This review summarizes the recent progress of the studies on acetyl CoA synthase (ACS), a metalloenzyme that plays an important role of fixing CO and CO 2 in the global carbon cycle. ACS forms a complex with CO dehydrogenase (CODH), in which ACS assembles acetyl CoA from CO, coenzyme A, and a methyl moiety derived from the methyl cobalamin of the corrinoid iron-sulfur protein (CFeSP), whereas CODH catalyzes the reversible conversion of CO 2 to CO. Since the structural elucidation of acetyl-CoA

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... ynthase (ACS), the model synthesis of the ACS active site (A-cluster) has come up for a challenging topic due to its important role in nature. In this review, the biochemical investigation on the ACS structures and functions are overviewed at first, and then each of the model studies is described in details. It also includes the ACS model studies that had been investigated before the report of the crystal structures as well as those after the structural elucidation.