Understanding the tethered unhooking and rehooking of S2B in the reaction domain of FeMo-co, the active site of nitrogenase.

The active site of the nitrogen fixing enzyme nitrogenase is an FeMoSC cluster, and investigations of the enigmatic chemical mechanism of the enzyme have focussed on a pair of Fe atoms, Fe2 and Fe6, and the S2B atom that bridges them. There are three proposals for the status of the Fe2-S2B-Fe6 bridge during the catalytic cycle: one that it remains intact, another that it is completely labile and absent during catalysis, and a third that S2B is hemilabile, unhooking one of its bonds to Fe2 or Fe6. This report examines the tethered unhooking of S2B and factors that affect it, using DFT calculations of 50 geometric/electronic possibilities with a 485 atom model including all relevant parts of surrounding protein. The outcomes are: (a) unhooking the S2B-Fe2 bond is feasible and favourable, but alternative unhooking of the S2B-Fe6 bond is unlikely for steric reasons, (b) energy differences between hooked and unhooked isomers are generally <10 kcal mol, usually with unhooked more stable, (c) ligation at the -Fe6 position inhibits unhooking, (d) unhooking of hydrogenated S2B is more favourable than that of bare S2B, (e) hydrogen bonding from the NεH function of His195 to S2B occurs in hooked and unhooked forms, and possibly stabilises unhooking, (f) unhooking is reversible with kinetic barriers ranging 10-13 kcal mol. The conclusion is that energetically accessible reversible unhooking of S2B or S2BH, as an intrinsic property of FeMo-co, needs to be considered in the formulation of mechanisms for the reactions of nitrogenase.