FTIR spectroscopic characterization of the cytochrome aa3 from Acidianus ambivalens:: Evidence for the involvement of acidic residues in redox coupled proton translocation

The aa(3)-type quinol oxidase from Acidianus ambivalens is a divergent member of the heme-copper oxidases superfamily, namely, concerning the putative channels for intraprotein proton conduction. In this study, we used electrochemically induced FTIR difference spectroscopy to identify residues involved in redox-coupled protonation changes. In the spectral region characteristic for the nu(C=O) mode from protonated aspartic or glutamic acid side chains, a number of prominent features can be observed between 1790 and 1710 cm(-1), clearly indicating the reorganization or protonation of more than four protonatable residues upon electron transfer. A direct comparison of the Fourier-transform infrared difference spectra at different pH values reveals the noteworthy high pK of >8 for some of these residues, and the protonation of two of them. These acidic residues may play a role in the proton transport to the oxygen reducing site, in proton pumping pathways, or in protonation reactions concomitant with quinone reduction. Whereas the residues contributing between 1790 and 1750 cm(-1) have the typical position of an aspartic/glutamic acid side chain buried in the protein, a position closer to the surface is suggested for the residues contributing below approximately 1730 cm(-1). The possible involvement of residues contributing between 1750 and 1720 cm(-1) in the quinone binding site is demonstrated on the basis of experiments in the presence and absence of ubiquinone-2 and of the native electron carrier of the A. ambivalens respiratory chain, caldariella quinone. Most signals seen here are not observable in comparable spectra of typical members of the heme copper oxidase superfamily and thus reflect unique features of the enzyme from the hyperthermoacidophilic archaeon A. ambivalens.