An electrophile-nucleophile interaction in metalloprotein structures.

A new attractive interaction in metalloprotein structures, between the thiolate anion of a metal-bound cysteine (acting as a nucleophile) and a carbonyl carbon of a peptide group (an electrophile), has been identified. From 82 cases extracted from 23 metalloprotein structures, the interacting S and C atoms are found to be at a distance of 3.2 (+/-2) A, such that the angle S ... C-O is 109 degrees (+/-15 degrees). Usually, the interacting atoms are from the same Cys residue, and to allow the S to interact with the carbonyl group the side-chain and the main-chain torsion angles deviate from those found in cysteines not bound by metals. There is a good correlation between the S...C distance and the angular deviation of the S...C vector from the normal to the peptide plane. Various data points may be envisaged to represent "snapshots" along the reaction coordinate for the intra-residue attack of Cys S on the CO group.