A motif of amino acid residues, located at the active site and specific β-strands in a-amylases, is recognized in α-1,3- and α-1,6-glucan-synthesizing glucosyltransferases, leading to the conclusion that these enzymes contain an α/β-barrel closely related to the (β/α)8-fold of the α-amylase superfamily. The secondary structure elements of the transferase barrel, however, are circularly permuted to start with an α-helix equivalent to helix 3 in the α-amylases. Thus, the transferase counterpart of the long third β → α connection — constituting a domain in the α-amylases — is divided to precede and succeed the barrel. This architectural arrangement may be coupled to sucrose scission and glucosyl transfer. The involvement in the mechanism in glucosyltransferases of active site residues recurring in amylolytic enzymes is discussed.
