Metal-cysteinyl thiolate clusters

X-ray crystal structures of CODH/ACS proteins show that they are large (300 kDa) homodimeric proteins with the two CODH domains j3 subunits) at the core, and the two ACS domains (a subunits) tethered to the side of each of these (3 subunits. The (3 subunits each contain one Ni and 10 Fe ions that are arranged into three FeS clusters (the so-called B-, C-, and D-clusters), whereas the a subunits each contains the active site A-cluster, a [Fe4S4] " cubane that is bridged by the sulfur of a cysteinyl residue to the proximal metal (Mp) of a binuclear center. This binuclear center contains a square planar nickel ion, referred to as the distal Ni (Ni(j), which is coordinated by the two thiolates and two backbone amides of a Cys-Gly-Cys motif There has been some debate as to the identity of Mp, as Ni, Cu, and Zn ions have been shown to occupy this site however, it is now generally accepted that the A-cluster is a binuclear Ni—Ni center bridged by a cysteine thiol to a [4Fe— 4S] cluster. " ... 

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