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

Abstract

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 reduction3–5. 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-co5‐7. 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).