Structures relevant to deoxy hemoglobins

The structure of the picket-fence porphyrin compound, Fe(PF)(2-MeIm), is shown in Figure 4.28. 172 Minus the pickets, it is essentially a magnified view of the active site of deoxymyoglobin, shown in Figure 4.29. 181 Some metrical details of these structures, of a very similar unsubstituted tetraphenylporphyrin, I 10 and of several other deoxyhemoglobins I Ie, 182-185 are listed in Table 4.7. In general they are all similar, but important differences exist.
 

In all structures, except deoxyerythrocruorin,183 the iron atom is displaced about 0.4 to 0.5 Afrom the plane of the porphyrin toward the axial base. For deoxyerythrocruorin the displacement is less than half this, perhaps because the water molecule is weakly coordinated to the iron center.

An imidazole group from a histidine residue-the distal histidine E7 in position 7 on helix labeled E-hovers over the binding site for most vertebrate hemoglobins, except for genetically engineered mutants of human hemoglobin (j3E7His -> Gly), pathological mutant hemoglobins, such as hemoglobin Zurich (j3E7His -> Arg), and some others, such as elephant hemoglobin. Long believed to be noncoordinating, this distal histidine may, in fact, coordinate weakly to the Fe center at low temperature. 159 In the a chains of human deoxyhemoglo-

bin, hemoglobin A, a water molecule is found in the binding cavity. 182 For many years the binding cavity has been referred to as the hydrophobic pocketliterally, water-hating. Although many hydrophobic groups, such as valine, leucine, isoleucine, and phenylalanine are positioned over the porphyrin, the immediate environment around the binding site is, in fact, polar, with the distal histidine and associated water molecules, as well as the heme group itself. As will be shown in the next section, the label "hydrophobic pocket" becomes more misleading when the interaction of coordinated ligands with distal groups is examined.
 

The orientation of the axial base, angle 1>1, is similar for Fe(PF)(2-MeIm) and for several vertebrate deoxyhemoglobins. On the other hand, Fe(TPP)(2Melm) and deoxyerythrocruorin have a similar eclipsed axial-base orientation. At least for five-coordinate species, where the iron center is substantially out of the porphyrin plane, orientation of the axial base does not invariably induce structural perturbations, e.g., doming, in the porphyrin skeleton. The conformation of the protein chain is such that the proximal histidine in
deoxyhemoglobin coordinates in a slightly tilted manner,182,186 comparable to the tilt that the sterically active 2-methyl substituent induces in the synthetic systems. 172 Clearly, coordination of the histidine to the heme in a symmetric manner, as would be expected in the absence of the protein constraints, does not produce the conformation of lowest free energy for the whole molecule.