Characterization of Heterogeneous Nickel Sites in CO Dehydrogenases from Clostridium thermoaceticum and Rhodospirillum rubrum by Nickel L-Edge X-ray Spectroscopy

• Published in: Journal of the American Chemical Society Vol. 122; no. 43; pp. 10553 - 10560
• Main Authors: Ralston, C. Y, Wang, Hongxin, Ragsdale, S. W, Kumar, M, Spangler, N. J, Ludden, P. W, Gu, W, Jones, R. M, Patil, D. S, Cramer, S. P
• Format: Journal Article
• Language: English
• Published: American Chemical Society 01-11-2000
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja0009469

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.