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Stereochemical effects of amino acid substitution in abnormal human haemoglobin.

Over 100 mutant human haemoglobins are now known; in most of them a single pair of amino acid residues is replaced in either the aor the ,B-chains. About half the replacements occur on the surface of the haemoglobin molecule and are innocuous, at least in heterozygotes. Replacements in one of the two pairs of contacts between aand ,B-chains (aIl92) tend to affect the interaction between the haem molecules, so that the oxygenequilibrium curve becomes hyperbolic rather than sigmoid (Perutz & Lehmann, 1968; Perutz, 1969). X-ray analyses of several ofthese mutant haemoglobins have been carried out by using a method that is very sensitive to small perturbations of the mutant as compared with the wild-type structure. The results show a great variety ofeffects, some predictable, others unexpected. Predictably, a replacement of an asparagine residue by a threonine residue in a position where the y-methyl group of the threonine residue gets in the way of the porphyrin ring causes perturbations throughout the fl-subunits, but unexpectedly these perturbations also extend to the a-subunits [haemoglobin Kansas, G4(102)fl]. Replacement of an arginine residue by a leucine residue at FG4(92)a causes no perturbations in the deoxy form, but extensive ones in the oxy form, of the molecule (Greer, 1970). The properties of these mutant haemoglobins suggest that the most important pathway of haem-haem interaction goes through the contact ocl92. Haem-haem interaction cannot occur if the structural transition between the oxy and deoxy forms is inhibited. We had therefore expected such inhibition to occur in the mutant haemoglobins where Hill's constant is close to unity. However, this is not true. It appears that small perturbations in either the oxy or the deoxy forms may be sufficient to stop haem-haem interaction, for reasons which are not yet clear. Perutz, M. F. (1969). Proc. R. Soc. B, 173, 113. Perutz, M. F. & Lehmann, H. (1968). Nature, Lond., 219, 902. Greer, J. (1970). Ph.D. Dissertation: University of Cambridge.