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

In 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 a ₃ reduction. When prokaryotic cytochrome c-552 oxidase (ba ₃) 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 a₃ ³⁺CNCuB⁺. Both states become the fully reduced a ₃²⁺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.