The receptor binding conformation of bombyxin is induced by alanine(B15).
Bombyxin is an insect hormone with an insulin-like structure which affects the reduction of stored carbohydrates in the silkworm Bombyx mori. The receptor binding surface of bombyxin includes a trough on the interface between the B chain helix and the N-terminal A chain helix. Alanine(B15) is located on the edge of this feature, whereas the bottom is formed by hydrophobic core residues Ile(A2) and Leu(B14). Replacement of alanine(B15) with bulkier residues produces a negative steric effect on bombyxin receptor binding; alpha-aminobutyric acid reduced the affinity to 6.5%, valine to 1.1%, norvaline to 0.88%, and leucine to 0.05%. CD spectra of these analogues were indistinguishable from each other and identical to that of bombyxin. Changing the backbone structure by replacing alanine with glycine and alpha-aminoisobutyric acid resulted in analogues with activities of 3.7 and 1.4%, respectively, but also a disturbed structure as determined by CD spectroscopy. Replacement of other residues on the periphery of the trough, i.e., arginines at positions B12 and B16, also reduced the level of receptor binding but to a lesser extent than the replacement of alanine(B15). The level of receptor binding for citrulline(B12) bombyxin was 17% and for citrulline(B16) bombyxin was 45%. When it is considered that glycine(A1) is located on the edge of the same trough but across from Ala(B15) and is required for maintenance of the overall structure of bombyxin, it is proposed that the bombyxin receptor binding site forms a contiguous hydrophobic area consisting of residues Ile(A2), Leu(B14), and Ala(B15).
Fullbright
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Department of Biochemistry and Molecular Biology, Medical University of South Carolina, 173 Ashley Avenue, P.O. Box 250509, Charleston, South Carolina 29425, USA.
Büllesbach
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EE
D000409:Alanine; D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D012831:Bombyx; D002942:Circular Dichroism; D007328:Insulin / Q000737:chemistry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009479:Neuropeptides / Q000737:chemistry / Q000378:metabolism*; D011487:Protein Conformation; D011956:Receptors, Cell Surface / Q000378:metabolism*; D018013:Receptors, Neuropeptide / Q000378:metabolism*
D007328:Insulin; D009479:Neuropeptides; D011956:Receptors, Cell Surface; D018013:Receptors, Neuropeptide; C096422:bombyxin receptor; C077536:bombyxin II; D000409:Alanine