Fe(III) and Co(III) centers with carboxamido nitrogen and modified sulfur coordination: lessons learned from nitrile hydratase.

Nitrile hydratase (NHase) is a non-heme Fe(III) or non-corrinoid Co(III) metalloenzyme with an unprecedented coordination sphere comprising deprotonated carboxamido nitrogens and modified Cys-S (-SO(-) and -SO(2)(-)) sulfurs. We have synthesized model complexes derived from designed ligands that contain these donor groups. The model complexes mimic almost all the intrinsic properties of the unique M(III) (M = Fe, Co) active site of NHase. Even a functional Co(III) model has been synthesized that hydrolyzes nitriles catalytically at pH close to the optimum pH of the enzyme. Our studies have provided insight into how the unusual donor atoms dictate the overall properties of the biological M(III) sites.