An experimental and theoretical study of the morphine binding capacity and kinetics of an engineered opioid receptor

Electrochemical real-time monitoring of ligand binding to an engineered opioid receptor specific for morphine is reported. In the particular systems studied, 90% of the binding was found to be completed after only 85-120 s. Thus, the binding kinetics has proven to be more rapid than previously believed. The observed association rate constant for the morphine binding reaction was calculated to be 215 M(-1)s(-1). A theoretical analysis of the experimental binding data suggested that the binding sites of the engineered opioid receptor could best be described by a model having two populations of binding sites: K(D)=40 microM (13 micromol/g) and K(D)=205 microM (29 micromol/g). Furthermore, a theoretical model was developed in order to explain the observed binding of the engineered opioid receptor. This model suggested that the binding sites on the polymer surface are up to 5.1A deep and they allow 100% of the ligand (morphine) to anchor itself into the site. The predicted theoretical maximum binding capacity for the reported receptor is calculated to be approximately 2 mmol/g polymer (based on an increase of cavity density).