Differential Effects of Glutamate-286 Mutations in the Aa3-Type Cytochrome C Oxidase from Rhodobacter Sphaeroides and the Cytochrome Bo3 Ubiquinol Oxidase from Escherichia Coli

Both the aa3-type cytochrome c oxidase from Rhodobacter sphaeroides (RsCcOaa3) and the closely related bo3-type ubiquinol oxidase from Escherichia coli (EcQObo3) possess a proton-conducting D-channel that terminates at a glutamic acid, E286, which is critical for controlling proton transfer to the active site for oxygen chemistry and to a proton loading site for proton pumping. E286 mutations in each enzyme block proton flux and, therefore, inhibit oxidase function. In the current work, resonance Raman spectroscopy was used to show that the E286A and E286C mutations in RsCcOaa3 result in long range conformational changes that influence the protein interactions with both heme a and heme a3. Therefore, the severe reduction of the steady-state activity of the E286 mutants in RsCcOaa3 to ~0.05% is not simply a result of the direct blockage of the D-channel, but it is also a consequence of the conformational changes induced by the mutations to heme a and to the heme a3-CuB active site. In contrast, the E286C mutation of EcQObo3 exhibits no evidence of conformational changes at the two heme sites, indicating that its reduced activity (3%) is exclusively a result of the inhibition of proton transfer from the D-channel. We propose that in RsCcOaa3, the E286 mutations severely perturb the active site through a close interaction with F282, which lies between E286 and the heme-copper active site. The local structure around E286 in EcQObo3 is different, providing a rationale for the very different effects of E286 mutations in the two enzymes. This article is part of a Special Issue entitled: Allosteric cooperativity in respiratory proteins.