Spectral properties of a cyanobacterial cytochrome c oxidase: Evidence for cytochrome a.a3

Membranes isolated from $\text{Nostoc}$ sp. strain Mac oxidised NAD(P)H and horse heart ferrocytochrome c in dark reactions inhibited by KCN, NaN₃, CO, and by anaerobiosis. Reduced $\text{minus}$ oxidised difference spectra revealed peaks at 603 and 445 nm which shifted to 590 and 430 nm, respectively, in reduced $\text{plus}\text{CO}\text{minus}$ reduced spectra. In presence of suitable electron mediators the pigment could be reduced also with NAD(P)H or ascorbate; KCN prevented this reduction. Photoaction spectra of CO-inhibited membranes showed peaks at 590 and 430 nm. From the results it is concluded that cytochrome a.a₃ is a functional respiratory oxidase in $\text{Nostoc}$ sp. strain Mac.