A novel alpha-amylase inhibitor from amaranth (Amaranthus hypocondriacus) seeds

The major alpha-amylase inhibitor (AAI) present in the seeds of Amaranthus hypocondriacus, a variety of the Mexican crop plant amaranth, is a 32-residue-long polypeptide with three disulfide bridges. Purified AAI strongly inhibits the alpha-amylase activity of insect larvae (Tribolium castaneum and Prostephanus truncatus) and does not inhibit proteases and mammalian alpha-amylases. AAI was sequenced with the automated Edman method, and the disulfide bridges were localized using enzymatic and chemical fragmentation methods combined with N-terminal sequencing. AAI is the shortest alpha-amylase inhibitor described so far which has no known close homologs in the sequence data bases. Its residue conservation patterns and disulfide connectivity are related to the squash family of proteinase inhibitors, to the cellulose binding domain of cellobiohydrolase, and to omega-conotoxin, i.e. a group of small proteins termed "knottins" by Nguyen, D. L., Heitz, A., Chiche, L., Castro, B., Boigegrain, R., Favel, A., and Coletti-Previero, M. ((1990) (Biochimie 72, 431-435) The three-dimensional model of AAI was built according to the common structural features of this group of proteins using side-chain replacement and molecular dynamics refinement techniques.