INHIBITION OF SICKLE BETA-CHAIN (BETA(S))-DEPENDENT POLYMERIZATION BYNONHUMAN ALPHA-CHAINS - A SUPERINHIBITORY MOUSE-HORSE CHIMERIC ALPHA-CHAIN

Horse alpha-chain inhibits sickle beta-chain-dependent polymerization; however, its inhibitory potential is not as high as that of mouse alp ha-chain. Horse alpha-(1-30) and alpha-(31-141) segments make, respect ively, minor and major con tributions to the inhibitory potential of h orse alpha-chain. The sum of the inhibitory potential of the two segme nts does not account for the inhibitory potential of the full-length h orse alpha-chain. Although the polymerization inhibitory potential of horse alpha-chain is lower than mouse alpha-chain, the inhibitory pote ntial of horse alpha-(31-141) is comparable to that of mouse alpha-(31 -141). When mouse alpha-(1-30) is stitched to horse alpha-(31-141), th e product is a chimeric alpha-chain with an inhibitory potential great er than mouse alpha-chain. In contrast, the stitching of horse alpha-( 1-30) with mouse alpha-(31-141) had no additional inhibitory potential . Molecular modeling studies of HbS containing the mouse-horse chimeri c alpha-chain indicate altered sidechain interactions at the alpha(1) beta(1) interface when compared with HbS, In addition, the AB/GH corne r perturbations facilitate a different stereochemistry for the interac tion of the epsilon-amino group of Lys-16(alpha) with the beta-carboxy l group of Asp-116(alpha), resulting in a decrease in the accessibilit y of the side chain of Lys-16(alpha) to the solvent. Based on molecula r modeling, we speculate that these perturbations by themselves, or in synergy with the altered conformational aspects of the alpha(1) beta( 1) interactions, represent the molecular basis of the superinhibitory potential of the mouse-horse chimeric alpha-chains.