title: Three-dimensional solution structure of bombyxin-II an insulin-like peptide of the silkmoth Bombyx mori: structural comparison with insulin and relaxin.
abstract: The three-dimensional solution structure of bombyxin-II, an insulin-like two-chain peptide produced by the brain of the silkworm Bombyx mori, has been determined by simulated annealing calculations based on 535 distance constraints and 24 torsion-angle constraints derived from NMR data and three distance constraints of the disulfide bonds. To our knowledge, this is the first three-dimensional structure determined for an invertebrate insulin-related peptide. The root-mean-square deviations between the best 10 structures and the mean structure are 0.58(+/- 0.15) A for the backbone heavy atoms (N, C alpha, C) and 1.03(+/- 0.18) A for all non-hydrogen atom if less well-defined N and C termini (A1, A20, B(-2) to B4 and B23 to B25) are excluded. The overall main-chain structure of bombyxin-II is similar to that of insulin. However, there are significant conformational and functional differences in their B-chain C-terminal parts. The B-chain C-terminal part of bombyxin-II adopts an extension of the B-chain central helix like that of relaxin and is not required for bombyxin activity, while the corresponding part of insulin adopts a sharp turn and a beta-strand and is essential for insulin activity. This structure demonstrates that bombyxin-II is more closely related to relaxin than to insulin, and suggests that insulin might have evolved the additional receptor-recognition site in the B-chain C-terminal beta-strand to distinguish itself from bombyxin and relaxin. The structure of bombyxin-II thus provides novel insights into the receptor recognition and divergent molecular evolution of insulin-superfamily peptides.
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D012831:Bombyx / Q000378:metabolism; D006801:Humans; D007328:Insulin / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009479:Neuropeptides / Q000737:chemistry* / Q000378:metabolism; D010802:Phylogeny; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D011956:Receptors, Cell Surface / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D012065:Relaxin / Q000737:chemistry; D013329:Structure-Activity Relationship
chemical_list: D007328:Insulin; D009479:Neuropeptides; D011956:Receptors, Cell Surface; D011993:Recombinant Fusion Proteins; C096422:bombyxin receptor; C077536:bombyxin II; D012065:Relaxin