Synergic binding of carbon monoxide and cyanide to the FeMo cofactor of nitrogenase: relic chemistry of an ancient enzyme?

The first electrochemical and infra-red data on the binding of cyanide to the isolated iron-molybdenum cofactor of nitrogenase, FeMoco, is described. It is shown that cyanide stabilises a hitherto unrecognised, low-spin, EPR-active (S= 1/2), superoxidised form of FeMoco, and we provide the first evidence that carbon monoxide and cyanide bind synergically to the oxidised and semireduced states of the isolated cofactor, states which are unreactive to carbon monoxide alone.