SOLUTION STRUCTURE OF A SYNTHETIC PEPTIDE CORRESPONDING TO A RECEPTOR-BINDING REGION OF FSH (HFSH-BETA-33-53)

The receptor binding surface of human follicle-stimulating hormone (hFSH) is mimicked by synthetic peptides corresponding to the hFSH-beta chain amino acid sequences 33-53 [Santa-Coloma, T. A., Dattatreyamurty, D., and Reichert, L. E., Jr. (1990), Biochemistry 29, 1194-1200], 81-95 [Santa-Coloma, T. A., and Reichert, L. E., Jr. (1990), J. Biol. Chem. 265, 5037-5042], and the combined sequence (33-53)-(81-95) (Santa-Coloma, T. A., Crabb, J. W., and Reichert, L. E., Jr. (1991), Mol. Cell. Endocrinol. 78, 197-204]. These peptides have been shown to inhibit binding of hFSH to its receptor. Circular dichroism (CD) and nuclear magnetic resonance (NMR) spectroscopy were used to determine the structure of the first peptide in this series, the 21 amino acid peptide hFSH-beta-(33-53), H2N-YTRDLVYKDPARPKIQKTCTF-COOH. Analysis of CD data indicated the presence of approximately equal amounts of antiparallel beta-pleated sheet, turns including a beta-turn, "other" structures, and a small amount of alpha-helix. The major characteristics of the structure were found to be relatively stable at acidic pH and the predominant effect of increased solvent polarity was a small increase in alpha-helical content. One- and two-dimensional NMR techniques were used to obtain full proton and carbon signal assignments in aqueous solution at pH 3.1. Analysis of NMR results confirmed the presence of the structural features revealed by CD analysis and provided a detailed picture of the secondary structural elements and global folding pattern in hFSH-beta-(33-53). These features included an antiparallel beta-sheet (residues 38-51 and 46-48), turns within residues 41-46, and 50-52 (a beta-turn) and a small N-terminal helical region comprised of amino acids 34-36. One of the turns is facilitated by prolines 42 and 45. Proline-45 was constrained to the trans conformation, whereas proline-42 favored the trans conformer (approximately 70%) over the cis (approximately 30%). Two resonances were observed for the single alanine residue (A-43) sequentially proximal to P-42, but the rest of the structure was minimally affected by the isomerization at proline-42. The major population of molecules, containing trans-42 and trans-45 prolines, presented 120 NOEs. Distance geometry calculations with 140 distance constraints and energy minimization refinements were used to derive a moderately well-defined model of the peptide's structure. The hFSH-beta-(33-53) structure has a highly polar surface composed of six cationic amino acid (arginine-35, lysine-40, arginine-44, lysine-46, glutamine-48, and lysine-49) and two anionic residues (aspartate-36 and aspartic acid-41). A hydrophobic region in the structure is composed of residues in the antiparallel beta-sheet and beta-turn which fold to produce a distorted "hairpin." The structure of this domain, together with the protruding and positively charged region in the vicinity of residues 42-45, may mimic the surface of hFSH that binds to the receptor.