16 ADP + 16 Pi + 2 NH3 + H2Structure and Function of Nitrogenase
The complete reaction for the fixation of dinitrogen by nitrogenase is accepted as follows:
N2 + 3 e- + 8 H+
16 ATP
16 ADP + 16 Pi + 2 NH3 + H2
Nitrogenase is a tetramer protein consisting of two identical Fe4S4 cluster and FeMo cluster subunits. It is beyond my CHIME abilities to fully display the protein, however this3 page does a superb job, and I will be referring to it from time to time (note: I had difficulty viewing CHIME plug-ins on some Internet Explorer browsers. . .if you have difficulty please try Netscape Navigator or a newer version of Internet Explorer -Adam). The structure is almost completely alpha helix in nature. The function of the Fe4S4 cluster seems to be general 1 electron redox cycles. The other main feature of the protein is a MoFe3S4-Fe4S4 bridged cubanes, as shown below.
FeMo cofactor showing cystine bridged cubes. Adapted from ref [4]
This is where it is accepted that N2 is reduced to NH3. There is disagreement over where exactly N2 binds in the literature1. Suffice to say that the matter is unresolved and under the mechanism section I will elaborate on the latest argument, being for Fe binding/activation. It seems that there are multiple "shuttle" routes to provide protons for reduction, although there appears to be a primary route involving His 1951. Solvent can access the cluster, and chelation experiments show that ADP inhibits demetallation by bypyridine1. It could be inferred from this that binding of ADP somehow closes off the solvent-accessible portion of the protein. What this means in terms of function isn't immediately clear and I could not locate any references that specifically attack this question. Perhaps the conformation change prevents solvent from reacting with the active site when it has been activated by ATP hydrolysis (thus leaving coordinated ADP) and before the energy generated by hydrolysis can be transferred to the substrate. Once the ADP dissociates the protein relaxes back to allow water access to re-protonate proton donors. The calculations in reference [5] emphasis that proton balance is key to the function of this protein. This is, of course, my uneducated reasoning however I present it for your consideration.
Next: An overview of the possible mechanisms of nitrogenase.
intro structure mechanism references