The Role of Iron Protein-Nucleotide Interaction and Conformational Change in Nitrogenase Catalysis

Abstract

There are a number of key questions in nitrogenase biochemistry pertaining to the biosynthesis of the metal clusters, the mechanism of reduction at the active site, and the role of MgATP in catalysis (Peters and Szilagyi, 2006). We have, for several years, been focusing on the last question, using a variety of biochemical and physical methods to probe Fe-protein structure in defined conformations. These include Fe proteins with or without bound nucleotides and variants resulting from site-specific amino-acid substitution. During catalysis, the Fe protein binds two MgATP molecules and couples their hydrolysis to the transfer of electrons from the Fe protein to the MoFe protein within a complex (Peters and Szilagyi, 2006; Howard and Rees, 1994). Multiple steps of complex formation, MgATP hydrolysis, and electron transfer are required to accumulate enough reducing equivalents to reduce N2 to ammonia. MgATP-dependent conformational changes in the Fe protein represent a potential mechanism to effectively gate the flow of electrons toward substrate reduction. The structure of the Fe protein confirmed it as an approx. 60 kD homodimer with a single [4Fe-4S] cluster located at the subunit interface. The Fe protein is structurally similar to a large number of proteins in which nucleotide binding and hydrolysis are coupled to conformational changes typically transduced within a macromolecular complex (Howard and Rees, 1994).