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Conformational analysis of cyclo(2,9)-Ac-QCRSVEGSCG-OH from the C-terminal loop of human growth hormone.

A 10 amino acid residue cyclic peptide, cyclo(2,9)-Ac-Gln1-Cys2-Arg3-Ser4-Val5-Glu6-Gly7 -Ser8-Cys9-Gly10, from the C-terminal region of human growth hormone (hGH) was synthesized and studied by 2D proton NMR and molecular dynamics (MD) simulations. The solubility of the peptide was low in water; hence, NMR studies were done in two solvent mixtures, water and deuterated dimethyl sulfoxide. NOE-constrained molecular dynamics and MD simulations resulted in major and minor conformers in solution. The major conformer has a type I beta-turn at Gln1-Cys2-Arg3-Ser4 and a loop structure around Glu6-Gly7-Ser8. Comparison of the conformation of this peptide with the peptide fragment 181-190 in the intact hGH protein X-ray crystal structure indicated that the synthetic peptide retains some structural similarity to the intact protein. Since the C-terminal region is important in binding the hGH protein to its receptor, the conformation of the synthetic peptide could be useful in understanding the binding mechanism.