Femtosecond dynamics of reduced cytochrome oxidase and its CO derivative

The photodissociation dynamics of reduced CO-ligated and reduced unligated cytochrome c oxidase (ferrocytochrome c:O₂ oxidoreductase, cytochrome aa₃, or CcO) and subsequent thermal reactivity have been studied with femtosecond UV-visible absorption spectroscopy. In both the unligated and the CO-ligated derivatives, we observe transients due to excited-state formation/decay and ligand photodissociation/rebinding reactions which occur at both of the heme centers (cytochromes a and a₃) in this enzyme. For cytochrome a, transients are observed that decay biexponentially with time constants of 3-5 and 20 ps. These are assigned to fast excited-state decay with loss of axial histidine imidazole and subsequent axial ligand rebinding. In the case of CO-ligated CcO additional transients due to cytochrome a₃ are observed. After the initial loss of CO (<100 fs), a transient is observed that decays with a 6-ps time constant. This is assigned to a relaxation occurring at the cytochrome a₃ site, perhaps caused by a ligand transfer to the cytochrome a₃ site as has been suggested or to a local protein conformational change. Additional photophysics and the implications of the observed thermal reactions for the function of the enzyme are discussed.