Characterization of Heterogeneous Nickel Sites in CO Dehydrogenases fromClostridium thermoaceticumandRhodospirillum rubrumby Nickel L-Edge X-ray Spectroscopy

Abstract

Carbon monoxide dehydrogenase from Clostridium thermoaceticum (Ct-CODH) is a nickel-containing enzyme that catalyzes acetyl-CoA synthesis and CO oxidation at two separate Ni sites, the A-cluster and C-cluster, respectively. Carbon monoxide dehydrogenase from Rhodospirillum rubrum (Rr-CODH) contains only a C-type cluster and catalyzes only CO oxidation. We have used L-edge X-ray absorption spectroscopy to study the Ni electronic structure of these two enzymes. The spectra indicate that most of the Ni in as-isolated Ct-CODH is low-spin Ni(II). Upon CO treatment, a fraction of the nickel is converted either to high-spin Ni(II) and/or to Ni(I). Ni in dithionite-reduced Rr-CODH also exhibits a clear low spin Ni(II) component, again mixed with either high-spin Ni(II) or Ni(I). The spectrum of Rr-CODH shifts to higher energy upon indigo carmine oxidation, suggesting either that most of the high-spin Ni(II) is converted to low-spin Ni(II) and/or that some Ni is oxidized between these two forms. These results are discussed and compared with recent L-edge spectra for the Ni site in hydrogenase.