STABILIZED NMR STRUCTURE OF HUMAN PARATHYROID HORMONE(1-34)

The structure of the biologically-active N-terminal region of human parathyroid hormone, PTH(l - 34), was investigated in the presence of 10% trifluoroethanol using two-dimensional proton NMR spectroscopy, distance geometry and dynamic simulated annealing. Complete assignments of all backbone and side chain hydrogens were made with the aid of totally correlated spectroscopy (TOCSY) experiments, providing through-bond H-1-H-1 connectivities, and NOESY providing through-space and sequential backbone connectivities. Distance and angle constraints were used in the distance geometry algorithm DIANA II to generate a family of structures satisfying all inputs. The lowest energy structures were further refined using the dynamic-simulated-annealing protocol XPLOR 3. The major structural features evident in 10% trifluoroethanol are two segments of alpha-helix extending from residues Glu4 to Lysl4/Hisl4 and Ser17 to Asp30. A short length of unordered structure joined the two spans of helix. The structures of the N-terminal regions (4-13/14) agreed closely with the structure found in human parathyroid-hormone-related protein (PTHrP)(1 - 34) obtained earlier by Ray, Barden and Kemp [19]. However, PTHrP(16-19) exhibited a reverse tum which is incorporated in an extended helix in PTH. The consequent interactions between several hydrophobic residues in the C-terminal region in PTHrP are absent in PTH. Moreover, the tum in PTHrP(22-25) at the start of the C-terminal helix is present as a more standard loop of helix in PTH. Comparisons between the structures of the two hormones has enabled the probable location and structure of the PTH receptor-binding site to be placed in the segment of amphiphilic alpha-helix (24-31).