Hemoglobin allosterism

STEREOCHEMICAL TERMINOLOGY, lUPAC RECOMMENDATIONS HETEROTROPIC EFFECT MONOD-WYMAN-CHANGEUX MODEL HEMOGLOBIN ALLOSTERISM... 

Gibson QH, Hoffman BM, Crepeau RJH, Edelstein SJ, Bull C. Manganese hemoglobin Allosteric effects in stopped flow flash photolysis and sedimentation measurements. Biochem Biophys Res Commun 1974 59 146. 

The allosteric effect is seen in hemoglobin which can exist in two quaternary stmctural states oxygenated (R) or deoxygenated (T). The binding of one O2 or some other effector to one of the subunits stabilizes the R form as compared to the T form. Binding of a second and third O2 stabilizes it even further. 

For many years hemoglobin was the only allosteric protein whose stereochemical mechanism was understood in detail. However, more recently detailed structural information has been obtained for both the R and the T states of several enzymes as well as one genetic repressor system, the trp-repressor, described in Chapter 8. We will here examine the structural differences between the R and the T states of a key enzyme in the glycolytic pathway, phosphofructokinase. 

In addition to enzymes, noncatalytic proteins may exhibit many of these properties hemoglobin is the classic example. The allosteric properties of hemoglobin are the subject of a Special Focus beginning on page 480. 

A model for the allosteric behavior of hemoglobin is based on recent observations that oxygen is accessible only to the heme groups of the a-chains when hemoglobin is in the T conformational state. Perutz has pointed out that the heme environment of /3-chains in the T state is virtually inaccessible because of steric hindrance by amino acid residues in the E helix. This hindrance dis-... 

FIGURE 15.36 The structure, in ionic form, of BPG or 2,3-bisphosphoglycerate, an important allosteric effector for hemoglobin. 

Thermodynamically it would be expected that a ligand may not have identical affinity for both receptor conformations. This was an assumption in early formulations of conformational selection. For example, differential affinity for protein conformations was proposed for oxygen binding to hemoglobin [17] and for choline derivatives and nicotinic receptors [18]. Furthermore, assume that these conformations exist in an equilibrium defined by an allosteric constant L (defined as [Ra]/[R-i]) and that a ligand [A] has affinity for both conformations defined by equilibrium association constants Ka and aKa, respectively, for the inactive and active states ... 

The 3D structure of the 2,3-diphosphoglycerate (DPG) complex of hemoglobin (Hb) served to derive simple aromatic dialdehydes that mimic the function of DPG as an allosteric modulator of the oxygen affinity of Hb. Some of the resulting compounds were as active and even more active than DPG, the natural ligand [1-3]. 

THE ALLOSTERIC PROPERTIES OF HEMOGLOBINS RESULT FROM THEIR QUATERNARY STRUCTURES... 

The properties of individual hemoglobins are consequences of their quaternary as well as of their secondary and tertiary structures. The quaternary structure of hemoglobin confers striking additional properties, absent from monomeric myoglobin, which adapts it to its unique biologic roles. The allosteric (Gk alios other, steros space ) properties of hemoglobin provide, in addition, a model for understanding other allosteric proteins (see Chapter 11). 

Subunit contacts need to be relatively extensive and stable if they are to ensure subunit association in the absence of a covalent link. However, in some cases a subunit contact can shift back and forth between two different stable positions, as has been demonstrated for oxy- versus deoxyhemoglobin (Perutz, 1970). Allosteric control can then be exerted by any factors which either affect the local conformation or bind between the subunits. A less elegant but even more extreme example is lamprey hemoglobin, which dissociates altogether in the oxy form (Hendrickson and Love, 1971). 

Since this structure was first proposed, Braunitzer and co-workers have determined the amino acid sequence of rhinoceros hemoglobin (23a). Its allosteric effector site shows only a single substitution compared to that of human hemoglobin—His NA2/8 — Glu—yet ATP lowers its oxygen affinity more than DPG, and GTP lowers it more than ATP, just as in teleost fish (R. Baumann, unpublished observations). This observation supports the hydrogen bond between N-6 of the adenine and Glu NA2 proposed in Fig. 6 in fact it can hardly be explained without that bond. 

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