The functional residues and their representation by a hypothetical 3D model of silkworm eclosion hormone

We have previously reported that an insect neuropeptide, eclosion hormone contained an alpha-helix in the N-terminal region and the helix was likely to play an important role in constructing an active globular structure. Furthermore, Met24 and Phe25 were found to be indispensable for the biological activity. On the other hand, no strict structure at the C-terminal side was found. In this paper, we predicted the secondary structure in the C-terminal side and analyzed the functional residues by a Gly-substitution technique. As a result, we speculated that the eclosion hormone contains three alpha-helices throughout the molecule which are essential for an active peptide structure. Moreover, we found four residues important for the biological activity of silkworm eclosion hormone: Phe29, Ile55, Phe58 and Leu59. In order to understand these results stereochemically, we have constructed a 3D structure using computer aided molecular modelling. The hypothetical 3D model showed that Phe25 and Phe58 interact together in a hydrophobic manner to keep a globular form. Met24, Phe29, and Ile55 are exposed to solvent to have a hydrophobic interaction with an eclosion hormone receptor. Leu59 can also play an important role by forming a functional conformation with Phe29 and Ile55.