Complexes of cytochrome caa3 from the thermophilic bacterium PS3 formed with ligands and during catalytic activity.

PS3 (thermophilic bacterium) cytochrome caa3 reacts slowly with cyanide which forms a low-spin complex with the CuBa3 centre. Partial reduction under catalytic conditions increases the rate of cyanide binding, and the reaction constant is rather similar to that of the mammalian enzyme, but the partially reduced complex dissociates more rapidly than does the corresponding eukaryotic complex. A simple biphasic reaction can account for the results obtained. The azide complex of partially reduced PS3 cytochrome caa3 shows an alpha-peak blue shift similar to that of the mammalian enzyme. PS3 cytochrome caa3 forms an oxyferri ("oxygenated") species like the mammalian enzyme, but does not undergo high- to low-spin changes during the aerobic steady state with ascorbate and N,N,N',N'-tetramethyl-p-phenylenediamine as substrates. Interactions seen in cytochrome oxidase-ligand reactions and its spin-state changes are therefore intrinsic to the enzyme's large catalytic subunits and do not require the presence of the small nuclear-encoded subunits found in eukaryotic systems.