Valence hybridization

The possible donor-acceptor interactions among the valence hybrids are constrained by diatomic symmetry to the five possible combinations shown in Table 3.14. For simplicity, we shall temporarily suppress the subscript atoms in the NBO symbols (which are now simply indicated as a, nx,...). We also introduce a special typographic convention to denote each type of NBO (curved bonds for 7tx and ny, opposed arrows for complementary y and y, and so forth), as shown in Table 3.14. (The corresponding typographic convention for nonbonded NBOs will consist of a short bar on the atomic symbol, A for hn, A for pA, and A for py.) We also employ the convention of attaching an asterisk ( ) to denote an unfilled orbital, i.e., an orbital that is formally serving as an acceptor in a donor-acceptor interaction (the star says I m empty ). Thus, the notation px —> pf denotes an interaction from a filled pA orbital on the first atom to an unfilled pA on the second atom.46... 

VI. H NMR Investigations of Structures and Properties of Symmetric Valency Hybrid Hemoglobins Models for Doubly... 

Mutation in the distal heme pocket of the a chain the heme iron atoms in the a chains are in the high-spin ferric state, i.e., (a2 p2), with five unpaired electrons per heme iron in the a chains and with four unpaired electrons per heme iron in the p chains a naturally occurring valency hybrid hemoglobin... 

Symmetric Valency Hybrid Hemoglobins Models for Doubly... 

Figures 51 and 52 show the hfs and exchangeable proton resonances of cross-linked valency hybrid Hbs with two cyanomet chains— (ot+CN0+CN)A(a0)cXL and (afi+CN)A(a+CNP)cXL—in both deoxy and CO forms from +7 to +20 ppm from H20. Overall, the 600-MHz...

H NMR spectra of these two valency hybrid Hbs are quite similar to each other. There are small spectral changes in these two hybrid Hbs in going from the deoxy to the CO form (Miura and Ho, 1982). 

Fig. 53. Oxygen equilibrium curves of cross-linked mixed-valency hybrid hemoglobin (a + CN3)A(ap)cXL, in 0.1 M phosphate plus 2 mM KCN at various pH values. (A) Hill plots (B) 02 binding constants. [From Miura et al. (1987)].

It is inferred that the affinity state of these doubly ligated species is different from that of fully ligated Hb, but is perhaps an intermediate affinity state. We do not understand why the cooperativity of these intermediate species is greatly reduced. The symmetric cyanomet valency hybrid Hbs [(a+CNp)2) and (a0+CN)2] have also been found to have reduced cooperativity (n = 1.1-1.2) and increased affinity for 02 compared to Hb A (Cassoly and Gibson, 1972 Maeda et al., 1972 Hofrichter et al., 1985). Thus, none of the four models for doubly ligated species can represent the features of K3 and of cross-linked Hb or Hb A, as pointed out by Miura et al. (1987). 

A number of functional studies have shown that the partially ligated species and valency hybrid Hbs exhibit slowly interconverting conformation states. Some of them have properties that are intermediate between those associated with the T- and R-quaternary conformations (Cassoly and Gibson, 1972 Samaja et al., 1987 Sharma, 1989 Berjis et al., 1990). Thus, one needs to be careful in making correlations between a given crystal structure of a specific Hb species and its functional properties. It should be kept in mind that crystallization is a selective procedure, i.e., it selects those proteins with specific structures that are crystallizable under a given set of crystallization conditions. The structures of these crystallized proteins may not be the dominant ones when function is measured under solution conditions different from those used in crystallizing the proteins. 

Our 02 equilibrium curves for the four cross-linked mixed-valency hybrid Hbs indicate that there are significant functional differences between (ot+CN0)A(a0)cXL and (ot0+CN)A(a0)cXL. The two asymmetrical hybrid Hbs with two cyanomet subunits show a low cooperativity and the 02 affinity of these two hybrids is much lower than that of the isolated a or 0 subunit of Hb A (Miura et al., 1987). The overall 02-binding property as measured by P50 is on the order of (a0)A(a0)cXL > (a+CN0)A(a0)cXL > (a0+CN)A(a0)cXL >... 

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