The role of nickel in acetyl-CoA synthesis by the bifunctional enzyme CO dehydrogenase/acetyl-CoA synthase: Enzymology and model chemistry

CO dehydrogenase/acetyl-CoA synthase (CODH/ACS) is one of the four known nickel enzymes. It is a bifunctional protein that catalyzes the oxidation of CO to CO2 at a nickel iron-sulfur cluster (Cluster C) and a remarkable condensation reaction between a methyl group (donated from a methylated corrinoid iron-sulfur protein), carbon monoxide, and coenzyme A to form acetyl-CoA at a separate nickel iron-sulfur cluster (Cluster A). This review focuses on the current understanding of the structure and function of Cluster A and on related model chemistry. It describes studies that uncovered the first example of a biological organometallic reaction sequence. The mechanism of acetyl-CoA synthesis includes enzyme-bound methylnickel, iron-carbonyl, and acylmetal intermediates. Discovery of the methylnickel species constituted the first example of an alkylnickel species in biology and unveiled a new biological role for nickel.