Neuroglobin: Enzymatic reduction and oxygen affinity

Neuroglobin (Ngb) is a hexacoordinate globin expressed in the nervous system of vertebrates, involved in neuroprotection. O-2 equilibrium measurements on mouse Ngb yielded significantly different P-50 values, ranging from similar to 2 torr to similar to 10 torr. By a kinetic approach minimizing the effects of protein autoxidation, we measured P-50 = 2.2 torr at 20 degrees C. As predicted from the structure, O-2 binds to the Y44D Ngb mutant more quickly (k = 2.2 s(-1) vs 0.15 s(-1)) and with slightly higher affinity (P-50 = 1.3 torr) than wild-type. In addition, we introduced a novel reduction protocol for metNgb based on NADH:flavorubredoxin oxidoreductase (FlRd-red) from Escherichia. coli, a candidate for the Ngb reducing activity recently identified in E coli extracts. Interestingly, E coli FlRd-red shares sequence similarity with the FAD-binding domain of the human apoptosis-inducing factor, a finding which may have unexpected significance with reference to the mechanism of neuroprotection by Ngb. (c) 2008 Elsevier Inc. All rights reserved.