Mechanism of proton transfer through the KC proton pathway in the Vibrio cholerae cbb3 terminal oxidase

Proton transfer in cytochrome bd-I from E. coli involves Asp-105 in CydB

Cytochrome bds are bacterial terminal oxidases expressed under low oxygen conditions, and they are important for the survival of many pathogens and hence potential drug targets. The largest subunit CydA contains the three redox-active cofactors heme b558, heme b595 and the active site heme d. One suggested proton transfer pathway is found at the interface between the CydA and the other major subunit CydB. Here we have studied the O2 reduction mechanism in E. coli cyt. bd-I using the flow-flash technique and focused on the mechanism, kinetics and pathway for proton transfer. Our results show that the peroxy (P) to ferryl (F) transition, coupled to the oxidation of the low-spin heme b558 is pH dependent, with a maximum rate constant (~104 s-1) that is slowed down at higher pH. We assign this behavior to rate-limitation by internal proton transfer from a titratable residue with pKa ~ 9.7. Proton uptake from solution occurs with the same P➔F rate constant. Site-directed mutagenesis shows significant effects on catalytic turnover in the CydB variants Asp58B➔Asn and Asp105B➔Asn variants consistent with them playing a role in proton transfer. Furthermore, in the Asp105B➔Asn variant, the reactions up to P formation occur essentially as in the wildtype bd-I, but the P➔F transition is specifically inhibited, supporting a direct and specific role for Asp105B in the functional proton transfer pathway in bd-I. We further discuss the possible identity of the high pKa proton donor, and the conservation pattern of the Asp-105B in the cyt. bd superfamily.

Vibrio cholerae requires oxidative respiration through the bd-I and cbb3 oxidases for intestinal proliferation

Vibrio cholerae respires both aerobically and anaerobically and, while oxygen may be available to it during infection, other terminal electron acceptors are proposed for population expansion during infection. Unlike gastrointestinal pathogens that stimulate significant inflammation leading to elevated levels of oxygen or alternative terminal electron acceptors, V. cholerae infections are not understood to induce a notable inflammatory response. To ascertain the respiration requirements of V. cholerae during infection, we used Multiplex Genome Editing by Natural Transformation (MuGENT) to create V. cholerae strains lacking aerobic or anaerobic respiration. V. cholerae strains lacking aerobic respiration were attenuated in infant mice 10⁵-fold relative to wild type, while strains lacking anaerobic respiration had no colonization defect, contrary to earlier work suggesting a role for anaerobic respiration during infection. Using several approaches, including one we developed for this work termed Comparative Multiplex PCR Amplicon Sequencing (CoMPAS), we determined that the bd-I and cbb₃ oxidases are essential for small intestinal colonization of V. cholerae in the infant mouse. The bd-I oxidase was also determined as the primary oxidase during growth outside the host, making V. cholerae the only example of a Gram-negative bacterial pathogen in which a bd-type oxidase is the primary oxidase for energy acquisition inside and outside of a host.

The bacterium that causes cholera, Vibrio cholerae, can grow with or without oxygen. When growing without oxygen it may use other molecules that serve the same purpose as oxygen, acting as a terminal electron acceptor in an energy-generating process known as respiration. Given the largely anaerobic nature of the gastrointestinal tract, and the lack of significant inflammation during cholera infection, a process that can stimulate elevated levels of oxygen and other terminal electron acceptors, we sought to understand the respiratory mechanisms of V. cholerae during infection. We used a powerful genome-editing method to construct mutant strains of V. cholerae lacking some or all of the complement of proteins required for aerobic or anaerobic respiration. By analyzing these mutants in the laboratory and in intestinal colonization of infant mice, we determined that the ability to respire without oxygen is completely dispensable for V. cholerae to thrive during infection. We determined that two of the four oxygen-dependent respiration mechanisms are essential for V. cholerae to grow during infection, with the other two dispensable for wild type levels of colonization.

On the evolution of cytochrome oxidases consuming oxygen

This review examines the current state of the art on the evolution of the families of Heme Copper Oxygen reductases (HCO) that oxidize cytochrome c and reduce oxygen to water, chiefly cytochrome oxidase, COX. COX is present in many bacterial and most eukaryotic lineages, but its origin has remained elusive. After examining previous proposals for COX evolution, the review summarizes recent insights suggesting that COX enzymes might have evolved in soil dwelling, probably iron-oxidizing bacteria which lived on emerged land over two billion years ago. These bacteria were the likely ancestors of extant acidophilic iron-oxidizers such as Acidithiobacillus spp., which belong to basal lineages of the phylum Proteobacteria. Proteobacteria may thus be considered the originators of COX, which was then laterally transferred to other prokaryotes. The taxonomy of bacteria is presented in relation to the current distribution of COX and C family oxidases, from which COX may have evolved.