Bacterial cytochrome oxidases.

A survey of the occurrence of bacterial cytochromes demonstrated that the presence of cytochrome oxidase isozymes was a common feature, and confirmed that the complements aa3+o and a1+a2+o, were present mainly in Gram-positives and Gram-negatives respectively. Cytochrome a2, which never occurred as the sole oxidase, was confined to highly adaptable organisms. There was a close correlation between cytochrome complement and general taxonomy. Bacteria were selected for experimental study: Kurthia zopfii, Xanthomonas hyacinthi and Acinetobacter Iwoffi (o); Acetobacter [T71] (a1+o); Chromobacterium violaceum (a2+o); Escherichia coli (a1+a2+o); Microbacterium thermosphactum and Bacillus subtilis (aa3), and a Keilin-Hartree preparation (aa3) from pig heart for comparison. Membrane preparations containing cytochrome oxidases a1,aa3 o were similarly inhibited both quantitatively and qualitatively by CO, indicating similarities in their active site. Each oxidase-CO complex was photodissociable, the Keilin-Hartree preparation being particularly sensitive. A CO-insensitive oxidase was present in K.zopfii. Inhibition, by cyanide with respect to electrons was uncompetitive in preparations containing a1 or aa3 and non-competitive in those containing o. Oxidases aa3 and o were inhibited non-competitively with respect to oxygen. Relative sensitivities were variable. Cytochromes a1 and o were only partially inhibited and multiple, independent sites were affected. Copper was present in the membranes equivalent to approximately 1 Cu : 1 a1, 1 o or 0.5 a3. Cytochrome a1 displayed a much higher second order velocity constant (∼ 23 x 107 M-1. S-1) than did aa3 and o (∼ 3 x 107 M-1. s-1) at 30°. No oxidase class bestowed a particularly high capacity for reaction. No preparation mediated the reduction of natural, alternative inorganic electron acceptors, but aa3 and a2 bound to nitrite. Estimates of P/O ratios in intact cells suggested only one site of oxidative phosphorylation in Ac. [T71] (containing a1 as major whereas three sites were present in cells containing aa3 or o. These findings were confirmed in E. coli utilising either o or a1+a2+o. The results allowed broad discussion and speculation on the reasons for cytochrome oxidase multiplicity.