Ligand Binding in a Docking Site of Cytochrome c Oxidase:  A Time-Resolved Step-Scan Fourier Transform Infrared Study

The description of reaction regulation in enzymes responsible for activating and catalyzing small molecules (O2, NO) requires identification of ligand movement into the binding site and out of the enzyme through specific channels and docking sites. We have used time-resolved step-scan Fourier transform infrared spectroscopy on CO-photolyzed cytochrome c oxidase ba 3 from T. thermophilus, which is responsible for the activation and reduction of both O2 and NO, to gain insight into the structure of ligand-binding intermediates at ambient temperature. We show that, upon dissociation, the photolyzed CO becomes trapped within a ligand docking site located near the ring A propionate of heme a 3. The 2131 cm-1 mode of the “docked” CO we have detected corresponds to the B1 state of Mb and persists for 35 μs. The release of CO from the docking site is not followed by recombination to the heme a 3 Fe. Our analysis indicates that this behavior reflects a mechanism in which the protein near ring A of heme a 3 propionate reorganizes about the released CO from the docking site, and establishes a transient barrier that inhibits the recombination process to the heme a 3 Fe for a few milliseconds. Rebinding to heme a 3 occurs with k 2 = 29.5 s-1. These results have implications for understanding the role of ligand binding/escape through docking sites and channels in heme-copper oxidases and, thus, in respiration.