Nitrile hydratase and related non-heme iron sulfur complexes

Iron-containing nitrile hydratases (NHases) are characterized by a low spin Fe(III) state and a high wavelength absorption at 700 nm. The first X-ray structure reveals a new type of N2S3(O) iron coordination sphere, all the ligands being part of a small peptide, Cys X Y Cys Ser Cys, the two nitrogens arising from the amides of the peptide main chain, and the three thiolates from the cysteines. Several iron complexes with a mixed nitrogen sulfur coordination sphere have been synthesized and characterized by UV, EPR and electrochemistry, the two nitrogens arising from amide residues or bipyridine moiety. Iron(III) complexes, with deprotonated carboxamido nitrogen donors, are pentacoordinated. They are stable over a wide range of potential, have a S = 3/2 spin state and exhibit a visible absorption between 470 and 550 nm. Iron(III) complexes, coordinated to the imine resonance form, of the amide or to a bipyridine moiety, are reduced to the Fe(II) state at a potential quite similar to that of NHase (-480 mV). Moreover the amidate complexes are water soluble and stable over the pH range 3-10 as NHase. One of them yields, upon reaction with NO, a stable iron(III) nitrosyl complex, [Fe(NO)(N2S2)]NEt4, characterized by a NO stretching at 1780 cm(-1) instead of 1850 cm(-1) for the [Fe(III)-NO] state of NHase. (C) 1999 Elsevier Science S.A. All rights reserved.