Molecular Pharmacology, Vol 13, 606-614, Copyright © 1977 by the American Society for Pharmacology and Experimental Therapeutics

Forces and Structural Limitations of Binding of Thyrotrophin-Releasing Factor to the Thyrotrophin-Releasing Receptor: the Pyroglutamic Acid Moiety

¹ Division of Medical Biochemistry, Health Sciences Centre, The University of Calgary, Calgary, Alberta, Canada T2N 1N4
² The Salk Institute, La Jolla, California 92112

Thyrotrophin-releasing factor (TRF) and analogues of its pyroglutamyl ring were tested for their potencies in eliciting thyrotrophin release in vivo. The ratio of potency of an analogue to the potency of TRF was used to predict changes in binding energy of association between the TRF molecule and the TRF-thyrotroph receptor. Assumptions made in this correlation were that the analogue has efficacy and stability to degradation in vivo equal to TRF, and a similar temperature dependence of binding. Types of noncovalent bonds were assigned according to the changes in binding energy. Using the potencies of 23 analogues of TRF in which only the pyroglutamyl residue was modified (some taken from the literature), the following inferences have been drawn about the TRF molecule-TRF-thyrotroph receptor interaction. (a) The ring amide proton is a hydrogen bond acceptor. The strength of this hydrogen bond is approximately 1.5 kcal/ mol. (b) The -ketone oxygen is a hydrogen bond donor. The strength of this hydrogen bond is approximately 3.5 kcal/mole. (c) The -methylene group is involved in formation of a hydrophobic bond whose strength is approximately 0.8 kcal/mole. (d) The receptor molecule in the region of the ring amide proton cannot accommodate a group larger than a hydrogen atom. (e) There is a tight fit between the receptor and the - and -carbons of the pyroglutamyl ring. (f) The 5-membered ring structure and its puckering are important in maintaining the right spatial location of the ring amide and the -ketone. (g) There is room for substitution on the -carbon of pyroglutamic acid for at least one methyl group. (h) The pyroglutamyl residue may contribute up to 50% of the total binding energy of TRF.