Controlled protonation of iron-molybdenum cofactor by nitrogenase: a structural and theoretical analysis

Qualitative molecular modelling has been used to identify possible routes for transfer of protons from the surface of the nitrogenase protein to the iron-molybdenum cofactor (FeMoco) and to substrates during catalysis. Three proton-transfer routes have been identified; a water-filled channel running from the protein exterior to the homocitrate ligand of FeMoco, and two hydrogen-bonded chains to specific FeMoco sulphur atoms. It is suggested that the water channel is used for multiple proton deliveries to the substrate, as well as in diffusion of products and substrates between FeMoco and the bulk solvent, whereas the two hydrogen-bonded chains each allow a single proton to be added to, and subsequently depart from, FeMoco during the catalytic cycle. Possible functional differences in the proton-transfer channels are discussed in terms of assessment of the protein environment and specific hydrogen-bonding effects. The implications of these observations are discussed in terms of the suppression of wasteful production of dihydrogen by nitrogenase and the Lowe-Thorneley scheme for dinitrogen reduction.