Crystal structure of amylomaltase from Thermus aquaticus, a glycosyltransferase catalysing the production of large cyclic glucans

Amylomaltase is involved in the metabolism of starch, one of the most impor tant polysaccharides in nature. A unique feature of amylomaltase is its abi lity to catalyze the formation of cyclic amylose. In contrast to the well s tudied cyclodextrin glucanotransferases (CGTases), which synthesize cycloam ylose with a ring size (degree of polymerization or DP) of 6-8, the amyloma ltase from Thermus aquaticus produces cycloamyloses with a DP of 22 and hig her. The crystal structure of amylomaltase from Thermus aquaticus was deter mined to 2.0 Angstrom resolution. It is a member of the alpha-amylase super family of enzymes, whose core structure consists of a (beta, alpha)(8) barr el. Ln amylomaltase, the 8-fold symmetry of this barrel is disrupted by sev eral insertions between the barrel strands. The largest insertions are betw een the third and fifth barrel strands, where two insertions form subdomain B1, as well as between the second and third barrel strands, forming the al pha-helical subdomain B2. Whereas part of subdomain B1 is also present in o ther enzyme structures of the alpha-amylase superfamily, subdomain B2 is un ique to amylomaltase. Remarkably, the C-terminal domain C, which is present in all related enzymes of the alpha-amylase family, is missing in amylomal tase. Amylomaltase shows a similar arrangement of the catalytic side-chains (two Asp residues and one Glu residue) as in previously characterized memb ers of the alpha-amylase superfamily, indicating similar mechanisms of the glycosyl transfer reaction. Ln amylomaltase, a conserved loop of around eig ht amino acid residues is partially shielding the active center. This loop, which is well conserved among other amylomaltases, may sterically hinder t he formation of small cyclic products. (C) 2000 Academic Press.