Restricted analogues provide evidence of a biologically active conformation of thyrotropin-releasing hormone

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.