On the function of nitrogenase FeMo cofactors and competitive catalysts: chemical principles, structural blue-prints, and the relevance of iron sulfur complexes for N-2 fixation

This article tries to rationalize why low molecular weight complexes have n ot yet been able to copy nitrogenase catalyzed reactions or to act as compe titive catalysts with nitrogenase-like activity. An answer is sought in tha t such complexes must rather fulfil the principles governing FeMoco functio n than duplicate its structure. Such principles, e.g. metal sulfur bonds, r eversible M-S bond dissociation, Bronsted basicity, vacant sites, redox act ivity, are illustrated with metal complexes of multidentate thioether thiol ate ligands, A structure-function relationship of metal sulfur [MS] centers is described, revealing that [MS] centers can stay structurally invariable in spite of considerable electronic changes. Complexes with [MS] centers f urther accomplish strong coupling of H+/e(-) fluxes, the heterolytic activa tion of H-2, and the stabilization of the N-2 reduction key intermediate di azene N2H2. Low-spin states of Fe(II) centers can be enforced by sterical c onstraints. These coordination chemistry results, combined with X-ray struc tural and biochemistry findings, form the basis of a model for the FeMoco f unction. It proposes the breakage of one Fe-S-Fe bridge of FeMoco and the f ormation of two unique five-coordinate low-spin Fe(II) centers when the enz yme passes from the resting into the turnover state. (C) 1999 Elsevier Scie nce S.A. All rights reserved.