Electron Transfer Kinetics between Hemoglobin Subunits*

The kinetics for electron transfer have been measured for samples of hemoglobin valency hybrids with initially one type of subunit, α or β, in the oxidized state. Incubation of these samples under anaerobic conditions tends to randomize the type of subunit that is oxidized. With a time coefficient of a few hours at pH 7, 25 °C, the Hb solution (0.1 mm heme) approaches a form with about 60% of β chains reduced, indicating a faster transfer rate in the direction α to β. There was no observable electron transfer for samples saturated with oxygen. The electron transfer occurs predominantly between deoxy and aquo-met subunits, both high spin species. Furthermore, electron transfer does not depend on the quaternary state of hemoglobin. Incubation of oxidized cross-linked tetramer Hb A with deoxy Hb S also displayed electron transfer, implying a mechanism via inter-tetramer collisions. A dependence on the overall Hb concentration confirms this mechanism, although a small contribution of transfer between subunits of the same tetramer cannot be ruled out. These results suggest that in vivo collisions between the Hb tetramers will be involved in the relative distribution of the methemoglobin between subunits in association with the reductase system present in the erythrocyte.