Simultaneous resonance Raman detection of the heme a3-Fe-CO and CuB-CO species in CO-bound ba3-cytochrome c oxidase from Thermus thermophilus: Evidence for a charge transfer CuB-CO transition

Abstract

Understanding of the chemical nature of the dioxygen and nitric oxide moiety of ba3-cytochrome c oxidase from Thermus thermophilus is crucial for elucidation of its physiological function. In the present work, direct resonance Raman (RR) observation of the Fe-C-O stretching and bending modes and the C-O stretching mode of the CuB-CO complex unambiguously establishes the vibrational characteristics of the heme-copper moiety in ba3-oxidase. We assigned the bands at 507 and 568 cm -1 to the Fe-CO stretching and Fe-C-O bending modes, respectively. The frequencies of these modes in conjunction with the C-O mode at 1973 cm -1 showed, despite the extreme values of the Fe-CO and C-O stretching vibrations, the presence of the α-conformation in the catalytic center of the enzyme. These data, distinctly different from those observed for the caa3-oxidase, are discussed in terms of the proposed coupling of the α-and β-conformations that occur in the binuclear center of heme-copper oxidases with enzymatic activity. The Cu B-CO complex was identified by its ν(CO) at 2053 cm-1 and was strongly enhanced with 413.1 nm excitation indicating the presence of a metal-to-ligand charge transfer transition state near 410 nm. These findings provide, for the first time, RR vibrational information on the EPR silent CuB(I) that is located at the O2 delivery channel and has been proposed to play a crucial role in both the catalytic and proton pumping mechanisms of heme-copper oxidases.