Nitrogenase mechanistic studies

Studt F, Tuczek F. Theoretical, spectroscopic, and mechanistic studies on transition-metal dinitrogen complexes implications to reactivity and relevance to the nitrogenase problem.J Comput Chem. 2006 27 1278-1291. 

The dependence of rate constants for approach to equilibrium for reaction of the mixed oxide-sulfide complex [Mo3((i3-S)((i-0)3(H20)9] 1+ with thiocyanate has been analyzed into formation and aquation contributions. These reactions involve positions trans to p-oxo groups, mechanisms are dissociative (391). Kinetic and thermodynamic studies on reaction of [Mo3MS4(H20)io]4+ (M = Ni, Pd) with CO have yielded rate constants for reaction with CO. These were put into context with substitution by halide and thiocyanate for the nickel-containing cluster (392). A review of the chemistry of [Mo3S4(H20)9]4+ and related clusters contains some information on substitution in mixed metal derivatives [Mo3MS4(H20)re]4+ (M = Cr, Fe, Ni, Cu, Pd) (393). There are a few asides of mechanistic relevance in a review of synthetic Mo-Fe-S clusters and their relevance to nitrogenase (394). 

The studies discussed so far deal - for good reasons - with the isolated FeMoco. However, qualitative molecular modeling has also been used to identify possible proton transfer routes from the surface of the nitrogenase protein to the FeMoco (40) and mechanistic aspects of biological nitrogen fixation are discussed in the light of such data (10-12). 

Mechanistic speculations about the molybdoenzymes must be considered to be in their infancy with the possible exception of those for xanthine oxidase. Although the detailed structural nature of the molybdenum site is unknown, there is sufficient information from biochemical and coordination chemistry studies to allow informed arguments to be drawn. Here we first discuss evidence for the nuclearity of the molybdenum site and then discuss both oxo-transfer and proton-electron transfer mechanisms for molybdenum enzymes. A final discussion considers the unique aspects of nitrogenase and the possible reasons for the use of molybdenum in enzymes. 

A combined approach using genetic, biochemical and biophysical methods has led to a better mechanistic understanding of the nitrogenase pathway over the last decade." In particular, site-directed mutagenesis experiments with individual acid substitutions have been applied with great success. These studies revealed that the residue at position a-70 within the... 

There are some molybdenum-containing enzymes, e.g. nitrogenase, with clearly defined and obviously vital biological roles and other such enzymes with much more nebulous roles. From a mechanistic point of view all molybdenum enzymes seem closely related to one another (Stiefel, 1973). Even so, it is perhaps a little unfortunate that the enzyme xanthine oxidase, which has proved to be the most amenable to study by EPR of molybdenum and is thus the enzyme whose catalytic mechanism is the best understood, is also the enzyme whose biological role is the least clear. 

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