Reduction of cyclopropene as criterion of active-site homology between nitrogenase and its Fe–Mo cofactor

Abstract

THE nitrogenase Fe–Mo cofactor (FeMo-co)¹ is usually characterised by its unique ability to restore N₂-fixing ability to extracts of certain nitrogenase-less mutants^1,2 and by its spectral properties, some of which can be correlated with those of the FeMo protein itself³. The role of FeMo-co in nitrogenase function is not yet clear. Recently, it was reported that uncomplemented FeMo-co catalysed BH⁻₄ reduction of C₂H₂ to C₂H₄ in a CO-inhibited, non-ATP-dependent process₄. This was taken as evidence that FeMo-co is the active site for N₂ reduction and binding in nitrogenase, although conversion of N₂ to NH₃ by cofactor-BH⁻₄ mixtures was not observed⁴. C₂H₂ is readily reduced by many homogeneous and heterogeneous catalysts and can give only a single 2 e⁻ reduction product, urging caution in exclusive reliance upon it as a model substrate^5,6. The observation that nitrogenase catalyses reduction of cyclopropene to both propene and cyclopropane in a ∼2:1 ratio in vitro⁷ and also in vivo⁸ suggests a more demanding criterion⁸ for evaluation of FeMo-co catalysis, one which still avoids the difficult energetic requirements for N₂ activation and reduction. This new criterion is strengthened by the fact that cyclopropene competitively inhibits N₂ (ref. 9). We find that incubation of FeMo-co with a suitable ‘apoprotein’ extract yields a reconstituted FeMo-protein activity for reduction of cyclopropene to propene and cyclopropane. FeMo-co alone, however, only catalyses cyclopropane formation in the conditions described⁴ for C₂H₂ reduction. This result and other considerations suggest that active site homology between FeMo-co and nitrogenase has not yet been fully demonstrated on a function basis.