Analysis of the molecular interaction of mitragynine from kratom with human α1-acid glycoprotein: biophysical and molecular modeling investigations

Mitragynine (MTG), the primary psychoactive alkaloid in Mitragyna speciosa (kratom), has garnered much attention for its therapeutic properties, which is attributed mainly to its selective action on opioid receptors. Despite its clinical potential, the molecular framework of its binding to plasma proteins remains incomplete. Specifically, no studies have thoroughly examined its interaction with α1-acid glycoprotein (AAG), a carrier protein in the circulatory system that influences drug disposition and bioavailability. Hence, this study aims to explore the binding dynamics between MTG and AAG using a combination of spectroscopic, calorimetric, microscopic, and computational methods. Based on isothermal titration calorimetric and fluorescence studies, an intermediate affinity for the MTG–AAG binding was determined (Ka ∼ 105 M−1). Despite evidence of microenvironmental changes around Trp residues, MTG binding did not disrupt the overall structural integrity of AAG. Thermodynamic analysis indicated that the MTG–AAG interaction was energetically favorable, and enthalpy driven mainly by hydrogen bonding and van der Waals forces, with negative entropy change suggesting a more ordered complex formation. Docking analysis showed MTG embedded more deeply within the central cavity of variant F1*S, enhancing complex stability, as opposed to binding near the cavity entrance in variant A. Molecular dynamics simulations supported the stable complexation of MTG with both AAG variants, with variant F1*S maintaining more structural compactness while variant A exhibited slight unfolding upon binding. These findings have clear significance on the potential therapeutic applications of kratom-derived drugs, especially those structurally related to MTG.