RESUMEN
The structure of human parathyroid hormone (PTH) related protein (residues 1-34) containing an Ala substituted for an Ile in position 15 was studied by two-dimensional proton nuclear magnetic resonance spectroscopy. This mutant retains quite high levels of adenylate cyclase activity based on slightly reduced PTH receptor binding capacity. Three segments of helix were revealed extending from His5 to Lys11, Lys13 to Arg19, and from Phe22 to Thr33/Ala34, with a decided kink between the first two helices around Gly12. N- and C-terminal helices were stabilized by charged and hydrophobic side chain interactions between His5 and Glu30, Asp17 and both His9 and His25, and between Leu8 and Ala29, resulting in a globular molecule occupying a single conformation. While the structure of the entire mid-molecule region differed greatly from the structure of the native peptide, the structure of both N- and C-terminal regions remains essentially unaltered. The residues responsible for initiating signal transduction in the mutant are located in the vicinity of the residues responsible for receptor binding. The C-terminal amphipathic helix forming the receptor binding site exhibits reduced binding as a result of the closely applied N-terminal signal transduction-activating region. Although not contributing directly to receptor binding, the N-terminal region can sterically affect hormone binding through modifications to certain N-terminal side chains.