Identification of FeMoco domains within the nitrogenase MoFe protein

Abstract

During nitrogenase turnover, the Fe protein (a homodimer, Mr ≈ 60,000) sequentially delivers single electrons to the MoFe protein (an α₂β₂ tetramer, Mr ≈ 220,000) upon which is located the substrate-reduction site (for a review, see Smith et al., this volume). Because the Fe protein is a single electron donor yet multiple electrons are required for substrate reduction, the MoFe protein must be able to accept and store multiple electrons. Very little is known about where and how these electrons are accumulated within the MoFe protein nor is anything known about their intramolecular delivery to the substrate-reduction site. The MoFe protein is known to contain redox centers of two distinct types; two iron-molybdenum cofactors, called FeMoco, and four proposed [4Fe-4S] centers, called P clusters. There is compelling biochemical and genetic evidence that FeMoco is the redox center which is, contains or is part of the substrate-reduction site. In its semi-reduced state, FeMoco exhibits a characteristic S = 3/2 electron paramagnetic resonance (EPR) signal that is unique to metallobiomolecules. This EPR signal can be recognized in whole cells, purified MoFe protein, and in FeMoco extracted from its polypeptide matrix.