Mechanism of Molybdenum Nitrogenase.

Biological nitrogen fixation is mediated by the nitrogenase enzyme system that catalyses the ATP dependent reduction of atmospheric dinitrogen to ammonia. Nitrogenase consists of two component metalloproteins, the MoFe-protein with the FeMo-cofactor that provides the active site for substrate reduction, and the Fe-protein that couples ATP hydrolysis to electron transfer. An overview of the nitrogenase system is presented that emphasizes the structural organization of the proteins and associated metalloclusters that have the remarkable ability to catalyse nitrogen fixation under ambient conditions. Although the mechanism of ammonia formation by nitrogenase remains enigmatic, mechanistic inferences motivated by recent developments in the areas of nitrogenase biochemistry, spectroscopy, model chemistry and computational studies are discussed within this structural framework.

Structural Basis of Biological Nitrogen Fixation.

Orbital interactions, electron delocalization and spin coupling in iron-sulfur clusters

The crystal structure of the tungsten-containing aldehyde ferredoxin oxidoreductase (AOR) from Pyrococcus furiosus, a hyperthermophilic archaeon (formerly archaebacterium) that grows optimally at 100 degrees C, has been determined at 2.3 angstrom resolution by means of multiple isomorphous replacement and multiple crystal form averaging. AOR consists of two identical subunits, each containing an Fe4S4 cluster and a molybdopterin-based tungsten cofactor that is analogous to the molybdenum cofactor found in a large class of oxotransferases. Whereas the general features of the tungsten coordination in this cofactor were consistent with a previously proposed structure, each AOR subunit unexpectedly contained two molybdopterin molecules that coordinate a tungsten by a total of four sulfur ligands, and the pterin system was modified by an intramolecular cyclization that generated a three-ringed structure. In comparison to other proteins, the hyperthermophilic enzyme AOR has a relatively small solvent-exposed surface area, and a relatively large number of both ion pairs and buried atoms. These properties may contribute to the extreme thermostability of this enzyme.

http://science.sciencemag.org/content/sci/267/5203/1463.full.pdf

Structure of a hyperthermophilic tungstopterin enzyme, aldehyde ferredoxin oxidoreductase

The coupling of ATP hydrolysis to electron transfer by the enzyme nitrogenase during biological nitrogen fixation is an important example of a nucleotide-dependent transduction mechanism. The crystal structure has been determined for the complex between the Fe-protein and MoFe-protein components of nitrogenase stabilized by ADP·AIF_4 –, previously used as a nucleoside triphosphate analogue in nucleotide-switch proteins. The structure reveals that the dimeric Fe-protein has undergone substantial conformational changes. The β-phosphate and AIF_4 – groups are stabilized through intersubunit contacts that are critical for catalysis and the redox centre is repositioned to facilitate electron transfer. Interactions in the nitrogenase complex have broad implications for signal and energy transduction mechanisms in multiprotein complexes.

Structure of ADP x AIF4(-)-stabilized nitrogenase complex and its implications for signal transduction.

Molybdenum Enzymes, Cofactors, and Model Systems.

Molybdenum enzymes, cofactors and chemistry - an introductory survey, Edward I. Stiefel. Part 1 Molybdenum cofactor enzymes: the reaction mechanism of xanthine oxidase, Russ Hille biochemistry of the molybdenum cofactors, K.V. Rajagopalan the bacterial Oxmolybdenum cofactor, O. Meyer et al. Part 2 Molybdenum cofactor models: models of pterin-containing molybdenum enzymes, Charles G. Young and Anthony G. Wedd pterins, quinoxalines and metallo-ene-dithiolates - synthetic approach to the molybdenum cofactor, Robert S. Pilato et al strategies for the synthesis of the cofactor of the oxomolybdoenzymes, C.D. Garner et al molybdenum complexes of reduced pterins, Sharon J. Nieter Burgmayer et al chemical and physical coupling of oxomolybdenum centres and iron porphyrins - models for the molybdenum-iron interaction in sulfite oxidase, Michael J. LaBarre et al. Part 3 Nitrogenase: nitrogenase structure, function and genetics, Barbara K. Burgess crystal structures of the iron protein and molybdenum-iron protein of nitrogenase, D.C. Rees et al structure and environment of metal clusters in the nitrogenase molybdenum-iron protein from clostridium pasteurianum, Jeffrey T. Bolin et al biosynthesis of the iron-molybdenum cofactor of nitrogenase, Paul W. Ludden et al role of the iron-molybdenum cofactor polypeptide environment in azotobacter vinelandii molybdenum-nitrogenase catalysis extended x-ray absorption fine structure and L-Edge spectroscopy of nitrogenase molybdenum-iron protein J. Chen et al redox properties of the nitrogenase proteins from azotobader vinelandii, G.D. Watt et al the molybdenum-iron protein of nitrogenase - structural and functional features of metal cluster prosthetic groups, W.H. Orme-Johnson protein component complex formation and adenosine triphosphate hydrolysis in nitrogenase, James Bryant Howard electron-transfer reactions associated with nitrogenase from klebsiellia pneumoniae R.N.F. Thorneley et al Part 4 Nitrogenase models: recent structure determinations of the molybdenum-iron protein of nitrogenase - impact on design of synthetic analogs for the iron-molybednum-sulfur analogs for the iron-molybdenum-sulfur active site and the iron-molybdenum cofactor, Dimitri Coucouvanis. Part of contents.

Molybdenum enzymes, cofactors, and model systems : developed from a symposium sponsored by the Division of Inorganic Chemistry at the 204th National Meeting of the American Chemical Society, Washington, DC, August 23-28, 1992

Dimitri Coucouvanis

Papers

Molybdenum Enzymes, Cofactors, and Model Systems.

Molybdenum enzymes, cofactors, and model systems : developed from a symposium sponsored by the Division of Inorganic Chemistry at the 204th National Meeting of the American Chemical Society, Washington, DC, August 23-28, 1992