Spectroscopic characterization of mutants supports the assignment of histidine-419 as the axial ligand of heme o in the binuclear center of the cytochrome bo ubiquinol oxidase from Escherichia coli

Abstract

The bo-type ubiquinol oxidase of Escherichia coli is a member of the superfamily of heme-copper oxidases which also includes the aa3-type cytochrome c oxidases. The oxygen-binding binuclear center of cytochrome bo is located in subunit I and consists of a heme (heme o; heme a3 in the aa3-type oxidases) and a copper (Cu(B)). Previous spectroscopic studies have shown that heme o is bound to the protein via a single histidine residue. Site-directed mutagenesis of conserved histidine residues in subunit I has identified two residues (H284 and H419) which are candidates for the ligand of heme o, while spectroscopic studies of mutants at H284 definitively demonstrated that this residue cannot be the axial ligand. Consequently, the single remaining conserved histidine in subunit I (H419) was assigned as the ligand for the heme of the binuclear center. In this paper, this assignment is tested by characterization of additional mutants in which the putative heme o axial ligand, H419, is replaced by other amino acids. All mutations at H419 result in the loss of enzyme activity. Analyses via UV-visible and Fourier transform infrared spectroscopies reveal that substantial perturbation has occurred at the binuclear center as a result of the amino acid substitutions. In contrast with the wild-type enzyme, the mutant enzymes bind very little carbon monoxide. Three other amino acid residues which are potential ligands for heme o are shown tob e nonessential for enzyme activity. Mutations in these residues do not perturb the UV-visible or FTIR spectroscopic characteristics of the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)