Cu I and H 2 O 2 Inactivate and Fe II Inhibits [Fe]‐Hydrogenase at Very Low Concentrations

[Fe]‐Hydrogenase (Hmd) catalyzes reversible hydride transfer from H 2 . It harbors an iron‐guanylylpyridinol as a cofactor with an Fe II that is ligated to one thiolate, two COs, one acyl‐C, one pyridinol‐N, and solvent. Here, we report that Cu I and H 2 O 2 inactivate Hmd (half‐maximal rates at 1 μ M Cu I and 20 μ M H 2 O 2 ) and that Fe II inhibits the enzyme with very high affinity ( K i =40 n M ). Infrared and EPR studies together with competitive inhibition studies with isocyanide indicated that Cu I exerts its inhibitory effect most probably by binding to the active site iron‐thiolate ligand. Using the same methods, it was found that H 2 O 2 binds to the active‐site iron at the solvent‐binding site and oxidizes Fe II to Fe III . Also it was shown that Fe II reversibly binds away from the active site iron, with binding being competitive to the organic hydride acceptor; this inhibition is specific for Fe II and is reminiscent of that for the [FeFe]‐hydrogenase second iron, which specifically interacts with H 2 .