Biochemical properties and structural features of the thermostable maltodextrin transglycosidases from Thermotoga maritima

Maltosyltransferase (MTase) is an extremely thermostable enzyme which, based on its primary structure, is classified into glycoside hydrolase family 13. The enzyme is a non-hydrolytic transglycosidase (maltodextrin glycosyltransferase, MGTase) which catalyses the transfer of maltosyl units from α-1,4-linked glucans or malto-oligosaccharides to other α-1,4-linked glucans, maltooligosaccharides or glucose. MTase represents the first exo-MGTase known. To date, the only organism known to produce a starch-converting enzyme with this unique reaction chemistry is the hyperthermophilic bacterium Thermotoga maritima, a strictly anaerobic heterotroph with a maximum growth temperature of 90°C. In addition to MTase, T. maritima possesses a second MGTase, 4-α-glucanotransferase (GTase), also a member of the glycoside hydrolase family 13. In contrast to MTase, GTase displays a broad transfer specificity. Recently, crystals of recombinant MTase and GTase have been obtained by the hanging-drop vapor-diffusion method, and the crystal structures of MTase and its complex with maltose have been determined at 2.4 and 2.1 Å resolution, respectively. In this communication, the enzymatic characteristics of MTase and GTase are reviewed, and structural features, possibly of importance for the unique transfer specificity and thermostability of MTase, are discussed.