The occurrence and function of a-type cytochromes in the aerobic respiratory chain of Comamonas percolans NCTC 1937 grown under O2-sufficient and O2-limited conditions

Abstract

Intact cells of Comamonas percolans (NCTC 1937) after growth under O2-limited conditions in batch culture exhibit, in addition to b- and c-type cytochromes, an unusual pigment absorbing variably at 588 to 594 nm in reduced minus oxidized difference spectra at room temperature. CO difference spectra suggest ligand binding by this pigment and also reveal a prominent 448 nm absorption minimum and CO-binding b- and/or c-type cytochromes. Although the 588 to 594 nm-absorbing component is reminiscent of ‘cytochrome a 1’, claimed to be a terminal oxidase in some bacteria, O2-limited cells of C. percolans contain no detectable haem A. In contrast, cells grown under O2-sufficient conditions exhibit a membrane-bound cytochrome aa 3 and contain haem A. Low-temperature photodissociation studies of O2-sufficient cells show cytochrome aa 3 to be functional in CO and O2 binding and suggest that aa 3 is the terminal oxidase of a cytochrome b- and c-containing aerobic respiratory chain. Analogous studies of O2-limited cells reveal a component absorbing, in its unliganded state, at 448 nm. Exposure of such CO-liganded cells to white actinic light is followed by oxidation of b- and/or c-type cytochromes and, although the functional oxidase has not been identified, we conclude that C. percolans does not utilize a cytochrome oxidase of the a 1 type.