Photodissociation of cytochrome c oxidase-nitric oxide complexes

The dissociation of cytochrome c oxidase-nitric oxide complexes was studied by optical spectroscopy at cryogenic temperatures (15 degrees K). With the reduced cytochrome c oxidase-nitric oxide complex, the observations that were reported by Yoshida et al. were confirmed. Photodissociation of the oxidized cytochrome c oxidase-nitric oxide complex did not induce any significant absorbance changes between 350 and 875 nm. With the azide-nitrosyl-cytochrome c oxidase complex, the illumination caused the dissociation of the a2+(3).NO complex to the unligated state a2+(3). Increasing the temperature to 77 degrees K led to the formation of a new complex, probably a3+(3).N3-. The N3(-)-NO-cytochrome c oxidase complex was the only compound for which appreciable photodissociation was achieved by continuous illumination at room temperature (20 degrees C). The effect of illumination was biphasic. In the first phase the a2+(3).NO complex is dissociated and cytochrome a3 oxidized by an electron transfer to CuB. In the second phase nitric oxide, which is still bound to CuB after the first phase, is expelled from the complex by azide, with a concomitant electron transfer from CuB to cytochrome a.