This article reviews an enzyme with two important catalytic activities, carbon monoxide dehydrogenase (CODH) (reaction 1) and acetyl-CoA synthase (ACS) (reaction 2). These reactions are key to an autotrophic pathway that has become known as the reductive acetyl-CoA or the Wood/Ljungdahl pathway. ACS also catalyzes two exchange reactions that have been valuable in elucidating the mechanism of acetyl-CoA synthesis: an exchange reaction between CO and the carbonyl group of acetyl-CoA (reaction 3) and an exchange reaction between free CoA and the CoA moiety of acetyl-CoA (reaction 4). Ten years ago, one of the authors (Ragsdale) and Harland Wood, first proposed that the CODH from acetogenic bacteria catalyzes the final steps in acetyl-CoA synthesis and, therefore, should be renamed acetyl-CoA synthase. It was previously accepted that these steps occurred on a cobalt-containing corrinoid protein. This was a controversial proposal that required the existence of carbonyl, methyl, and acetyl enzyme adducts. Proof of this postulate was nontrivial since it required the characterization of enzyme-bound intermediates. With the goal of detecting and characterizing intermediates in the pathway, the authors` laboratory and others began to probe the enzyme in the resting state and at different stages of the catalytic cycle with sensitive spectroscopic methods.more » This review summarizes the fruits of the combined labor of the laboratories working on this interesting problem. 237 refs.« less