The Mechanism of Dinitrogen Reduction by Molybdenum Nitrogenases

Abstract

The recent descriptions of the crystal structure of the molybdenum-iron protein and the iron protein of Azotobacter vinelandii nitrogenase [ 11 and of Clostridium pasteurianum nitrogenase [2] have been exciting and frustrating, exciting because the structures have been sought for more than 30 years, and frustrating because the structures do not obviously reveal the active site, nor do they really imply a specific active site.This has not prevented speculation and calculation to determine how nitrogenase actually works [3, 41. Simple speculation is easier, but should always attempt to remain consistent with the great deal we do know about the process of biological nitrogen fixation. Calculation is more dangerous. In the past, theoreticians have paid relatively little attention to chemical facts, and relied upon their imagination. The results of the calculations may indeed indicate a preferred pathway among those considered. However, there is no guarantee that the pathways considered include the one that nitrogenase uses. We are limited by our imagination and our knowledge. Consequently, it behoves us, when considering a mechanism, to use all the data we can. This short review is designed to do just that.The structure of the metal clusters at the core of the molybdenum-iron protein in both A. vinelandii [l] and C. pasteurianum [2] are shown in Fig. 1. The smaller eight-iron system, which is believed to act as an electron-transfer agent to the ironmolybdenum cofactor/cluster (FeMoco), is not believed to be involved directly with binding and reducing dinitrogen. There are at least three genetically distinct kinds of nitrogenase [5] based on iron and molybdenum, iron and vanadium, and iron and no other evident metal. These three nitrogenases, with a common iron constituent, have belled the speculation that iron, and not molybdenum or vanadium, is at the active site of nitrogenase [6]. Currently, there is no definitive answer to this problem, but this has not curbed inventive minds.As an example, there have been at least three sets of calculations performed investigating the ways in which dinitrogen might bind to the seven-iron residue of the FeMoco. One 131 was a bit previous, in that the residue initially not identified, but labelled Y, was postulated to be N,. Now, the consensus appears to be that Y is actually sulfur [7]. Another two calculations discuss N, binding to iron clusters [4, 81.

Late Metal Scaffolds that Activate Both, Dinitrogen and Reduced Dinitrogen Species NxHy

“…It should be noted, that the real mechanism might be much more complex, as in situ studies during catalysis did not lead to the successful trapping and isolation of any intermediate. In the proposed mechanism, (which is also suggested to take place at Mo of FeMoco) [6,23,24] the distal nitrogen atom of coordinated N 2 is hydrogenated in three steps until the first equivalent of NH 3 is liberated, and a nitrido species is formed. The remaining nitrido-N atom is hydrogenated in three further steps to yield the second NH 3 molecule.…”

Great Metalloclusters in Enzymology

“…Nevertheless, a variety of mechanistic proposals have been presented, inspired by model chemistry and the cofactor structure (see the collection of commentaries in 57-61). Much attention has been focused on the possible role of molybdenum as the site for substrate binding and reduction (see 62), although since the unusual coordination environment of the irons in the FeMo-cofactor was crystallographically established, the possibility that ligands may bind to one or more iron sites on the cofactor has received increasing attention (63). It is fair to say at this moment, however, that the debate is still more philosophical than scientific, as no compelling experimental studies on nitrogenase or model systems have been presented that conclusively identify the modes of substrate binding, much less the sequence of electron and proton transfer to substrates.…”

New complexes of platinum(0) with cyclopropenes

“…Now that the structures of classical molybdenum nitrogenases are reasonably well established, 1 it is evident that the 'active site' is not an obvious entity. 2 Indeed it is likely to consist of several metal atoms working in conjunction. It is also becoming clear that the reductions of substrates by nitrogenases may be very complex, involving as they do many electrons and protons, and also not necessarily occurring at the site at which dinitrogen is itself reduced.…”