Progress in Nitrogenase-Related Research

Abstract

This session of the Congress comprises the latest results and thoughts with respect to the structure and function of nitrogenase and its components. Major success stories in the last five years have included the solution of the structures of the Fe protein (Georgiadis et al, 1992) and MoFe protein (Kim, Rees, 1992; Bolin et al, 1993; Peters et al., 1997). However, the three-dimensional structures of the MoFe protein have given rise to a major controversy, namely, the structure of the P cluster. Two related structures, both composed of two (whole or partial) [4Fe-4S] cubes with four terminal Cys-Fe bonds and two cysteinyl bridges, have been proposed. The first model has a disulfide bond as the third bridge between the cubes, whereas the second model, which comprises a [8Fe-7S] cluster, has a hexacoordinate S (i. e., one S atom common to both partial cubes) forming the third bridge. The second model is now preferred and the structural differences have been rationalized by different redox states having different structural P-cluster forms with the first structure solved on crystals in two (or more) states. The structure and composition of FeMo-cofactor have not been items of dispute. It is bound to the alpha-subunit by only alpha-Cys-275 and alpha-His-442, although many putative hydrogen-bonding interactions exist. Many of these interactions were predicted through mutagenesis studies, which predated the structure determination (Brigle et al, 1985; Dean et al., 1990; Scott et al., 1990; Kent et al., 1990).