FeMo-cofactor substrate binding

Although FeMo-cofactor is clearly knpHcated in substrate reduction cataly2ed by the Mo-nitrogenase, efforts to reduce substrates using the isolated FeMo-cofactor have been mosdy equivocal. Thus the FeMo-cofactor s polypeptide environment must play a critical role in substrate binding and reduction. Also, the different spectroscopic features of protein-bound vs isolated FeMo-cofactor clearly indicate a role for the polypeptide in electronically fine-tuning the substrate-reduction site. Site-directed amino acid substitution studies have been used to probe the possible effects of FeMo-cofactor s polypeptide environment on substrate reduction (163—169). Catalytic and spectroscopic consequences of such substitutions should provide information concerning the specific functions of individual amino acids located within the FeMo-cofactor environment (95,122,149). 

It is still debated if the overall oxidation state of the FeMo cofactor in its resting state (or, without substrate binding) has to be described as... 

The second example is the measurement of the kinetics of a substrate binding to extracted FeMo-cofactor of nitrogenase by a double-mixing stopped-flow approach. In a typical experiment, solutions of FeMoco-L and CN are rapidly mixed and held together for a known length of time. Subsequently, this solution is mixed with a solution of [NEt4]SPh whereupon the thiolate reacts with the cofactor. 

C. Substrate Binding to the FeMo-Cofactor Future Outlook... 

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... 

However, this model does not explain why, in the comparable experiment performed under HD, no D2 forms, nor why substrates other than N2 do not promote HD formation. Also, if H2 can interact with the active site, why is a substrate of any kind needed to promote HD formation Displacement of H2 is not a necessity for binding N2, but why does HD form only when N2 is being reduced One simple answer proposed by Helleren et al. is that HD formation and N2 binding occur at different places.54 It is possible that different substrates bind to and are transformed at different parts of the large FeMoco (FeMo cofactor) site of N-ases discussed below. CO inhibits nitrogen fixation in N-ases but not H2 evolution. A single site that binds H2 and N2 equivalently should be poisoned by CO for both H2 and N2 activity, and evidence increasingly points to multisite processes in the FeMoco cluster. However, a possible model (10) for HD formation at the same site as N2 activation is discussed below. 

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