The terminal oxidase cytochrome bd-I confers carbon monoxide resistance to Escherichia coli cells

Carbon monoxide (CO) plays a multifaceted role in the physiology of organisms, from poison to signaling molecule. Heme proteins, including terminal oxidases, are plausible CO targets. Three quinol oxidases terminate the branched aerobic respiratory chain of Escherichia coli. These are the heme‑copper cytochrome bo3 and two copper-lacking bd-type cytochromes, bd-I and bd-II. All three enzymes generate a proton motive force during the four-electron oxygen reduction reaction that is used for ATP production. The bd-type oxidases also contribute to mechanisms of bacterial defense against various types of stresses. Here we report that in E. coli cells, at the enzyme concentrations tested, cytochrome bd-I is much more resistant to inhibition by CO than cytochrome bd-II and cytochrome bo3. The apparent half-maximal inhibitory concentration values, IC50, for inhibition of O2 consumption of the membrane-bound bd-II and bo3 oxidases by CO at ~150 μM O2 were estimated to be 187.1 ± 11.1 and 183.3 ± 13.5 μM CO, respectively. Under the same conditions, the maximum inhibition observed with the membrane-bound cytochrome bd-I was 20 ± 10% at ~200 μM CO. We compared the effect of CO on O2 consumption of E. coli cells of three mutants each expressing only one terminal cytochrome complex, bd-I, bd-II or bo3. CO inhibits respiration of bd-II- and bo3-only cells to a similar extent. Much lower inhibition percentage is observed in case of bd-I-only cells. [Display omitted] •CO plays a multifaceted role in microbial physiology.•Bacterial terminal oxidases are plausible CO targets.•E. coli respiratory chain has three terminal oxidases, cytochromes bd-I, bd-II, bo3.•In E. coli cells, bd-I is much more resistant to CO inhibition than bd-II and bo3.