Proton Transfer Pathways in Nitrogenase with and without Dissociated S2B

Nitrogenase is the only enzyme that can convert N2 to NH3. Crystallographic structures have indicated one of sulfide ligands active-site FeMo cluster, S2B, be replaced by an inhibitor, like CO and OH−, it has been suggested may displaced also during normal reaction. We investigated possible proton transfer pathways within cluster conversion N2H2 two molecules NH3, assuming protons enter at S3B, S4B or S5A ions are then transferred substrate. use combined quantum mechanical molecular (QM/MM) calculations with TPSS B3LYP functionals. The indicate barriers for these reactions reasonable if S2B ligand remains bound but they become prohibitively high dissociated. This suggests unlikely reversibly dissociates reaction cycle.