Molecular modeling of residues 38-57 of the beta-subunit of human lutropin.
Several regions on both the alpha- and beta-subunits of human LH comprise the receptor-binding domain of the hormone. One of these, a disulfide loop peptide containing residues 38-57 on the beta-subunit, also stimulated steroidogenesis in rat Leydig cells. Circular dichroism analysis and a Schiffer-Edmundson helical wheel projection of beta-(38-57) revealed the possibility of an amphipathic alpha-helical structure through its N-terminal region. Secondary structure prediction algorithms do not predict alpha-helix in beta-(38-57), but, rather, suggest a sheet-coil-sheet topology. Homology searches between this peptide and proteins with known structure revealed that the two best matches are with prealbumin-(10-30) and melittin-(1-26). Based on hydrophobic moment calculations, we suggest that beta-(38-57) more closely resembles melittin, a known example of an amphipathic helix. Molecular models were constructed that included an alpha-helix between Pro-39 and Pro-50 producing a hydrophilic face involving Thr-40, Arg-43, and Gin-46. Loop closure was performed either visually or by an incremental minimization procedure, using distance constraints to patch in a disulfide bond. Molecular dynamics at 300, 360, and 1000 K were used to explore the local conformational space, and dynamic structures were minimized. The most reasonable structures were found with the 300 and 360 K simulations, with those at 360 K consistently producing structures with lower conformational energies. In each of these simulations, the N-terminus of the alpha-helix unraveled to form a reverse turn (predicted by the GOR algorithm) which include Cys-38, Pro-39, Thr-40, and Met-41. Simulations at 1000 K produced the most variation in structure, but these were deemed unreasonable. Although not all possible conformations were explored, several models were found that comply with the assumption of an amphipathic helix in the N-terminal half of the peptide.