Relationships Between Structure and Activity in the α‐Amylase Family of Starch‐metabolising Enzymes

Abstract α‐Amylases are known to be multidomain proteins, i.e., the molecules consist of several folding units. Each α‐amylase is believed, however, to have a catalytic domain consisting, of a barrel of eight parallel α‐strands surrounded by eight α‐strands. with an extra helix inserted after the sixth γ‐strands. The α‐strands and helices alternate along the polypeptide chain and are linked together by irregular loops. Amino acid residues situated on the loops joining the C‐terminal end of each α‐strand to the N‐terminal end of the following helix make up the active site of the enzymes. A similar structure has been found in cyclodextrin glucanotransfcrases and it is now believed that such a (α/α)8‐barrel also constitutes the catalytic domain of enzymes active on α‐1.6‐glucosidic bonds, and of enzymes with dual specificity for both α‐1.4‐ and α‐1.6‐ bonds. Knowledge of the three‐dimensional structure of α‐amylases and cyclodextrin glucanotransferase has made possible identification of structural features important for enzymic activity and specificity. By analogy, some general conclusions are reached concerning pullulanase, isoamylase. oligo‐1,6‐glucosidase, neopullulanase and branching enzymes.