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

摘要

Abstract: Time-resolved electronic absorption, infrared,resonance Raman, and magnetic circular dichroismspectroscopies are applied to characterization of theintermediate which is formed within 20 ps afterphotodissociation of CO from cytochrome a$-3$/ ofreduced cytochrome oxidase. This intermediate decayswith the same halflife (ca. 1 $mu@s) as the post-photodissociation Cu$-B$/$PLU@-CO species previouslyobserved by time-resolved infrared. The transientUV-Vis spectra, kinetics, infrared, and Raman evidencesuggest that an endogenous ligand is transferred fromCu$-B$/ to Fe$-a3$/ when CO binds to Cu$-B$/, forming acytochrome a$-3$/ species with axial ligation whichdiffers from the reduced unliganded enzyme. Thetime-resolved magnetic circular dichroism resultssuggest that this transient is high spin and thereforefive coordinate. Thus it is inferred that the ligandfrom Cu$-B$/ binds on the distal side of cytochromea$-3$/ and displaces the proximal histidine imidazole.This remarkable mechanistic feature is an additionalaspect of the previously proposed 'ligand shuttle'activity of the Cu$-B$//Fe$-a3$/ pair. The authorssuggest that the ligand shuttle may play a functionalrole in redox-linked proton translocation by theenzyme. More detail on this work is presentedelsewhere.!