Stabilization of charges on isolated ionic groups sequestered in proteins by polarized peptide units.

Electrostatic interactions are of considerable importance in protein structure and function, and in a variety of cellular and biochemical processes. Here we report three similar findings from highly refined atomic structures of periplasmic binding proteins. Hydrogen bonds, acting primarily through backbone peptide units, are mainly responsible for the involvement of the positively charged arginine 151 residue in the ligand site of the arabinose-binding protein, for the association between teh sulphate-binding protein and the completely buried sulphate dianion, and for the formation of the complex of the leucine/isoleucine/valine-binding protein with the leucine zwitterion. We propose a general mechanism in which the isolated charges on the various buried, desolvated ionic groups are stabilized by the polarized peptide units. This mechanism also has broad application to processes requiring binding of uncompensated ions and charged ligands and stabilization of enzyme reaction charged intermediates, as well as activation of catalytic residues.