title: X-ray Absorption Spectroscopy Reveals an Organometallic Ni-C Bond in the CO-Treated Form of Acetyl-CoA Synthase.
abstract: Acetyl-CoA synthase (ACS) is a key enzyme in the Wood-Ljungdahl pathway of anaerobic CO2fixation, which has long been proposed to operate by a novel mechanism involving a series of protein-bound organometallic (Ni-CO, methyl-Ni, and acetyl-Ni) intermediates. Here we report the first direct structural evidence of the proposed metal-carbon bond. We describe the preparation of the highly active metal-replete enzyme and near-quantitative generation of the kinetically competent carbonylated intermediate. This advance has allowed a combination of Ni and Fe K-edge X-ray absorption spectroscopy and extended X-ray absorption fine structure experiments along with density functional theory calculations. The data reveal that CO binds to the proximal Ni of the six-metal metallocenter at the active site and undergoes dramatic structural and electronic perturbation in forming this organometallic Ni-CO intermediate. This direct identification of a Ni-carbon bond in the catalytically competent CO-bound form of the A cluster of ACS provides definitive experimental structural evidence supporting the proposed organometallic mechanism of anaerobic acetyl-CoA synthesis.
authors.affiliation: Department of Biological Chemistry, University of Michigan , Ann Arbor, Michigan 48109-0606, United States.
authors.forename: Mehmet
authors.initials: M
authors.lastname: Can
authors.affiliation: Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory , Menlo Park, California 94025, United States.|Department of Chemistry, Stanford University , Stanford, California 94306, United States.
authors.forename: Logan J
authors.initials: LJ
authors.lastname: Giles
authors.affiliation: Department of Biological Chemistry, University of Michigan , Ann Arbor, Michigan 48109-0606, United States.
authors.forename: Stephen W
authors.identifier: 0000-0003-3938-8906
authors.initials: SW
authors.lastname: Ragsdale
authors.affiliation: Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory , Menlo Park, California 94025, United States.
authors.forename: Ritimukta
authors.initials: R
authors.lastname: Sarangi
mesh_terms: D002248:Carbon Monoxide / Q000378:metabolism*; D020134:Catalytic Domain; D003066:Coenzyme A Ligases / Q000737:chemistry / Q000378:metabolism*; D008958:Models, Molecular; D009532:Nickel / Q000737:chemistry*; D009942:Organometallic Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D011789:Quantum Theory; D056928:X-Ray Absorption Spectroscopy
chemical_list: D009942:Organometallic Compounds; D009532:Nickel; D002248:Carbon Monoxide; D003066:Coenzyme A Ligases