Amylosucrase, a glucan-synthesizing enzyme from the alpha-amylase family

Amylosucrase (E,C, 2.4.1.4) is a member of Family 13 of the glycoside hydro lases (the alpha -amylases), although its biological function is the synthe sis of amylose-like polymers from sucrose. The structure of amylosucrase fr om Neisseria polysaccharea is divided into five domains: an all helical N-t erminal domain that is not similar to any known fold, a (beta/alpha)(8)-bar rel A-domain, B- and B'-domains displaying alpha/beta -structure, and a C-t erminal eight-stranded beta -sheet domain. In contrast to other Family 13 h ydrolases that have the active site in the bottom of a large cleft, the act ive site of amylosucrase is at the bottom of a pocket at the molecular surf ace. A substrate binding site resembling the amylase 2 subsite is not found in amylosucrase, The site is blocked by a salt bridge between residues in the second and eight loops of the (beta/alpha)(8)-barrel. The result is an exo-acting enzyme. Loop 7 in the amylosucrase barrel is prolonged compared with the loop structure found in other hydrolases, and this insertion (form ing domain B') is suggested to be important for the polymer synthase activi ty of the enzyme. The topology of the B'-domain creates an active site entr ance with several ravines in the molecular surface that could be used speci fically by the substrates/ products (sucrose, glucan polymer, and fructose) that have to get in and out of the active site pocket.