The mixed valence state of the oxidase binuclear centre: how Thermus thermophilus cytochrome ba3 differs from classical aa3 in the aerobic steady state and when inhibited by cyanide

AbstractIn the aerobic steady state of the classical eukaryotic cytochrome c oxidase, three aa3 redox metal centres (cytochrome a, CuA and CuB) are partially reduced while the fourth, cytochrome a3, remains almost fully oxidized. Turnover depends primarily upon the rate of cytochrome a3 reduction. When prokaryotic cytochrome c-552 oxidase (ba3) of Thermus thermophilus turns over, three different metal centres (cytochromes b, a3 and CuA) share the steady state electrons; it is the fourth, CuB, that apparently remains almost fully oxidized until anaerobiosis. Cytochrome a3 stays partially reduced during turnover and a possible P/F state may also be populated. Cyanide traps the aerobic ba3 CuB centre in the a32+CNCuB2+ state; the corresponding eukaryotic cyanide trapped state is a33+CNCuB+. Both states become the fully reduced a32+CNCuB+ upon anaerobiosis.The different reactivities of the aa3 and ba3 binuclear centres may be correlated with the very different proximal histidine εN-Fe distances in the two enzymes (3.3 Å for ba3 compared to 1.9 Å for aa3) which may in turn relate to the functioning of thermophilic Thermus cytochrome ba3 in vivo at a very elevated temperature. But the differences may also just exemplify how evolution can find surprisingly different solutions to the common problem of electron transfer to oxygen. Some of these alternatives were potentially enshrined in a model of the oxidase reaction already adopted by Gerry Babcock in the early 1990s