Ultrafast and not-so-fast dynamics of cytochrome oxidase: the ligand shuttle and its possible functional significance

Time-resolved electronic absorption, infrared, resonance Raman, and magnetic circular dichroism spectroscopies are applied to characterization of the intermediate which is formed within 20 ps after photodissociation of CO from cytochrome a₃ of reduced cytochrome oxidase. This intermediate decays with the same halflife (ca. 1 μs) as the post- photodissociation Cu_B+-CO species previously observed by time-resolved infrared. The transient UV-Vis spectra, kinetics, infrared, and Raman evidence suggest that an endogenous ligand is transferred from Cu_B to Fe_a3 when CO binds to Cu_B, forming a cytochrome a₃ species with axial ligation which differs from the reduced unliganded enzyme. The time-resolved magnetic circular dichroism results suggest that this transient is high spin and therefore five coordinate. Thus it is inferred that the ligand from Cu_B binds on the distal side of cytochrome a₃ and displaces the proximal histidine imidazole. This remarkable mechanistic feature is an additional aspect of the previously proposed 'ligand shuttle' activity of the Cu_B/Fe_a3 pair. The authors suggest that the ligand shuttle may play a functional role in redox-linked proton translocation by the enzyme. More detail on this work is presented elsewhere.