RELATIONSHIPS BETWEEN STRUCTURE AND ACTIVITY IN THE ALPHA-AMYLASE FAMILY OF STARCH-METABOLIZING ENZYMES

Alpha-amylases are known to be multidomain proteins, i. e., the molecu les consist of several folding units. Each alpha-amylase is believed, however, to have a catalytic domain consisting of a barrel of eight pa rallel beta-strands surrounded by eight alpha-helices, with an extra h elix inserted after the sixth beta-strand. The beta-strands and helice s alternate along the polypeptide chain and are linked together by irr egular loops. Amino acid residues situated on the loops joining the C- terminal end of each beta-strand to the N-terminal end of the followin g helix make up the active site of the enzymes. A similar structure ha s been found in cyclodextrin glucanotransferases and it is now believe d that such a (beta/alpha)8-barrel also constitutes the catalytic doma in of enzymes active on alpha-1,6-glucosidic bonds, and of enzymes wit h dual specificity for both alpha-1,4- and alpha-1,6- bonds. Knowledge of the three-dimensional structure of alpha-amylases and cyclodextrin glucanotransferase has made possible identification of structural fea tures important for enzymic activity and specificity. By analogy, some general conclusions are reached concerning pullulanase, isoamylase, o ligo-1,6-glucosidase, neopullulanase and branching enzymes.