Structures of deoxy and carbonmonoxy haemoglobin Kansas in the deoxy quaternary conformation.

Haemoglobin Kansas (Asn102(G4)/? + Thr) is the only widely studied mutant or modified haemoglobin having four functional haems and displaying lower than normal oxygen aBnity. Two forms of this mutant have been investigated by X-ray diffraction. The deoxy form, whose crystals are isomorphous with the native form, has been examined directly by the difference Fourier technique (3.4 A). In addition to the replaced residue itself, the difference electron density map shows signs of slight movements throughout the region between a and fl haem pockets. However, neither type of chain shows stereochemical evidence of a very abnormal oxygen affmity in the tetramer. The nature of the perturbation is different from that proposed in the earlier, low-resolution study of Greer (1971a). Exposure of deoxy crystals to carbon monoxide produces two new crystal forms similar but not identical to the native deoxy form. One of these structures has been solved by rotation and translation function methods and a difference map between carbonmonoxy haemoglobin Kansas in the deoxy quaternary structure and native deoxy hsemoglobin has been calculated at 3.6 A resolution. Carbon monoxide molecules are observed at three of the four haems, and two unidentiiled large peakss appear next to the sulphydryl groups of Gys93k None of the interchain salt bridges which stabilize the deoxy quaternary state appears to be broken, but large tertiary structural changes are seen in the liganded chains. It seems that when the molecule is subjected to the lattice constraints of the deoxy crystal, the salt bridges do not break upon ligand binding, even though the pH dependence of the first Adair constant and the linearity of proton release with ligend uptake both imply that this does happen to stripped HbA in solution.