Epitopes of monoclonal antibodies which inhibit ubiquinol oxidase activity of Escherichia coli cytochrome d complex localize functional domain

The aerobic respiratory chain of Escherichia coli contains two terminal oxidases: the cytochrome d complex and the cytochrome o complex. Each of these enzymes catalyzes the oxidation of ubiquinol-8 within the cytoplasmic membrane and the reduction of molecular oxygen to water. Both oxidases are coupling sites in the respiratory chain; electron transfer from ubiquinol to oxygen results in the generation of a proton electrochemical potential difference across the membrane. The cytochrome d complex is a heterodimer (subunits I and II) that has three heme prosthetic groups. Previous studies characterized two monoclonal antibodies that bind to subunit I and specifically block the ability of the enzyme to oxidize ubiquinol. In this paper, the epitopes of both of these monoclonal antibodies have been mapped to within a single 11-amino acid stretch of subunit I. The epitope is located in a large hydrophilic loop between the fifth and sixth putative membrane-spanning segments. Binding experiments with these monoclonal antibodies show this polypeptide loop to be periplasmic. Such localization suggests that the loop may be close to His186, which has been identified as one of the axial ligands of cytochrome b558. Together, these data begin to define a functional domain in which ubiquinol is oxidized near the periplasmic surface of the membrane.