Quantum Biochemistry Insights into Ligand Recognition at the a_1A-Adrenoceptor

Understanding the molecular basis of ligand recognition at α_1A-adrenoceptor (α_1A-AR) is essential for developing highly selective therapeutic agents. In this study, we applied a quantum biochemistry approach combining density functional theory (DFT) with the molecular fractionation with conjugate caps (MFCC) method to perform a detailed energetic characterization of the interactions between α_1A-AR and three ligands with distinct pharmacological profiles: the endogenous nonselective agonist noradrenaline, the partial and selective α_1A-AR agonist oxymetazoline, and the selective α_1A-AR antagonist tamsulosin. Our calculations of total binding energy accurately reproduced the experimental relative affinity ranking (tamsulosin > oxymetazoline > noradrenaline), supporting the reliability of the MFCC-DFT protocol in modeling receptor-ligand interactions at quantum resolution. A total of 81, 88, and 93 amino acid residues of α_1A-AR interacted with noradrenaline, oxymetazoline, and tamsulosin, respectively. The most energetically relevant residues were located within 4 Å. A comprehensive residue-level analysis revealed that ASP106, VAL107, PHE288, and PHE312 are key contributors to the total binding energy of all ligands, corroborating evidence from structural and mutagenesis studies. Specifically for oxymetazoline, this ligand contains a tert-butyl group that establishes nonpolar interactions with residues such as VAL185 and ALA189, which are not observed in the noradrenaline-α_1A-AR complex. Additionally, unlike noradrenaline, oxymetazoline exhibits an attractive interaction with MET292 and does not engage in polar interactions with SER188. These differential interaction patterns may contribute to the distinct pharmacological profile of oxymetazoline relative to noradrenaline. Tamsulosin also exhibited a distinct interaction profile compared to agonists noradrenaline and oxymetazoline, interacting with residues located in the extracellular vestibule, including SER83, PHE86, GLU87, TRP102, CYS176, and LYS309. These additional interactions play a pivotal role in stabilizing tamsulosin within the binding pocket, contributing to its high selectivity and antagonistic behavior at the α_1A-AR. Altogether, these findings provide a robust theoretical framework for understanding the molecular determinants of functional selectivity and subtype specificity at α_1A-AR, offering valuable insights for the rational design of new ligands with improved selectivity, efficacy, and safety profiles.