The bacterial quinol oxidase, cytochrome o, is an enzyme which is highly analogous to the better known cytochrome c oxidase, cytochrome aa3 but with the important difference that it lacks the near infra-red absorbing pigment CuA. In this article we report an absorption band in the near IR spectrum of cytochrome o with a maximal absorption at 738 nm, and which is attributable to the ferrous high-spin haem. The 758 nm band has an extinction coefficient or 0.2–0.3 mM−1·cm−1 at 758–800 nm. This region in cytochrome aa3 , is dominated by the CuA absorption. The 758 nm absorption is lost on addition of CO or cyanide to the reduced enzyme. The carbon monoxide compound of cytochrome o also has absorbance bands in the near infra-red, and these may be attributable to a low-spin ferrous haem compound.
