Movement of the iron-sulfur head domain of cytochrome bc(1) transiently opens the catalytic Q(o) site for reaction with oxygen.

Cytochrome bc(1), a key enzyme of biological energy conversion, generates or uses a proton motive force through the Q cycle that operates within the two chains of cofactors that embed two catalytic quinone oxidation/reduction sites, the Q(o) site and the Q(i) site. The Q(o) site relies on the joint action of two cofactors, the iron-sulfur (FeS) cluster and heme b(L). Side reactions of the Q cycle involve a generation of superoxide which is commonly thought to be a product of an oxidation of a highly unstable semiquinone formed in the Q(o) site (SQ(o)), but the overall mechanism of superoxide generation remains poorly understood. Here, we use selectively modified chains of cytochrome bc(1) to clearly isolate states linked with superoxide production. We show that this reaction takes place under severely impeded electron flow that traps heme b(L) in the reduced state and reflects a probability with which a single electron on SQ(o) is capable of reducing oxygen. SQ(o) gains this capability only when the FeS head domain, as a part of a catalytic cycle, transiently leaves the Q(o) site to communicate with the outermost cofactor, cytochrome c(1). This increases the distance between the FeS cluster and the remaining portion of the Q(o) site, reducing the likelihood that the FeS cluster participates in an immediate removal of SQ(o). In other states, the presence of both the FeS cluster and heme b(L) in the Q(o) site increases the probability of completion of short-circuit reactions which retain single electrons within the enzyme instead of releasing them on oxygen. We propose that in this way, cytochrome bc(1) under conditions of impeded electron flow employs the leak-proof short-circuits to minimize the unwanted single-electron reduction of oxygen.