K3036.58 in the μ opioid (MOP) receptor is important in conferring selectivity for covalent binding of β-funaltrexamine (β-FNA)

β-funaltrexamine (β-FNA) is an irreversible μ opioid (MOP) receptor antagonist and a reversible agonist of κ opioid (KOP) receptor. β-FNA binds covalently to the MOP receptor at Lys233 5.39 , which is conserved among opioid receptors. Molecular docking of β-FNA showed that K303 6.58 in the MOP receptor and E297 6.58 in the KOP receptor played distinct roles in positioning β-FNA. K303 6.58 E MOP receptor and E297 6.58 K KOP receptor mutants were generated. The mutations did not affect β-FNA affinity or efficacy. K303 6.58 E mutation in the MOP receptor greatly reduced covalent binding of [ 3 H]β-FNA; however, E297 6.58 K did not enable the KOP receptor to bind irreversibly to β-FNA. Molecular modeling demonstrated that the e-amino group of K303 6.58 in the MOP receptor interacted with C O of the acetate group of β-FNA to facilitate covalent bond formation with Lys233 5.39 . Replacement of K303 6.58 with Glu in the MOP receptor resulted in repulsion between the COOH of Glu and the C O of β-FNA and increased the distance between K233 5.39 and the fumarate group, making it impossible for covalent bond formation. These findings will be helpful for design of selective non-peptide MOP receptor antagonists.