Carbon monoxide binding and inhibition of hydrogenase I from Clostridium pasteurianum characterized by x-ray crystallography

The binding and inhibition of hydrogenase I from Clostridium pasteurianum by carbon monoxide was characterized by kinetic and spectroscopic methods and the results have mechanistic implications. Initial characterizations of carbon monoxide inhibition of hydrogenase I revealed that it acts as a competitive inhibitor of hydrogen oxidation and proton reduction and that inhibition of the enzyme can be reversed by illumination.; Given the complex structure of the active site and the presence of carbonyl and cyanide ligands on the iron atoms in the active site of the native enzyme, it was of considerable interest to characterize the binding site of exogenously added carbon monoxide. How illumination reverses its inhibitory effects was also investigated.; The binding of exogenously added carbon monoxide to the enzyme and the photochemistry that occurs at the active site of the carbon monoxide-bound enzyme were examined by X-ray diffraction methods. In each set of experiments, two sets of data were collected from isomorphous protein crystals and used for the calculation of difference electron density maps. In the first set of experiments, an evaluation of the structural changes near the active site manifest by the differences in electron density between the native and carbon monoxide-bound enzyme indicates the displacement of an iron bound water molecule, originally observed in the native structure, with carbon monoxide. Similarly, in a second set of experiments, two data sets, collected before and during illumination with a laser, were collected on a single crystal of carbon monoxide-treated hydrogenase. The two data sets were used to calculate difference electron density maps and the changes in electron density near the active site indicate the loss of the inhibitory carbon monoxide ligand upon illumination. Also presented in this work is a summary of ongoing efforts to characterize a dithiolate moiety present in the active site of hydrogenase I.; The results presented in this work provide evidence for a ligand-exchangeable coordination site in the active site of the hydrogenase. They implicate that site, or the availability of that site for binding during catalysis, as a salient feature of the mechanism of hydrogen oxidation and proton reduction.