The ?-adrenergic antagonists (?-blockers) constitute a class of drugs that have well-established roles in treatments of various cardiovascular diseases. Despite a 50 year history, there are two clinically important subtypes of ?-adrenergic receptors (?ARs) called ?1AR and ?2AR that still are promising drug targets. Our study maps the interactions between nebivolol?one of the most efficient ?-blocking agents?and the ?2-adrenergic receptor by simulating two optical isomers of nebivolol: ssss-nebivolol and srrr-nebivolol. The srrr-configuration binds preferentially to ?1AR and ?2AR. The ssss-form has much lower binding affinity to both of them. Our work indicates that water is a very important component of the binding site of the ?2AR receptor. We found that the higher stereoselectivity of the srrr-configuration is due to interactions with water molecules, which extensively hydrate the binding site of ?2AR. By lowering the energy of binding, water enhanced the affinity of the srrr-form to ?2AR. We also address the problem of ?1AR/?2AR selectivity. At higher concentrations, all ?-blocking agents lose their specificity and bind nonselectively, causing many adverse effects. Our simulations indicate that PHE194, TYR308, and ILE309 of the ?2AR and the corresponding residues of the ?1AR receptor may be important determinants of ?1AR versus ?2AR selectivity.
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