Substitution of Asparagine for Aspartate-135 in Subunit I of the Cytochrome bo Ubiquinol Oxidase of Escherichia coli Eliminates Proton-Pumping Activity

The terminal quinol oxidase, cytochrome bo, of Escherichia coli is a member of the large terminal oxidase family, which includes cytochrome aa₃-type terminal oxidases from bacteria, plants, and animals. These enzymes conserve energy by linking electron transfer to vectorial proton translocation across mitochondrial or bacterial cell membranes. Site-directed mutagenesis of the five most highly conserved acidic amino acids in subunit I of cytochrome bo was performed to study their role in proton transfer. Mutation of only one of these sites, Asp135, to the corresponding amide, results in a dramatic decrease in proton pumping but with little change in electron-transfer activity. However, the conservative mutation Asp135Glu is active in proton translocation. It is proposed that an acidic residue at position 135 in subunit I may be important to form a functional proton input channel of the proton pump.