J. Ky. Acad. Sci. 66(2): 107-1 17. 2005. 



Development of the Human Mu, Kappa, and Delta Opioid Receptors 



and Docking with Morphine 



Debra L. Bautista,* Wesley Asher, and Lisa Carpenter 



Department of Chemistry, Eastern Kentucky University, Richmond, Kentucky 40475 



ABSTRACT 



Opioid receptors belong to the superfamily of G protein coupled receptors and are primarily responsive 

 to opiates to produce analgesia, but opiates also produce a variety of side effects. One goal of computational 

 chemistry is to determine the interactions between a ligand and protein. This knowledge could allow for the 

 development of opioid agonists without current side effects. Homology models of human mu, kappa, and 

 delta receptors were developed based on a previously validated homology model of the endothelial differ- 

 entiation gene. Docking of native hgand, morphine, was performed. The results indicate that the docking 

 studies identified the actual active site in the model. Morphine had hydrogen bonds to Asp211, His361, and 

 Ser381 in the mu receptor, and hydrogen bonds to Aspl38 and His291 in the kappa receptor. Morphine 

 had hydrogen bonds to Asp 128 and His278 in the delta receptor. This correlates well with experimental 

 data. We predict, based on the models, that mutation of Ser319 to alanine in the mu receptor would confer 

 delta type binding. We further predict that mutation of Tyr312 to tryptophan in the kappa receptor would 

 confer mu type binding. If Tyr312 were mutated to leucine, the resulting receptor would have delta type 

 binding. 



INTRODUCTION 



G protein coupled receptors (GPCRs) com- 

 prise one of the largest receptor famiUes. This 

 family has over 1000 members and continues 

 to grow (Klabunde and Hessler 2002). GPCRs 

 mediate cellular responses including vision, 

 chemotaxis, pain, allergy responses, and blood 

 pressure. Pharmaceutical companies frequent- 

 ly target these receptors and current drugs tar- 

 get opioid, histamine, dopamine, and adren- 

 ergic receptors. Drug sales in 2000 for the top 

 five drugs exceeded $10 billion (Klabunde and 

 Hessler 2002). 



GPCRs have the same molecular architec- 

 ture and similar function to bovine rhodopsin 

 (van Rhee and Jacobson 1996). The crystal 

 structure was published in 2001 and elucidates 

 the overall structure of this family (Palczewski 

 et al. 2000). These receptors have seven trans- 

 membrane alpha helices that span the lipid bi- 

 layer. The amino terminus is extracellular and 

 the carboxy terminus is intracellular. The ag- 

 onist binding domain is within the helical bun- 

 dle (van Rhee and Jacobson 1996). 



Opioid receptors, one GPCR subclass, are 

 divided into three subclasses, the mu, kappa, 

 and delta. These divisions are based on phar- 



* Corresponding author; contact by e-mail at: 

 debra.bautista@eku.edu 



macology and physiology (Raynor et al. 1994). 

 The three receptors have different affinity for 

 morphine (Ki values are mu 6.55 nM, kappa 

 113 nM, and delta 217 nM) (Lattanzi et al. 

 2005). If the differences in affinity are based 

 on differences in the binding site, computa- 

 tional methods could be used to determine 

 these differences. 



Morphine, an opiate that causes analgesia, 

 can be used for management of pain. Unfor- 

 tunately, morphine also has side effects in- 

 cluding respiratory depression and decreased 

 gastrointestinal motility (Raynor et al. 1994). 

 This alkaloid is also subject to abuse since eu- 

 phoria is associated with its use. The devel- 

 opment of an opioid receptor agonist that re- 

 lieves pain without side effects and that is not 

 subject for abuse has long been sought (Kief- 

 fer and Evans 2002; Stevens 1994). One step 

 in that process is to understand the molecular 

 interactions between the ligand and the recep- 

 tor. 



The focus of our study was the development 

 of the mu, kappa, and delta opioid receptors 

 and the docking of those models with mor- 

 phine. Interactions between the receptor and 

 hgand were analyzed to select the best com- 

 plex. These complexes were subjected to mo- 

 lecular dynamics to test the stability of the 

 complex. The receptor/ligand interactions 



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