Representations of the structures of the catalysts discussed in this article. Representations of the structures of the catalysts discussed in this article. Bound substrates or intermediates are in red and parts of the supramolecular structure that are implicated in the mechanism in gray. (1A) Active site of a [NiFe]-hydrogenase shown with a bridging hydride (8). (1B) Active site of a [FeFe]-hydrogenase, showing the likely position of bound H2 (8). (2) Active site of a CODH (structure from a crystallographically defined intermediate) (14). (3) Active site of a tungsten-containing FDH (15). (4) Active site of complex I with NADH stacked above the central ring of the flavin (20). (5) Active site of laccase shown with a bound O–O species assigned as peroxide (38). (6) Generic structure of the bisdiphosphine nickel complexes that are functional hydrogenase analogues (46). Fraser A. Armstrong, and Judy Hirst PNAS 2011;108:34:14049-14054 ©2011 by National Academy of Sciences