Ligand structures CODH

The crystal structure of a CODH/ACS enzyme was reported only in 2002.43,44 It reveals a trio of Fe, Ni, and Cu at the active site (6). The Cu is linked to the Ni atom through two cysteine-S, the Ni being square planar with two terminal amide ligands. Planarity and amide coordination bear some resemblance to the Ni porphinoid in MCR. A two-metal ion mechanism is likely for acetyl CoA synthesis, in which a Ni-bound methyl group attacks an adjacent Cu—CO fragment with formation of a Cu-acyl intermediate. A methylnickel species in CODH/ACS has been identified by resonance Raman spectroscopy.45... 

The history of CODH crystallography is one illustration of how the limitations of protein X-ray crystallography can lead to erroneous interpretation. The initial structural report of CODH (19) described the active site of CODH as having several important differences from the other members of its family. It did not possess the apical sulfido ligand but instead a second oxo ligand. Second, a... 

There has been some controversy over the active site structure of the Mo-CODH. Dobbek et al. [74] determined the crystal structure to 2.2 A, which revealed an active site where molybdenum is coordinated by two 0x0 ligands, a hydroxo group, and one pyranopterin cofactor (Fig. 9.2a) [74]. 

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