Photodissociation of oxygenated cytochrome o(s) (Vitreoscilla) and kinetic studies of reassociation

Abstract

Oxygenated cytochrome o(s) from Vitreoscilla was photodissociated by a laser flash but the quantum yield was low. The rebinding of oxygen to the ferrous cytochrome proceeded monophasically, and the second order rate constant was 7.8 X 10(7) M-1 s-1, the off rate constant 5.6 X 10(3) s-1, and the calculated dissociation constant for the oxygenated compound 7.2 X 10(-5) M at pH 7.3 and 25 degrees C. Rapid scanning spectroscopy revealed the formation of chytochrome o-O2 directly from ferrous chytochrome o and oxygen without any evidence for an intermediary formation of Compound D, another type of oxygenated chytochrome o. Photodissociation in solution containing CO/O2 mixtures resulted in rapid binding of oxygen followed by slow replacement by CO. This property as well as the photodissociability of chytochrome o-O2 suggests that the heme iron of the compound is in the ferrous state. In addition, the primary oxygen compound was fairly stable and did not decay further in the absence of CO, in marked contrast with that of mammalian cytochrome oxidase primary oxygen compound which rapidly decayed. This result suggests a possible role of this cytochrome as an oxygen carrier or storage.