Solution Structure of the Osteogenic 1−31 Fragment of the Human Parathyroid Hormone

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Abstract
The solution conformations of a selectively osteogenic 1−31 fragment of the human parathyroid hormone (hPTH), hPTH(1−31)NH2, have been characterized by use of very high field NMR spectroscopy at 800 MHz. The combination of the CαH proton and 13Cα chemical shifts, 3JNHα coupling constants, NH proton temperature coefficients, and backbone NOEs reveals that the hPTH(1−31)NH2 peptide has well-formed helical structures localized in two distinct segments of the polypeptide backbone. There are also many characteristic NOEs defining specific side-chain/backbone and side-chain/side-chain contacts within both helical structures. The solution structure of hPTH(1−31)NH2 contains a short N-terminal helical segment for residues 3−11, including the helix capping residues 3 and 11 and a long C-terminal helix for residues 16−30. The two helical structures are reinforced by well-defined capping motifs and side-chain packing interactions within and at both ends of these helices. On one face of the C-terminal helix, there are side-chain pairs of Glu22−Arg25, Glu22−Lys26, and Arg25−Gln29 that can form ion-pair and/or hydrogen bonding interactions. On the opposite face of this helix, there are characteristic hydrophobic interactions involving the aromatic side chain of Trp23 packing against the aliphatic side chains of Leu15, Leu24, Lys27, and Leu28. There is also a linear array of hydrophobic residues from Val2, to Leu7, to Leu11 and continuing on to residues His14 and Leu15 in the hinge region and to Trp23 in the C-terminal helix. Capping and hydrophobic interactions at the end of the N-terminal and at the beginning of the C-terminal helix appear to consolidate the helical structures into a V-shaped overall conformation for at least the folded population of the hPTH(1−31)NH2 peptide. Stabilization of well-folded conformations in this linear 1−31 peptide fragment and possibly other analogues of human PTH may have a significant impact on the biological activities of the PTH peptides in general and specifically for the osteogenic/anabolic activities of bone-building PTH analogues.