Site of the Amino-acid Substitution in Haemoglobin Seattle (α^A_β β^{70 Asp}₂)

Abstract

MANY abnormal haemoglobins show changed oxygen affinities. By examining the structural effects of specific amino-acid substitutions in the atomic models of oxy and deoxyhaemoglobin it has become possible to interpret such functional changes in stereochemical terms^1,2. Substitutions in the interior of the subunits, and especially those in the haem pocket, are liable to change the oxygen affinity, often by altering the relative stabilifies of the oxy and deoxy forms. Substitutions at the surface of the molecule, on the other hand, are always harmless, at least in heterozygotes. One exception was haemoglobin Seattle, a mutant with an abnormally low oxygen affinity, increased rate of methaemoglobin formation in vitro, and increased lability at high temperatures, causing mild to moderate chronic haemolytic anaemia in heterozygotes. These abnormalities were reported to be the result of replacement of alanine E20(76)β by glutamic acid^3,4. Ala E20 is external and has no contact with the haem; it is also a site that is highly variable in different species (Asn, Gin, Lys, His and Ser have all been found there⁵), which is against its having any functional importance. We have crystallized haemoglobin Seattle and re-examined the site of the substitution by X-ray analysis. We found it to be at Ala E14(70)β which is a haem contact where only alanine or serine normally occur. In the accompanying paper, Kurachi et al.⁶ show that alanine 70 is in fact replaced by aspartate.