Infrared and EPR spectroscopic characterization of a Ni(I) species formed by photolysis of a catalytically competent Ni(I)-CO intermediate in the acetyl-CoA synthase reaction.
Acetyl-CoA synthase (ACS) catalyzes the synthesis of acetyl-CoA from CO, coenzyme A (CoA), and a methyl group from the CH(3)-Co(3+) site in the corrinoid iron-sulfur protein (CFeSP). These are the key steps in the Wood-Ljungdahl pathway of anaerobic CO and CO(2) fixation. The active site of ACS is the A-cluster, which is an unusual nickel-iron-sulfur cluster. There is significant evidence for the catalytic intermediacy of a CO-bound paramagnetic Ni species, with an electronic configuration of [Fe(4)S(4)](2+)-(Ni(p)(+)-CO)-(Ni(d)(2+)), where Ni(p) and Ni(d) represent the Ni centers in the A-cluster that are proximal and distal to the [Fe(4)S(4)](2+) cluster, respectively. This well-characterized Ni(p)(+)-CO intermediate is often called the NiFeC species. Photolysis of the Ni(p)(+)-CO state generates a novel Ni(p)(+) species (A(red)*) with a rhombic electron paramagnetic resonance spectrum (g values of 2.56, 2.10, and 2.01) and an extremely low (1 kJ/mol) barrier for recombination with CO. We suggest that the photolytically generated A(red)* species is (or is similar to) the Ni(p)(+) species that binds CO (to form the Ni(p)(+)-CO species) and the methyl group (to form Ni(p)-CH(3)) in the ACS catalytic mechanism. The results provide support for a binding site (an "alcove") for CO near Ni(p), indicated by X-ray crystallographic studies of the Xe-incubated enzyme. We propose that, during catalysis, a resting Ni(p)(2+) state predominates over the active Ni(p)(+) species (A(red)*) that is trapped by the coupling of a one-electron transfer step to the binding of CO, which pulls the equilibrium toward Ni(p)(+)-CO formation.
Bender
Güneş
G
Department of Biological Chemistry, University of Michigan, Ann Arbor, Michigan 48109-0606, USA.
Stich
Troy A
TA
Yan
Lifen
L
Britt
R David
RD
Cramer
Stephen P
SP
Ragsdale
Stephen W
SW
D000106:Acetate-CoA Ligase / Q000737:chemistry* / Q000378:metabolism; D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism; D001665:Binding Sites; D002248:Carbon Monoxide / Q000737:chemistry* / Q000378:metabolism; D002384:Catalysis; D004578:Electron Spin Resonance Spectroscopy; D007506:Iron-Sulfur Proteins / Q000737:chemistry / Q000378:metabolism; D007700:Kinetics; D009532:Nickel / Q000737:chemistry*; D010084:Oxidation-Reduction; D010782:Photolysis; D046968:Thermoanaerobacter / Q000201:enzymology
D001426:Bacterial Proteins; C080269:C-Fe-SP protein, Moorella thermoacetica; D007506:Iron-Sulfur Proteins; D009532:Nickel; D002248:Carbon Monoxide; D000106:Acetate-CoA Ligase