Production of superoxide from hemoglobin-bound oxygen under hypoxic conditions

Low temperature electron paramagnetic resonance reveals the formation of a superoxide radical from the hemoglobin-bound oxygen blocked in the hydrophobic heme pocket where dismutation is slow. The formation of the free radical at intermediate oxygen pressures occurs when the rates of hemoglobin autoxidation for partly oxygenated intermediates increases. The flexibility of the heme pocket, which is high under hypoxia, possibly contributes to the formation of the superoxide.