'''SOLUTION STRUCTURE OF THE MAIN ALPHA-AMYLASE INHIBITOR FROM AMARANTH SEEDS'''
===SOLUTION STRUCTURE OF THE MAIN ALPHA-AMYLASE INHIBITOR FROM AMARANTH SEEDS===
==Overview==
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The most abundant alpha-amylase inhibitor (AAI) present in the seeds of Amaranthus hypochondriacus, a variety of the Mexican crop plant amaranth, is the smallest polypeptide (32 residues) known to inhibit alpha-amylase activity of insect larvae while leaving that of mammals unaffected. In solution, 1H NMR reveals that AAI isolated from amaranth seeds adopts a major trans (70%) and minor cis (30%) conformation, resulting from slow cis-trans isomerization of the Val15-Pro16 peptide bond. Both solution structures have been determined using 2D 1H-NMR spectroscopy and XPLOR followed by restrained energy refinement in the consistent-valence force field. For the major isomer, a total of 563 distance restraints, including 55 medium-range and 173 long-range ones, were available from the NOESY spectra. This rather large number of constraints from a protein of such a small size results from a compact fold, imposed through three disulfide bridges arranged in a cysteine-knot motif. The structure of the minor cis isomer has also been determined using a smaller constraint set. It reveals a different backbone conformation in the Pro10-Pro20 segment, while preserving the overall global fold. The energy-refined ensemble of the major isomer, consisting of 20 low-energy conformers with an average backbone rmsd of 0.29 +/- 0.19 A and no violations larger than 0.4 A, represents a considerable improvement in precision over a previously reported and independently performed calculation on AAI obtained through solid-phase synthesis, which was determined with only half the number of medium-range and long-range restraints reported here, and featured the trans isomer only. The resulting differences in ensemble precision have been quantified locally and globally, indicating that, for regions of the backbone and a good fraction of the side chains, the conformation is better defined in the new solution structure. Structural comparison of the solution structure with the X-ray structure of the inhibitor when bound to its alpha-amylase target in Tenebrio molitor shows that the backbone conformation is only slightly adjusted on complexation, while that of the side chains involved in protein-protein contacts is similar to those present in solution. Therefore, the overall conformation of AAI appears to be predisposed to binding to its target alpha-amylase, confirming the view that it acts as a lid on top of the alpha-amylase active site.
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{{ABSTRACT_PUBMED_11298757}}
==About this Structure==
==About this Structure==
1HTX is a [[Single protein]] structure of sequence from [http://en.wikipedia.org/wiki/Amaranthus_hypochondriacus Amaranthus hypochondriacus]. Full crystallographic information is available from [http://oca.weizmann.ac.il/oca-bin/ocashort?id=1HTX OCA].
1HTX is a [[Single protein]] structure of sequence from [http://en.wikipedia.org/wiki/Amaranthus_hypochondriacus Amaranthus hypochondriacus]. Full experimental information is available from [http://oca.weizmann.ac.il/oca-bin/ocashort?id=1HTX OCA].
==Reference==
==Reference==
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[[Category: Wodak, S J.]]
[[Category: Wodak, S J.]]
[[Category: Cysteine knot]]
[[Category: Cysteine knot]]
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Revision as of 05:40, 1 July 2008
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Solution structure of the main alpha-amylase inhibitor from amaranth seeds., Martins JC, Enassar M, Willem R, Wieruzeski JM, Lippens G, Wodak SJ, Eur J Biochem. 2001 Apr;268(8):2379-89. PMID:11298757
