Carbon monoxide dehydrogenases are enzymes in the process of carbon fixation and knowledge of the local electronic and physical structure of the metal clusters in these enzymes is essential for the understanding of the mechanisms of CO oxidation and acetyl-CoA synthesis. The detection of the K _ fluorescence with high energy resolution reveals information about oxidation and spin states of first-row transition metal compounds. We propose to use the spin-state sensitivity of the K _ fluorescence to investigate the CODH enzyme from R. rubrum to extend the current understanding of the Ni-Fe metal cluster by quantifying the amount of high and low spin Ni present. We plan to use site-selective x-ray absorption to study one of the Ni-Fe centers (Center A, the site of acetyl-CoA synthesis) in C. thermo-aceticum CODH to study the ligand environment of Ni(I) and Ni(II) and to determine if CO binds to Ni.