"[1] The term was coined by Bert Vallee and R. J. P. Williams,[2] following work on the catalytic activity of carbonic anhydrase.
Each oxidation state prefers a distinct coordination geometry: whereas copper(II) is normally square planar and prefers hard bases such as oxygen and nitrogen ligands, copper(I) is normally tetrahedral and binds preferentially to soft bases such as sulphur ligands.
In fact, the Cu site is neither planar nor tetrahedral it is considered a distorted tetrahedral, with two nitrogen ligands from histidine residues and two sulphur ligands from methionine and cysteine residues, and can therefore be considered an entatic state.
Under the entatic state hypothesis, the distortion results from strain caused by binding to ligands with relative orientation that is pre-arranged by the protein.
Some theoretical calculations show that a model system can have a geometry similar to that observed in the protein without any strain; these results, however, remain controversial.