RT Journal Article
SR Electronic
T1 Restricted analogues provide evidence of a biologically active conformation of thyrotropin-releasing hormone.
JF Molecular Pharmacology
JO Mol Pharmacol
FD American Society for Pharmacology and Experimental Therapeutics
SP 1092
OP 1096
VO 49
IS 6
A1 L Laakkonen
A1 W Li
A1 J H Perlman
A1 F Guarnieri
A1 R Osman
A1 K D Moeller
A1 M C Gershengorn
YR 1996
UL http://molpharm.aspetjournals.org/content/49/6/1092.abstract
AB Thyrotropin-releasing hormone (TRH) is a tripeptide (< Glu-His-Pro-NH2) that signals through a G protein-coupled receptor. TRH is a highly flexible molecule that can assume many conformations in solution. To attempt to delineate the biologically active conformation of TRH, we synthesized a pair of conformationally restricted cyclohexyl/Ala2-TRH analogues. The diastereomeric analogues use a lactam ring to restrict two of the six free torsional angles of TRH and constrain the X-Pro-NH2 peptide bond to trans. Unrestricted cyclohexyl/Ala2-TRH exhibited a 650-fold lower affinity than TRH for TRH receptor and was 430-fold less potent than TRH in stimulating inositol phosphate second messenger formation. One diastereomer exhibited higher affinity and potency than the unrestricted analogue despite the presence of the methylene bridge and fused ring, whereas the other showed lower affinity and potency. Computer simulations predicted that the positions of the cyclohexyl/Ala2 and Pro-NH2 moieties relative to < glutamate were different in the two analogues and that the conformation of the higher affinity analogue is different from that of trans-TRH in solution but is superimposable on that of trans-TRH found in a model of the TRH/TRH receptor complex. These experimental findings identify a favored relative position of < glutamate and Pro-NH2 in the more active conformation of two diastereomeric analogues of TRH and provide independent support for the model of the TRH/TRH receptor complex.