2 Структура и функция белка (1160071), страница 24
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Each heme is partially buried in apocket lined with hydrophobic amino acid side chains. It is bound to itspolypeptide chain through a coordination bond of the iron atom to theR group of a His residue (see Fig. 7-18). The sixth coordination bond ofthe iron atom of each heme is available to bind O2.Closer examination of the quaternary structure of hemoglobin,with the help of molecular models, shows that although there are fewcontacts between the two a chains or between the two (3 chains, thereare many contact points between the a and /3 chains.
These contactpoints consist largely of hydrophobic side chains of amino acid residues, but also include ionic interactions involving the carboxyl-terminal residues of the four subunits.Naturally occurring changes in the amino acid sequence of hemoglobin provide some useful insights into the relationship betweenstructure and function in proteins. More than 300 genetic variants ofhemoglobin are known to occur in the human population.
Most of thesevariations are single amino acid changes that have only minor structural or functional effects. An exception is a substitution of valine forglutamate at position 6 of the /3 chain. This residue is on the outersurface of the molecule, and the change produces a "sticky" hydrophobic spot on the surface that results in abnormal quaternary associationof hemoglobin. When oxygen concentrations are below a critical level,the subunits polymerize into linear arrays of fibers that distort cellshape.
The result is a sickling of erythrocytes (Fig. 7-27), the cause ofsickle-cell anemia.(b)Figure 7-26 The three-dimensional (quaternary)structure of deoxyhemoglobin, revealed by x-ray diffraction analysis, showing how the four subunitsare packed together, (a) A ribbon representation.(b) A space-filling model. The a subunits are shownin white and light blue; the /3 subunits are shownin pink and purple.
Note that the heme groups,shown in red, are relatively far apart.Figure 7—27 Scanning electron micrographs of(a) normal and (b) sickled human erythrocytes.The sickled cells are fragile, and their breakdowncauses anemia.187Part II Structure and Catalysis188Conformational Changes in HemoglobinAlter Its Oxygen-Binding CapacityVenousblood pO2ArterialbloodpO203COFigure 7-28 The oxygen-binding curves of myoglobin (Mb) and hemoglobin (Hb). Myoglobin has amuch greater affinity for oxygen than does hemoglobin. It is 50% saturated at oxygen partial pressures (pO2) of only 0.15 to 0.30 kPa, whereas hemoglobin requires a pO2 of about 3.5 kPa for 50%saturation.
Note that although both hemoglobinand myoglobin are more than 95% saturated at thepO2 in arterial blood leaving the lungs (—13 kPa),hemoglobin is only about 75% saturated in restingmuscle, where the pO2 is about 5 kPa, and onlyHemoglobin is an instructive model for studying the function of manyregulatory oligomeric proteins. The blood in a human being must carryabout 600 L of oxygen from the lungs to the tissues every day, but verylittle of this is carried by the blood plasma because oxygen is onlysparingly soluble in aqueous solutions. Nearly all the oxygen carriedby whole blood is bound and transported by the hemoglobin of theerythrocytes.
Normal human erythrocytes are small (6 to 9 ^im), biconcave disks (Fig. 7-27a). They have no nucleus, mitochondria, endoplasmic reticulum, or other organelles. The hemoglobin of the erythrocytesin arterial blood passing from the lungs to the peripheral tissues isabout 96% saturated with oxygen. In the venous blood returning to theheart, the hemoglobin is only about 64% saturated.
Thus blood passingthrough a tissue releases about one-third of the oxygen it carries.The special properties of the hemoglobin molecule that make itsuch an effective oxygen carrier are best understood by comparing theO2-binding or O 2 -saturation curves of myoglobin and hemoglobin (Fig.7-28).
These show the percentage of O2-binding sites of hemoglobin ormyoglobin that are occupied by O2 molecules when solutions of theseproteins are in equilibrium with different partial pressures of oxygenin the gas phase. (The partial pressure of oxygen, abbreviated pO2, isthe pressure contributed by oxygen to the overall pressure of a mixtureof gases, and is directly related to the concentration of oxygen in themixture.)From its saturation curve, it is clear that myoglobin has a veryhigh affinity for oxygen (Fig.
7-28). Furthermore, the 0 2 -saturationcurve of myoglobin is a simple hyperbolic curve, as might be expectedfrom the mass action of oxygen on the equilibrium myoglobin +O2 ^± oxymyoglobin. In contrast, the oxygen affinity of each of the fourO2-binding sites of deoxyhemoglobin is much lower, and the (^-saturation curve of hemoglobin is sigmoid (S-shaped) (Fig. 7-28).
