The Interstitial Carbon of the Nitrogenase FeMo Cofactor is Far Better Stabilized than Previously Assumed.

The first quantum-mechanical calculations of all relevant potential constants in both the iron-molybdenum cofactor and the iron-vanadium cofactor of nitrogenase suggest that the carbide is bound to the center of the enzyme much more strongly than hitherto assumed. Previous studies seemed to indicate a dummy function of the interstitial carbon, with a weak force constant (ca. 0.32 N cm-1 ). Our new investigations confirm a different picture: the central carbon atom binds the iron-sulfur cluster through six covalent C-Fe bonds. With a potential constant of more than 1.3 N cm-1 , the interstitial carbon also appears to be dynamically persistent. According to our investigations, the values for the elasticity within the iron-sulfur cluster have to be corrected too. These new details on the mechano-chemical properties of the FeMo cofactor will be important for elucidating the catalytic cycle of nitrogen fixation. By implementing our new algorithm in the freely available COMPLIANCE program, the dependence on the coordinates during the calculation of Hesse matrices is eliminated completely.