116 



Journal of the Kentucky Academy of Science 66(2) 



decrease the binding of morphine in the rat 

 mu receptor, Ilel98 (262 in the human) and 

 Tyr326 (390 in the human). From our models, 

 we propose that these residues are needed for 

 helical packing and not for binding. Ile262 in 

 helix 4 interacts with Tyr212 (helix 3) and 

 Val300 (helix 5). Tyr390 interacts with Vall42, 

 Cysl43, Glyl46, all in helix 1, and Aspl78, 

 AlalSl, and Thrl82 in helix 2. 



In a study of the rat mu receptor, Bot et al. 

 (1998) mutated residues that in our study are 

 directly needed for Ugand/receptor interac- 

 tions. They mutated Asp 114 to asparagine and 

 binding of a variety of ligands decrease from 

 8 to 41 fold. Agonist and peptide agonist were 

 affected but partial agonists and antagonists 

 were not. They also mutated His297 to aspar- 

 agine however this mutation was less effective 

 in reducing binding affinity of agonists but did 

 affect partial agonists and antagonists. Mor- 

 phine binding was decreased by two fold. 

 These two residues are indicated in the bind- 

 ing of morphine in all three models. 



Based on the above studies, the models pre- 

 sented here are in good correlation with ex- 

 perimental data and are expected to have good 

 predictive ability. Our models predict that in 

 the mu receptor, Ser319 mutation to alanine 

 would confer delta type binding. If Tyr312 of 

 the kappa receptor were mutated to trypto- 

 phan, the resulting receptor would have a mu 

 type binding profile. If the same residue (kap- 

 pa Tyr312) were mutated to leucine, the re- 

 sulting receptor would have delta type bind- 

 ing. 



SUMMARY 



The models presented here are in good 

 agreement with experimental literature. The 

 models identified an aspartate and a histidine 

 that are required by all three receptors for 

 binding of morphine. The models also identi- 

 fied residues that if mutated could impart a 

 different binding profile. The models were 

 also in agreement with mutagenesis data re- 

 garding two residues required for binding, an 

 aspartic acid in helix 3 and a histidine in helix 

 6, and two residues required for helical pack- 

 ing, tyrosine in helix 7 and an isoleucine in 

 helix 5. While these models represent only the 

 active state of the receptor, it does offer some 

 insight into an important biological process. 



ACKNOWLEDGMENTS 



Support received from the NIH/NCRR/, 

 Grant # 1 P20 RR16841-01 and by NSF KY 

 EPSCoR REG is gratefully acknowledged. 



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