The Molecular Mechanism of Autoxidation for Human Oxyhemoglobin

Abstract

Human oxyhemoglobin showed a biphasic autoxidation curve containing two rate constants, i.e. k f for the fast autoxidation due to the α chains, and k s for the slow autoxidation of the β chains, respectively. Consequently, the autoxidation of the HbO 2 tetramer produces two different curves from the pH dependence of k f and k s. The analysis of these curves revealed that the β chain of the HbO 2 tetramer 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 HbO 2 tetramer, 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 α 1 β 1 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 HbO 2 tetramer.