How feasible is the reversible S-dissociation mechanism for the activation of FeMo-co, the catalytic site of nitrogenase?

The active site of the enzyme nitrogenase (N→ NH) is a FeMoSC cluster that contains three doubly-bridging μ-S atoms around a central belt. A vanadium nitrogenase variant has a slightly different cluster, containing two μ-S atoms. Recent crystal structures have revealed substitution of one μ-S (S2B, bridging Fe2 and Fe6), by CO in Mo-nitrogenase and an uncertain light atom in V-nitrogenase. These systems retained catalytic activity, and were able to recover the lost μ-S atom. Electron density attributed to the dissociated S is displaced by 7 Å in the crystal structure of the non-standard V-protein. The hypothesis arising from these observations is that the chemical mechanism of nitrogenase involves reversible dissociation of S2B, leaving Fe2 and Fe6 seriously under-coordinated and reactive in trapping N and binding reaction intermediates. Accumulated experimental evidence points to the Fe2-S2B-Fe6 domain as the centre of catalytic hydrogenation of N. Using DFT simulations of a large model (>488 atoms) containing all relevant surrounding protein residues, I have investigated the chemical steps that could allow dissociation of S2B. The participation of H atoms is crucial, as is involvement of the nearby side chain of His195 that can function as proton donor to S2B and hydrogen-bonding supporter of displaced S2B. A significant result is that after ingress and binding of N at Fe2 the breaking of the Fe2-S2B bond can be strongly exergonic with negligible kinetic barrier. Subsequent extension of the Fe6-S2B bond and dissociation as HS (or SH) is endergonic by 20-25 kcal mol, partly because the separating HS is restricted by surrounding amino-acids. I present a number of reaction sequences and energy landscapes, and derive thirteen chemical principles relevant to the postulated S-dissociation mechanism. A key conclusion is that unhooking of S2BH or S2BH from Fe2 is favourable, likely, and propitious for subsequent H transfer to bound N or reaction intermediates. The space between Fe2 and Fe6 supports two bridging ligands, and another H atom on Fe6 can move without kinetic barrier to occupy the bridging position vacated by S2B.