Nitrogenase proton binding

In general there are few reproducible data on binding of reducible substrates to the isolated MoFe proteins. However, the S = EPR signal from the FeMoco centers of Kpl is pH dependent, the g values changing with a pKa of 8.7 (50). Of course, the proton is a substrate of nitrogenase however, there is no direct evidence for the proton associated with the pKa being bound directly to FeMoco. Nevertheless, this pKa can be perturbed by addition of the analog substrate acety-... 

Many mutant forms of nitrogenase have been investigated. Substitutions of His 195, Lys 191, and Gly 69 of the a chain affect reactions with various substrates.45 54 For example, the mutant obtained by substitution of His 195, whose imidazole forms an N-H--S hydrogen bond to a central bridging sulfide atom of FeMo-co (Fig. 24-3A), with glutamine (H195Q mutant) reduces N2 only very slowly.45 However, it still reduces both acetylene and protons.27/44a 45b Thus, it may be that different modes of substrate binding are needed for the individual steps of Eq. 24-10. 

Whether or not N2 binds at Mo, a possible role for this metal at the active site of the enzyme has been recently pointed out and is now discussed. In the absence of N2, aU conventional nitrogenases reduce protons to dihydrogen. In the presence of N2, only a fraction of electrons are used to reduce dinitrogen while the other fraction is still involved in dihydrogen formation, and is therefore wasted. The limiting stoichiometries for all three types of nitrogenase are as follows (equivalent of dihydrogen formed per equivalent... 

In addition to the details of dinitrogen binding to the FeMo-cofactor, the number and sequence of electrons and protons transferred to the substrate is a critical question. Since all known substrates of nitrogenase are reduced by an even number of electrons, most mechanistic... 

Scheme 1. The catalytic cycle for the reduction of N2 by the Mo nitrogenase. Eq represents the resting state of the MoFe protein of K. pneumoniae and species E -E represent intermediate forms of this protein following sequential one-electron reduction steps. The arrows between each state represent complex formation between the Fe protein and MoFe protein, electron transfer, and protonation, followed by protein dissociation. N2 binds to species 3, accounting for the stoichiometry of Eq. (1) the displacement of N2 from this species accounts for the competitive inhibition of N2 reduction by H2 (see Ref. 50 for a detailed presentation of this scheme).

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