aLeft: Enzymatic mechanism of acetyl-CoA synthesis by the Ni-metalloenzyme ACS. In the CODH/ACS complex, CO2 is reduced by CODH to generate CO, which is channeled to the ACS active site, where it combines with a methyl group and CoA to generate acetyl-CoA. Right: Mechanism of acetic acid synthesis by the Monsanto process, showing only the key steps and fragments at the metal center. It consists of “organo” and a “metallo” cycles, the later having steps similar to those for ACS.