Spectral evidence for the existence of a second cytochrome o in whole cells of Vitreoscilla

Abstract

Cytochrome o, a protoheme IX pigment, has been proposed as the terminal oxidase of the filamentous bacterium, Vitreoscilla. Aerobic and anaerobic photolysis of CO-liganded whole cells demonstrated the presence of a second CO-reactive pigment, cytochrome o'. At temperatures lower than -100 degrees C, anaerobic photolysis dissociated only about 25% of the total CO-liganded components to reveal the unliganded cytochrome o'. At these temperatures, the photolysis of cytochrome o could not be demonstrated. At warmer temperatures, recombination of CO with the reduced cytochrome o' occurred with an apparent energy of activation of 5.8 kcal/mol. Aerobic photolysis of whole cells demonstrated two oxygen-bound intermediates. At temperatures lower than -95 degrees C, a spectrally distinct compound with absorption maxima at 428, 534, and 564 nm appeared (form I'); the apparent second order rate constant (k+1) for the formation of this intermediate was found to be 9.1 M-1 s-1, the reverse rate (k-1) was 9.9 X 10(-5) s-1, and the equilibrium constant (Kd) was 1.1 X 10(-5) M. This oxygen intermediate of cytochrome o' is spectrally and kinetically similar to the oxygen intermediate of cytochrome o seen in Escherichia coli. At temperatures warmer than -90 degrees C, photolysis of aerobic samples resulted in the immediate formation of a second oxygen-bound intermediate (form I) with absorption maxima at 422, 534, and 564 nm. This second intermediate results from the binding of oxygen to the cytochrome o (oxygenated cytochrome o). These data support the proposal that whole cells of Vitreoscilla contain two alternative pathways of electron transport, one terminating with cytochrome o and the other with cytochrome o'.