EPR Investigation of [NiFe] Hydrogenases

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 addnl. 3 small inorg. diat. 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 no. 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. The available EPR data are interpreted in terms of a simple model, based on ligand field theory. The model indicates that the paramagnetic Ni-A, Ni-B, and Ni-C states are best described as formal NiIII low-spin species with a spin of S = 1/2 and a dz2 ground state. The dz2 orbital is oriented along the mol. z axis (gz axis, gz ≈ ge) and points to the open coordination site of the Ni. The "EPR-silent" states are all NiII species. XAS spectroscopy provides evidence that these states are high-spin (S = 1) states; however, supporting EPR spectra have not yet been reported. The light-induced Ni-L states are characterized by a nickel dz2 ground state with an admixt. of the dx2-y2 orbital. The identity of the third bridging ligand X between nickel and iron changes upon going from Ni-A to Ni-B to Ni-C and to Ni-L. ENDOR and HYSCORE data indicate that a μ-OH- bridge is present in Ni-B, for Ni-C a formal μ-H- has been identified, while for Ni-L the bridge is empty. The bridging ligand of the Ni-A state is still under debate. The identification of the electronic and geometric structure of the reaction intermediates employing spectroscopy and quantum chem. calcns. form the basis for setting up a reaction mechanism for the [NiFe] hydrogenase. [on SciFinder(R)]