Recent electrostatics calculations on the cytochrome c oxidase from Paracoccus denitrificans revealed an unexpected coupling between the redox state of the heme-copper center and the state of protonation of a glutamic acid (E78(II)) that is 25 Å away in subunit II of the oxidase. Examination of more than 300 sequences of the homologous subunit in other heme-copper oxidases shows that this residue is virtually totally conserved and is in a cluster of very highly conserved residues at he 'negative' end (bacterial cytoplasm or mitochondrial matrix) of the second transmembrane helix. The functional importance of several residues in this cluster (E89(II), W93(II), T94(II), and P96(II)) was examined by site-directed mutagenesis of the corresponding region of the cytochrome bo3 quinol oxidase from Escherichia coli (where E89(II) is the equivalent of residue E78(II) of the P. denitrificans oxidase). Substitution of E89(II) with either alanine or glutamine resulted in reducing the rate of turnover to about 43 or 10% of the wild-type value, respectively, whereas E89D has only about 60% of the activity of the control oxidase. The quinol oxidase activity of the W93V mutant is also reduced to about 30% of that of the wild-type oxidase. Spectroscopic studies with the purified E89A and E89Q mutants indicate no perturbation of the heme-copper center. The data suggest that E89(II) (E. coli numbering) is critical for the function of the heme copper oxidases. The proximity to K362 suggests that this glutamic acid residue may regulate proton entry or transit through the K-channel. This hypothesis is supported by the finding that the degree of oxidation of the low-spin heme b is greater in the steady state using hydrogen peroxide as an oxidant in place of dioxygen for the E89Q mutant. Thus, it appears that the inhibition resulting from the E89(II) mutation is due to a block in the reduction of the heme- copper binuclear center, expected for K-channel mutants.
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