NITROGENASE STRUCTURE AND FUNCTION - A BIOCHEMICAL-GENETIC PERSPECTIVE

Biological nitrogen fixation is catalyzed by nitrogenase, an enzyme co mposed of two component proteins called the Fe protein and the MoFe pr otein. During catalysis, electrons are delivered one at a time from th e Fe protein to the MoFe protein in a process involving component-prot ein association and dissociation and hydrolysis of at least two MgATP for each electron transfer. The Fe protein contains the sites for MgAT P binding and hydrolysis, whereas the site for substrate binding and r eduction is located on the MoFe protein. Among the important aspects o f nitrogenase enzymology discussed here are (a) the structures of the metal centers that participate in electron transfer, (b) the organizat ion of the metalloclusters within the polypeptides and their contribut ions to substrate binding and electron transfer, (c) the nature of the dynamic interactions between the two component proteins that lead to nucleotide hydrolysis and intermolecular electron transfer, (d) the me chanism by which the multiple electrons necessary for substrate reduct ion are distributed within the MoFe protein, (e) the nature of the int ramolecular electron path within the MoFe protein, and (f) where and h ow substrate and various inhibitors become bound to the substrate-redu ction site. This chapter summarizes biochemical-genetic strategies use d to address these questions and discusses them in the context of the recently proposed three-dimensional models for both the Fe protein and MoFe protein from Azotobacter vinelandii.