Crystallographic Analysis of the MoFe Protein of Nitrogenase from a nifV Mutant of Klebsiella pneumoniaeIdentifies Citrate as a Ligand to the Molybdenum of Iron Molybdenum Cofactor (FeMoco)

Abstract

The x-ray crystal structure of NifV(-) Klebsiella pneumoniae nitrogenase MoFe protein (NifV(-) Kp1) has been determined and refined to a resolution of 1.9 A. This is the first structure for a nitrogenase MoFe protein with an altered cofactor. Moreover, it is the first direct evidence that the organic acid citrate is not just present, but replaces homocitrate as a ligand to the molybdenum atom of the iron molybdenum cofactor (FeMoco). Subsequent refinement of the structure revealed that the citrate was present at reduced occupancy.