| Popis: |
When the α and β chains were separated from human oxyhemoglobin (HbO2), each individual chain was oxidized easily to the ferric form, their rates being almost the same with a very strong acid-catalysis. In the HbO2 tetramer, on the other hand, both chains become considerably resistant to autoxidation over a wide range of pH values (pH 5–11). Moreover, HbA showed a biphasic autoxidation curve containing the two rate constants, i.e. kf for the fast oxidation due to the α chains, and ks for the slow oxidation to the β chains. The kf/ks ratio increased from 3.2 at pH 7.5–7.3 at pH 5.8, but became 1 : 1 at pH values higher than 8.5. In the present work, we used the valency hybrid tetramers such as (α3+)2(βΟ2)2 and (αΟ2)2(β3+)2, and demonstrated that the autoxidation rate of either the α or β chains (when O2-ligated) is independent of the valency state of the corresponding counterpart chains. From these results, we have concluded that the formation of the α1β1 or α2β2 contact suppresses remarkably the autoxidation rate of the β chain and thus plays a key role in stabilizing the HbO2 tetramer. Its mechanistic details were also given in terms of a nucleophilic displacement of O2– from the FeO2 center, and the emphasis was placed on the proton-catalyzed process performed by the distal histidine residue. |
