A CIRCULARLY PERMUTED ALPHA-AMYLASE-TYPE ALPHA BETA-BARREL STRUCTURE IN GLUCAN-SYNTHESIZING GLUCOSYLTRANSFERASES/

A motif of amino acid residues, located at the active site and specifi c beta-strands in alpha-amylases, is recognized in alpha-1,3- and alph a-1,6-glucan-synthesizing glucosyltransferases, leading to the conclus ion that these enzymes contain an alpha/beta-barrel closely related to the (beta/alpha)(8)-fold of the alpha-amylase superfamily. The second ary structure elements of the transferase barrel, however, are circula rly permuted to start with an alpha-helix equivalent to helix 3 in the alpha-amylases. Thus, the transferase counterpart of the long third b eta --> alpha connection - constituting a domain in the alpha-amylases - is divided to precede and succeed the barrel. This architectural ar rangement may be coupled to sucrose scission and glucosyl transfer, Th e involvement in the mechanism in glucosyltransferases of active site residues recurring in amylolytic enzymes is discussed.