Nitrogenase reactivity

Fe-hydrogenase, which usually functions in the direction of hydrogen evolution, has been known for over 30 years. This enzyme contains a highly reactive complex Fe-S center in which one of the Fe atoms is complexed with CO and CN [5], The Fe hydrogenases have extremely high turnover numbers 6,000 s 1 for C. pasteuriamm and 9,000 s 1 for Desulfovibrio spp. Note that this is a thousand times faster than the turnover number of nitrogenase ... 

The trapping of reactive intermediates at low temperatures in a rigid medium prevents them from reacting, and allows a leisurely examination. This, combined with esr examination, has been important in studies of certain metalloproteins, e.g. nitrogenase. 

Of the alternative substrates of nitrogenase, many contain triple bonds In at least one of their resonance forms. As discussed In the next section, the reactivity of model molecules such as acetylene could give insights into the manner in which such unsaturated molecules bind to transition metal sulfur systems. 

Transition metal sulfide units occur in minerals in nature and play an important role in the catalytic activity of enzymes such as hydrogenase and nitrogenase. Industrial processes use transition metal sulfides in hydroprocessing catalysis. Both the metal and the sulfur sites in these compounds can undergo redox reactions which are an important part of their reactivity. Thus, the electronic situation of the ReS4 anion and related complexes is of considerable interest and has been evaluated applying quantum chemical methods. 

Cluster models of the polymetallic aggregates in nitrogenase should (1) contain structural features akin to those defined by the crystal structure (2) possess or approximate the observed stoichiometry (3) provide coordination sites at which substrate binding and activation may occur. A model need not mimic protein-bound cofactor reactivity, since isolated cofactors do not reduce sub-... 

Recently, Brill and co-workers (43, 44) have isolated mutant strains of Azotobacter vinelandii which produce an inactive nitrogenase component. This component can be reactivated by treatment with the neutralized acid-hydrolysis products of other nitrogenases (which themselves become inactive on such a treatment) but not apparently with any other molybdenum enzymes. This may either reflect a difference between the cofactor in nitrogenase and other molybdenum enzymes or may be caused by the reconstitution conditions used which may not have been sufficiently varied to allow for different molybdenum oxidation states to be attained. In any event, the chemical characterization and authentication of the molybdenum cofactor should reveal some of the intimate details of the molybdenum site. 

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