-Wyman-Changeux or Adair do that more faithfully. Carbon monoxide binding, fetal hemoglobin, and the sickle-cell variant are all outside this sim. A few further limits belong to the controls themselves. The Hill coefficient appears here as a dial that can be turned, which is a useful fiction rather than a physical operation: in a real molecule n is not an input but an outcome of the allosteric equilibrium, and there is no way to set it independently of everything else about the protein. The pH slider likewise moves P50 through an empirical Bohr coefficient rather than by protonating His146 and the chain N-termini, so the curve responds correctly while the mechanism behind the response is asserted rather than computed. Carbon dioxide, 2,3-BPG and temperature have no separate controls at all, so whatever they would contribute is already folded into whichever P50 the pH setting produces. That is worth remembering before reading the displayed curve as a complete account of a red cell in a real capillary. The connections run outward in two directions. Toward chemistry, the ferrous iron, its spin state and its coordination geometry tie this article to heme chemistry and to transition-metal coordination generally, and the distal-histidine trick recurs wherever a protein has to handle oxygen without being destroyed by it. Toward physiology and medicine, the P50 shifts described above are the working content of altitude acclimatisation, of the storage lesion in banked blood, of carbon monoxide poisoning and of the switch from fetal to adult hemoglobin after birth, while the globin genes themselves are the setting for sickle cell disease and the thalassaemias. ## References 1. Perutz, M. F.; Rossmann, M. G.; Cullis, A. F.; Muirhead, H.; Will, G.; North, A. C. T. "Structure of Haemoglobin: A Three-Dimensional Fourier Synthesis at 5.5 Angstrom Resolution, Obtained by X-Ray Analysis." *Nature*, 1960, volume 185, pages 416-422. 2. Perutz, M. F. "Stereochemistry of Cooperative Effects in Haemoglobin." *Nature*, 1970, volume 228, pages 726-739. 3. Monod, J.; Wyman, J.; Changeux, J.-P. "On the Nature of Allosteric Transitions: A Plausible Model." *Journal of Molecular Biology*, 1965, volume 12, pages 88-118. 4. Koshland, D. E., Jr.; Nemethy, G.; Filmer, D. "Comparison of Experimental Binding Data and Theoretical Models in Proteins Containing Subunits." *Biochemistry*, 1966, volume 5, pages 365-385. 5. Hill, A. V. "The Possible Effects of the Aggregation of the Molecules of Haemoglobin on its Dissociation Curves." *Journal of Physiology*, 1910, volume 40 (Supplement), pages iv-vii. 6. Bohr, C.; Hasselbalch, K.; Krogh, A. "Ueber einen in biologischer Beziehung wichtigen Einfluss, den die Kohlensaeurespannung des Blutes auf dessen Sauerstoffbindung uebt." *Skandinavisches Archiv fuer Physiologie*, 1904, volume 16, pages 402-412. 7. Benesch, R.; Benesch, R. E. "The Effect of Organic Phosphates from the Human Erythrocyte on the Allosteric Properties of Hemoglobin." *Biochemical and Biophysical Research Communications*, 1967, volume 26, pages 162-167. 8. Chanutin, A.; Curnish, R. R. "Effect of Organic and Inorganic Phosphates on the Oxygen Equilibrium of Human Erythrocytes." *Archives of Biochemistry and Biophysics*, 1967, volume 121, pages 96-102. 9. Bunn, H. F.; Briehl, R. W. "The Interaction of 2,3-Diphosphoglycerate with Various Human Hemoglobins." *Journal of Clinical Investigation*, 1970, volume 49, pages 1088-1095. 10. Severinghaus, J. W. "Simple, Accurate Equations for Human Blood O2 Dissociation Computations." *Journal of Applied Physiology*, 1979, volume 46, pages 599-602. 11. Eaton, W. A.; Henry, E. R.; Hofrichter, J.; Mozzarelli, A. "Is Cooperative Oxygen Binding by Hemoglobin Really Understood?" *Nature Structural Biology*, 1999, volume 6, pages 351-358. 12. Henry, E. R.; Bettati, S.; Hofrichter, J.; Eaton, W. A. "A Tertiary Two-State Allosteric Model for Hemoglobin." *Biophysical Chemistry*, 2002, volume 98, pages 149-164. 13. Antonini, E.; Brunori, M. *Hemoglobin and Myoglobin in Their Reactions with Ligands*. Amsterdam: North-Holland Publishing, 1971. 14. Imai, K. *Allosteric Effects in Haemoglobin*. Cambridge: Cambridge University Press, 1982. 15. Bunn, H. F.; Forget, B. G. *Hemoglobin: Molecular, Genetic and Clinical Aspects*. Philadelphia: W. B. Saunders, 1986. 16. Berg, J. M.; Tymoczko, J. L.; Gatto, G. J.; Stryer, L. *Biochemistry*, 9th edition, chapter 7, "Hemoglobin: Portrait of a Protein in Action". New York: W. H. Freeman, 2019. 17. West, J. B.; Luks, A. M. *West's Respiratory Physiology: The Essentials*, 10th edition. Philadelphia: Wolters Kluwer, 2016. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Hemoglobin) : [Wikitube](https://en.wikitube.io/wiki/Hemoglobin) ## Previous hub tags Tree parent: [[Oxygen]]. Legacy hubs: `REACTION`. --- *Created 2026-08-05 - append-only - hand-authored to WIKI_REPOPULATION_PROTOCOL v1.0 section 5 - 0 deletions*