Showing papers by "Edward I. Stiefel published in 1980"

Chemical Properties of the Fe-Mo Cofactor from Nitrogenase

Abstract: The existence of a molybdenum cofactor common to molybdenumcontaining enzymes was first postulated by Pateman et al.1 who discovered a common genetic determinant for nitrate reductase and xanthine dehydrogenase in Aspergillus nidulans. The cofactor con cept was further developed by Nason and co-workers who studied a mutant of Neurospora crassa (nit-1), which was blocked in the terminal portion of its nitrate reductase electron transport chain (where molybdenum is located), and which also lacked xanthine dehydrogenase activity.2,3 A key discovery in the field was that crude extracts prepared from the nit-1 mutant could be restored to activity by treatment with an acid hydrolysis product obtainable from any molybdenum-containing enzyme,4,5 including the molybdenumiron protein of nitrogenase.6 Additional studies showed that although molybdate could enhance the activity produced by the nit-1 extract plus the acid-treated products of the molybdoenzymes, neither molybdate nor any of the simple molybdenum complexes tested could activate the nit-1 extracts alone. Furthermore, when 99Mo04 2− was used, it became incorporated into the newly activated nitrate reductase, but only in the presence of the acid-treated products of the molybdoenzymes. These data led to the hypothesis of a labile molybdenum complex being one of the products of all acid-treated molybdoenzymes.7

HD Formation by Nitrogenase: A Probe for N2 Reduction Intermediates

Abstract: Nitrogenase is a complex of two separately purifiable proteins, the molybdenum-iron protein [MoFe] and the iron protein [Fe].1,2 In addition to catalyzing the reduction of N2 to ammonia, nitro genase has ATP-hydrolyzing activity,3 ATP-dependent H2-evolution activity,4 and supports a reaction between D2 and protons (from H2O) to form HD.5 In the absence of other substrates, all the reductant consumed is used to reduce protons to H2. When N2 is added as a substrate, an apparent maximum of 75% of the electrons reduce N2 while the remainder still reduce protons.6

Mo(VI) Complexes of N,S-Donor Ligands: Relevance to Molybdenum Enzymes

Abstract: The molybdenum enzymes other than nitrogenase have a common molybdenum cofactor1,2 and spectroscopic studies of their molybdenum sites reveal these to be similar although not identical.3 EPR* studies on xanthine oxidase in the Mo(V) oxidation state,4 together with results from model compounds,5 led to the suggestion of sulfur as a donor atom to molybdenum. However, in these original studies and in subsequent investigations, 3,6 the inorganic compounds used in the comparison were not structurally and, at times, not even stoichiometrically defined. In order to provide a comprehensive set of structurally defined oxo-molybdenum complexes containing sulfur-donor ligands for comparison with the enzymes by EPR*, EXAFS* and other spectroscopic probes, we have embarked on an exten sive synthetic and isolation program to obtain relevant compounds in the Mo(IV), Mo(V) and MoiVI) oxidation states. Recently, EXAFS studies on xanthine oxidase and sulfite oxidase8 have confirmed the presence of sulfur and terminal oxo ligands in the coordination sphere of molybdenum and have revealed distinct similarities to some of the model compounds discussed here.9

The Structures and Spectra of Molybdoezyme Active Sites and Their Models

Abstract: Publisher Summary This chapter reviews those well understood structural and spectroscopic parameters that are at present or may in the future be relevant to the process of structural and mechanistic elucidation of the molybdenum sites in enzymes. The molybdenum sites at present are found to be of two fundamentally different types. By far, the greatest amount of structural information about enzymic Mo sites has been provided by the techniques of electron paramagnetic resonance spectroscopy and X-ray absorption spectroscopy, including especially the analysis of extended X-ray absorption fine structure. X-ray absorption spectroscopy is likely to assume a central position in future structural study of Mo enzymes and is applicable to biometallic molecules. Although only one bond or grouping in any given complex may be similar to that of the enzyme, a dissection of the Mo enzyme site as guided by the properties of these compounds can bring insight into the nature of that site and the teleonomy of its choice by the enzyme.