Fig. 7.

Acetyl-coenzyme A (acetyl-S-CoA) synthase/decarbonylase (ACS). (A) The reaction catalyzed by ACS. (B) ACS structure and active sites. ACS (cartoon view, PDB access code 2z8y, Morella thermoacetica) contains a CODH homodimer (yellow and sand) with two [1Ni-4Fe-4S] clusters (side chains shown in stick view, bottom left) and three [4Fe-4S] clusters, including one bridging the subunits, along with two subunits (cyan and gray cartoon) each containing a [Ni-M-4Fe-4S] cluster (side chains shown in stick view, bottom right). The structure shown is for an inactive protein where M is Cu (purple sphere), but the active enzyme possesses Ni at this site. (C) ACS mechanism. In this oversimplified view of the ACS mechanism, the Ni located distal to the [4Fe-4S] cluster retains its divalent state throughout catalysis whereas the proximal Ni cycles between the Ni²⁺ and Ni¹⁺ states. Alternative mechanisms invoke Ni³⁺ or Ni⁰ states for the proximal metal site.

(2 column width)