Molecular dynamics simulation of human neurohypophyseal hormone receptors complexed with oxytocin-modeling of an...

The neurohypophyseal hormone oxytocin (CYIQNCPLG-NH(2), OT) is involved in the control of labor, secretion of milk and many social and behavioral functions via interaction with its receptors (OTR) located in the uterus, mammary glands and peripheral tissues, respectively. In this paper we propose the interactions responsible for OT binding and selectivity to OTR versus vasopressin ([F3,R8]OT, AVP) receptors: V1aR and V2R, all three belonging to the Class A G protein-coupled receptors (GPCRs). Three-dimensional models of the activated receptors were constructed using a multiple sequence alignment and the activated rhodopsin-transducin (MII-Gt) prototype [Slusarz and Ciarkowski, 2004] as a template. The 1 ns unconstrained molecular dynamics (MD) of three pairs of receptor-OT complexes (two complexes per each receptor) immersed in the fully hydrated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) lipid bilayer was conducted in the AMBER 7.0 force field. The relaxed models of ligand-receptor complexes were used to identify the putative binding sites of OT. The stabilizing interactions with conserved Gln residues in all complexes were identified. The nonconserved hydrophobic residues were proposed as responsible for OTR-OT selectivity and ligand recognition. These results provide guidelines for experimental site-directed mutagenesis and if confirmed, they may be helpful in designing new selective OT analogs with both agonistic or antagonistic properties.