Axial ligand bonding in blue copper proteins

Abstract

Self consistent field-Xα-scattered wave (SCF-Xα-SW) calculations have been used to characterize the bonding between the Cu(II) ion and the axial methionine residue in blue copper proteins. In addition, the interaction of an axial carbonyl oxygen of a glycine residue found at ∼3 Å. from the copper center in azurins has also been considered. Seven blue copper model sites were constructed to probe various changes in copper coordination and to examine the effects of Zn(II) substituted for Cu(II). It was found that the methionine group covalently binds to the Cu(II) ion site at 2.90 Å with bond strengths calculated to be ∼30% that of a ‘normal’ ligand-metal bond. The carbonyl oxygen atom of a glycine esidue was found to have essentially no covalent interaction with the blue copper site. The carbonyl oxygen atom does have a weak ionic attraction to the copper ion which is ∼ 1 4 that of the covalent stabilization of the methionine group. The ligand environment in blue copper proteins is best characterized as four coordinate, with three strong ‘in-plane’ ligands (two His and Cys) and a weaker axial bond to methionine. Substitution of Cu(II) by Zn(II) leads to a loss of covalency and an increase in the ionic interaction and results in a reasonable metal-glycine bond.