Modification of alpha-chain or beta-chain heme pocket polarity by Val(E11) --> thr substitution has different effects on the steric, dynamic, and functional properties of human recombinant hemoglobin. Deoxy derivatives.

The dynamic and functional properties of mutant deoxyhemoglobins in which either the beta-globin Val67(E11) or the alpha-globin Val62(E11) is replaced by threonine have been investigated through the thermal evolution of the Soret absorption band in the temperature range 300 to 20 K and through the kinetics of CO rebinding after flash photolysis at room temperature. The conformational properties of the modified alpha chain and beta chain distal heme pockets were also studied through x-ray crystallography and molecular modeling. The data obtained with the various techniques consistently indicate that the polar isosteric mutation in the distal side of the alpha chain heme pocket has a larger effect on the investigated properties than the analogous mutation on the beta chain. We attribute the observed differences to the presence of a water molecule in the distal heme pocket of the modified alpha chains, interacting with the hydroxyl of the threonine side chain. This is indicated by molecular modeling which showed that the water molecule present in the alpha chain distal heme pocket can bridge by H bonding between Thr62(E11) and His58(E7) without introducing any unfavorable steric interactions. Consistent with the dynamic and functional data, the presence of a water molecule in the distal heme pocket of the modified beta chains is not observed by x-ray crystallography.