Deoxygenation-linked association of a tetrameric component of chicken hemoglobin

Deoxygenation-dependent association of hemoglobin tetramers appears to be widespread among amphibians, reptiles, and possibly all or most birds. The evidence for this conclusion depends largely on oxygen equilibria of whole blood which have Hill coefficients that reach values as high as 5-7 at 80-90% oxygenation. Computer simulation of the sedimentation velocity behavior of the major components A and D of chicken hemoglobin shows that component D but not A self-associates to form dimers of tetramers. The gradient profiles at pH 7.5 were satisfactorily fitted with an association constant of 1.26 × 10⁴M^-1 and sedimentation coefficients of 4.63 and 7.35 S for tetramer and (tetramer)₂, respectively. Since components A and D share common β chains we conclude that tetramer-tetramer contacts must depend on surface residues of the α chains. Comparison of the amino acid sequences of the α^D and α^A chains of the hemoglobins from 12 avian species ranging from sparrow to ostrich shows that 20 residues are conserved in the α^D chains but not in the α^A chains. Nine of these (45%) are clustered between positions E20 and FG2. Four of the latter, Lys ⁷¹(E20), Asn⁷⁵(EF4), Gln⁷⁸(EF7), and Glu⁸²(F3) are conserved in all α^D chains even though they do not appear to participate in intratetramer contacts. Molecular modeling indicates that residues Lys⁷¹, Gln⁷⁸, and Glu⁸² of the α chain are strong candidates for the primary tetramer-tetramer contacts.