This shapeindicates that whereas the affinity of hemoglobin for binding the firstO2 molecule (to any of the four sites) is relatively low, the second, third,and fourth O2 molecules are bound with a very much higher affinity.This accounts for the steeply rising portion of the sigmoid curve. Theincrease in the affinity of hemoglobin for oxygen after the first O2 molecule is bound is almost 500-fold. Thus the oxygen affinity of each hemepolypeptide subunit of hemoglobin depends on whether O2 is bound toneighboring subunits. The conversion of deoxyhemoglobin to oxyhemoglobin requires the disruption of ionic interactions involving thecarboxyl-terminal residues of the four subunits, interactions that con-10% saturated in working muscle, where the pO2 isonly about 1.5 kPa.
Thus hemoglobin can release itsoxygen very effectively in muscle and other peripheral tissues. Myoglobin, on the other hand, is stillabout 80% saturated at a pO2 of 1.5 kPa, and therefore unloads very little oxygen even at very lowpO2. Thus the sigmoid O2-saturation curve of hemoglobin is a molecular adaptation for its transportfunction in erythrocytes, assuring the binding andrelease of oxygen in the appropriate tissues.Chapter 7 The Three-Dimensional Structure of Proteinsstrain the overall structure in a low-affinity state. The increase in affinity for successive O2 molecules reflects the fact that more of theseionic interactions must be broken for binding the first O2 than for binding later ones.Once the first heme-polypeptide subunit binds an O2 molecule, itcommunicates this information to the remaining subunits through interactions at the subunit interfaces.
The subunits respond by greatlyincreasing their oxygen affinity. This involves a change in the conformation of hemoglobin that occurs when oxygen binds (Fig. 7-29). Suchcommunication among the four heme-polypeptide subunits of hemoglobin is the result of cooperative interactions among the subunits.Because binding of one O2 molecule increases the probability that further O2 molecules will be bound by the remaining subunits, hemoglobin is said to have positive cooperativity.
Sigmoid binding curves,like that of hemoglobin for oxygen, are characteristic of positive cooperative binding. Cooperative oxygen binding does not occur with myoglobin, which has only one heme group within a single polypeptide chainand thus can bind only one O2 molecule; its saturation curve is therefore hyperbolic. The multiple subunits of hemoglobin and the interactions between these subunits result in a fundamental difference between the O2-binding actions of myoglobin and hemoglobin.Positive cooperativity is not the only result of subunit interactionsin oligomeric proteins. Some oligomeric proteins show negative cooperativity: binding of one ligand molecule decreases the probability that further ligand molecules will be bound.
These and additional regulatory mechanisms used by these proteins are considered inChapter 8.Hemoglobin Binds Oxygen in the Lungsand Releases It in Peripheral TissuesIn the lungs the pO2 in the air spaces is about 13 kPa; at this pressurehemoglobin is about 96% saturated with oxygen. However, in the cellsof a working muscle the pO2 is only about 1.5 kPa because muscle cellsuse oxygen at a high rate and thus lower its local concentration. As theblood passes through the muscle capillaries, oxygen is released fromthe nearly saturated hemoglobin in the erythrocytes into the bloodplasma and thence into the muscle cells.
As is evident from the O2saturation curve in Figure 7-28, hemoglobin releases about a third ofits bound oxygen as it passes through the muscle capillaries, so thatwhen it leaves the muscle, it is only about 64% saturated. When theblood returns to the lungs, where the pO2 is much higher (13 kPa), thehemoglobin quickly binds more oxygen until it is 96% saturated again.Now suppose that the hemoglobin in the erythrocyte were replacedby myoglobin. We see from the hyperbolic O 2 -saturation curve of myoglobin (Fig.
7-28) that only 1 or 2% of the bound oxygen can be releasedfrom myoglobin as the pO2 decreases from 13 kPa in the lungs to 3 kPain the muscle. Myoglobin therefore is not very well adapted for carrying oxygen from the lungs to the tissues, because it has a much higheraffinity for oxygen and releases very little of it at the pO2 in musclesand other peripheral tissues. However, in its true biological functionwithin muscle cells, which is to store oxygen and make it available tothe mitochondria, myoglobin is in fact much better suited than hemoglobin, because its very high affinity for oxygen at low pO2 enables it tobind and store oxygen effectively. Thus hemoglobin and myoglobin arespecialized and adapted for different kinds of O2-binding functions.189Figure 7-29 Conformational changes induced inhemoglobin when oxygen binds. (The oxygen-boundform is shown at bottom.) There are multiple structural changes, some not visible here; most of thechanges are subtle.
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