Glucan transferase

TABLE 28.2. List of a-l,4-glncan-processing glucan transferases. 

E. A. MacGregor, H. M. Jespersen, and B. Svensson, A circularly permuted alpha-amylase-type alpha/beta-barrel structure in glucan-synthesizing glucosyl-transferases, FEBS Lett., 378 (1996) 263-266. 

Hrmova, M., Farkas, V., Lahnstein, J., and Fincher, G.B. (2007) A barley xyloglucan xyloglucosyl transferase covalently links xyloglucan, cellulosic substrates, and (l,3 l,4)-D-glucans. J. Biol. Chem. 282, 12951-12962. 

Also referred to as UDP-glucose-/3-D-glucan glucosyl-transferase and UDP-glucose-cellulose glucosyltransfer-... 

The first ten years of this work has produced a plethora of novel, interesting and biologically active compounds. The focus has been on gaining an understanding of the mechanism of action of various -glucan endohydrolases. We hope, in the next decade, to be able to transfer this experience emd knowledge to other important enzymes, such as glycosyl transferases. 

In the field of polymer science, the most extensively used transferase is phosphorylase (systematic name (1 4)-a-D-glucan phosphate a-D-glucosyltransferase EC 2.4.1.1). Although this enzyme is responsible for the depolymerization of linear a-( 1 4) glycosidic chains in vivo it can also be used to synthesize linear a-( 1 4) glycosidic chains (amylose) in vitro. 

Heller and Villemez299,300 solubilized, but did not separate, the D-mannosyltransferase and D-glucosyltransferase activities from this particulate-enzyme system, and found that, when only GDP-D-mannose is provided as the substrate, a/ -(1— 4)-linked D-mannan is synthesized and, when only GDP-D-glucose is present, ap-(l— 4)-linked D-glucan is produced. They further noted, with this transferase system, that incorporation of D-glucose from GDP-D-glucose into polysaccharide is a shortlived reaction that can be greatly extended if GDP-D-mannose is also included in the reaction mixture. 

The enzymes involved are nucleoside diphosphohexosetransferases, probably GDP-D-mannosyltransferase (EC 2.4.1.32) and UDP-D-galactosyl-transferase. For a structurally related polymer, (1 - 6)-a-D-xylosyl-(l - 4)-/3-D-glucan, synthesis was shown to involve concurrent incorporation of D-xylose and D-glucose, and not substitution by D-xylose of a preformed (1 - 4)-/3-D-glucan chain.34 The insolubility of (1 - 4)-/3-D-glucan and man-nan would make the latter mechanism unlikely. 

The pathogenesis of dental caries may involve three distinct processes (1) adherence of the bacteria to the tooth, (2) formation of glycocalyx due to synthesis of a sticky glucan by the action of the bacterial enzyme glucosyl transferase on sucrose, and (3) accumulation of biobUm (plaque), within which there is continuing acid production by constituent bacteria (including streptococci and lactobacflli) able to metabolize carbohydrates at low pH values. This acid demineralizes an enamel. 

We have tested various 4aGTs for their ability to produce cyclic a-1,4 glucan from amylose, and now have the view believe that a cyclization reaction is the common feature of this enzyme (Fujii et al., 2003 Yanase et al., 2002). However, the smallest cyclic glucan produced is greatly dependent on the type of enzyme employed (Table 28.2). In addition to two transferase activities described above, some 4aGTs also exhibit week hydrolitic activity (Fujii et al., 2005a). 

Baecker, P. A., Greenberg, E., and Preiss, J. 1986. Biosynthesis of bacterial glycogen Primary structure of Escherichia coli 1,4-a-D-glucan 1,4-a-D-glucan 6-a-D-(glucano)-transferase as deduced from the nucleotide sequence of the glgB gene. J. Biol. Chem. 261,8738-8743. 

Borovsky, D., Smith, E. E., and Whelan, W. J. 1975. Purification and properties of potato 1,4-a-D-glucan 1,4-a-D-glucan, 6-a-(l,4-a-D-glucano)-transferase. Evidence a dual catalytic function in amylose branching enzyme. Eur. J. Biochem. 59, 615-625. 

Boyer, C. D., and Preiss, J. 1978a. Multiple forms of (1—>4)-a-D-glucan 6-glycosyl transferase from developing Zea mays L. kernels. Carbohydr. Res. 61, 321-334. 

Synthesis of the 1,6-a-D-glucose bonds in amylopectin is catalyzed by a branching enzyme [E.C. 2.4.1.18 1,4-a-D-glucan 6-a-(l,4-a-glucano)-transferase reaction 3]. Amylopectin contains longer chains (about 20-24 glucosyl units) and has less... 

Boyer CD, Preiss J. Multiple forms of a-l,4-glucan, a-l,4-glucan-6-glycosyl transferase from develoing Zea mays L. kernels. Carbohyd. Res. 1978 61 321-332. 

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