Nucleotide-iron-sulfur cluster signal transduction in the nitrogenase iron-protein: the role of Asp125.

Electron transfer in nitrogenase involves a gating process initiated by MgATP (magnesium adenosine triphosphate) binding to Fe-protein. The redox site, an 4Fe:4S cluster, is structurally separated from the MgATP binding site. For MgATP hydrolysis to be coupled to electron transfer, a signal transduction mechanism is proposed that is similar to that in guanosine triphosphatase proteins. Based on the three-dimensional structure of Fe-protein, Asp125 is likely to be part of a putative transduction path. Altered Fe-protein with Glu replacing Asp has been prepared and retains the ability for the initial nucleotide-dependent conformational change. However, either MgADP or MgATP can induce the shift and Mg binding to the nucleotide is no longer essential.