Similar carbon monoxide binding sites in bacterial cytochrome bo and mammalian cytochrome c oxidase

Abstract

Properties of the active site of terminal oxidation may be studied by examining the stable carbyn monoxide-bound adducts. The resonance Raman and the infrared absorption spectra of CO-bound mammalian cytochrome c oxidase have revealed several structural characteristics of the heme pocket. The cytochrome bo complex is a terminal ubiquinol oxidase from aerobically grown Escherichia coli. The complex contains three known redox centers: two heme groups and one copper atom. One of the hemes, termed cytochrome b[sub 563.5], is low spin and is analogous to cytochrome a in mammalian oxidases. The other heme (cytochrome o) and a nearby copper atom form a binuclear binding site for O[sub 2] or CO that is analogous to the cytochrome a[sub 3]-Cu[sub B] binuclear binding site in mammalian oxidases. The frequencies of the CO-sensitive modes which are detected in cytochrome bo are qualitatively similar to the corresponding frequencies found in the CO-bound cytochrome a[sub 3]. Thus, these data confirm the similarity in the properties of the active sites in these two terminal oxidases. The data substantiate the validity of utilizing site-directed mutagenesis experiments in cytochrome bo to draw inferences concerning the properties of cytochrome aa[sub 3]. Furthermore, the frequencies of the modes involving the boundmore » CO underscore the conclusion that if the proximal ligand in the CO adduct is histidine, features of its bonding to the iron atom of the heme are unique among the histidine-coordinated heme proteins that have been studied to date. The relatively high frequencies for both the Fe-CO and the C-O stretching modes indicate an anomalously weak proximal ligand-iron bond. The small frequency differences between the CO-isotope-sensitive modes in cytochrome bo and cytochrome aa[sub 3] do indicate that the binuclear site of the bacterial enzyme, while similar, is not identical to its mammalian counterpart. 15 refs., 1 fig.« less