A molecular mechanism for the cleavage of a disulfide bond as the primary function of agonist binding to G-protein-coupled receptors based on theoretical calculations supported by experiments

A model of the binding site of delta-opioids in the extracellular region of the G-protein-coupled opioid receptor based on modelling studies is presen ted. The distance between Asp288 and the disulfide bridge (Cys121-Cys198) f ormed between the first and second extracellular loops was found to be shor t. This model is consistent with site-directed mutagenesis studies. The arr angement of the ligands found in the receptor led to the development of a r eaction mechanism for the cleavage of the disulfide bond catalysed by the l igands. Semi-empirical quantum chemical PM3 and AM1 calculations as well as ab initio studies showed that the interaction between the carboxylic acid side chain of aspartic acid and the disulfide bond leads to the polarizatio n of, and withdrawal of a proton from, the protonated nitrogen of the ligan d to one of the sulfur atoms. A mixed sulfenic acid and carboxylic acid anh ydrate is formed as an intermediate as well as a thiol. The accompanying cl eavage of the disulfide bond may produce a conformational change in the ext racellular loops such that the pore formed by the seven-helix bundle opens allowing entrance of the ligand, water and ions into the cell. Cleavage of the disulfide bond after opioid administration was demonstrated experimenta lly by flow-cytometric measurements employing CMTMR and monobromobimane-bas ed analyses of membrane-located thiols. The suggested mechanism may explain , in a consistent way, the action of agonists and antagonists and is assume d to be common for many G-protein coupled receptors.