Metallo-enzyme catalysis: the entatic state

Abstract

The best of all possible homogeneous catalysts must have carefully designed coordination spheres such that their acid/base or redox ground state properties are correctly tuned for the optimum expression of function, and such that the sites retain sufficient mobility to allow the catalyst atoms to pass easily along the reaction coordinates. Biological evolution has had the opportunity to generate such catalysts through the use of advantageous mutations. The coordination sphere in metallo-enzymes is controlled by the local amino acid composition, the fold energy of the protein and the use of a variety of synthesised special ligands. The question arises as to how near to perfection have the metallo-protein catalysts become. Specially and specifically strained coordination centres of many metallo-enzymes in their ground states have already been identified and called ‘entatic’. We shall show that biological evolution has done a remarkable series of special syntheses, and that indeed the resultant coordination compounds can be described as very well designed for catalytic purposes, with respect to both structure and mobility. They may fall just short of perfection in that metallo-proteins are not simple catalysts but must perform one or two other functions.