The Molecular Mechanism of Autoxidation for Human Oxyhemoglobin

Human oxyhemoglobin showed a biphasic autoxidation curve containing two rate constants, i.e. kf for the fast autoxidation due to the α chains, and ks for the slow autoxidation of the β chains, respectively. Consequently, the autoxidation of the HbO2tetramer produces two different curves from the pH dependence ofkf and ks. The analysis of these curves revealed that the β chain of the HbO2tetramer does not exhibit any proton-catalyzed autoxidation, unlike the α chain, where a proton-catalyzed process involving the distal histidine residue can play a dominant role in the autoxidation rate. When the α and β chains were separated from the HbO2tetramer, however, each chain was oxidized much more rapidly than in the tetrameric parent. Moreover, the separated β chain was recovered completely to strong acid catalysis in its autoxidation rate. These new findings lead us to conclude that the formation of the α1β1 contact produces in the β chain a conformational constraint whereby the distal histidine at position 63 is tilted away slightly from the bound dioxygen, preventing the proton-catalyzed displacement of O·̄2 by a solvent water molecule. The β chains have thus acquired a delayed autoxidation in the HbO2 tetramer.