Identification of the V factor needed for synthesis of the iron-molybdenum cofactor of nitrogenase as homocitrate

Nitrogenase catalyses the ATP-dependent reduction of N₂ to NH₃, and is composed of two proteins, dinitrogenase (MoFe protein or component I) and dinitrogenase reductase (Fe protein or component II) ^1,2. Dinitrogenase contains a unique prosthetic group (iron-molybdenum cofactor, FeMoco) comprised of Fe, Mo and S, which has been proposed as the site of N₂ reduction^3-5. Biochemical and genetic studies of Nif^- (nitrogen fixation) mutants of Klebsiella pneumoniae which are defective in nitrogen fixation, have shown that the nifB, nifQ, nifN, nifE and nifV genes are required for the biosynthesis of FeMo-co^5-7. Recently, a system for in vitro synthesis of FeMoco was described⁸. The assay requires at least the nifB, nifN and nifE gene products⁸, and a low-molecular-weight factor (V factor) produced in the presence of the nifV gene product⁹. We have used this system to study FeMoco biosynthesis. We report here the isolation of V factor and identify it as homocitric acid ([R]2-hydroxy-l,2,4-butanetricarboxylic acid).