Abstract
EPR studies of the [NiFe] hydrogenases are reviewed. These enzymes contain a heterobimetallic [NiFe] center as the active site. The nickel is ligated to four cysteine residues, two of which form a bridge to the iron. The iron carries additionally 3 small inorganic diatomic ligands (2CN−, CO). A third small ligand X is situated in the bridge between Ni and Fe. In the catalytic cycle the enzyme passes through a number of redox states, several of which are paramagnetic. The iron remains in the divalent low-spin (FeII, S = 0) state, whereas the nickel changes its valence and spin state during this cycle. Nickel is believed to bind the hydrogen and to be directly involved in the catalytic process.
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