NATURES CARBONYLATION CATALYST - RAMAN-SPECTROSCOPIC EVIDENCE THAT CARBON-MONOXIDE BINDS TO IRON, NOT NICKEL, IN CO DEHYDROGENASE

Carbon monoxide dehydrogenase catalyzes the synthesis of acetyl-coenzy me A from coenzyme A, a methyl group, and carbon monoxide. The carbon monoxide binds to a mixed metal center of the enzyme, which contains n ickel bridged to an iron-sulfur cluster. Resonance Raman spectroscopy has been used to identify both C-O stretching and metal-CO stretching vibrations of the carbon monoxide adduct of the enzyme. This adduct wa s shown by isotopic exchange to be on the pathway for acetyl-coenzyme A synthesis. The metal to which carbon monoxide is bound was establish ed to be iron, not nickel, by preparation of enzyme from bacteria grow n on iron-54 and nickel-64. The Fe-CO frequency is low, 360 wave numbe rs, implying a weak bond, probably because of electron donation from s ulfide and thiolate ligands of the iron. A bimetallic mechanism is pro posed, in which carbon monoxide binds to an iron atom and is subsequen tly attacked by a methyl group on a nearby nickel atom, forming an ace tyl ligand, which is then transferred to coenzyme A.