Branching enzyme, polysaccharide synthesis

Guan H. P., Kuriki, T., Sivak, M., and Preiss, J. 1995. Maize branching enzyme catalyzes synthesis of glycogen-like polysaccharide in glgB-deficient Escherichia coli. Proc. Natl. Acad. Science USA 92, 964-967. 

Branching enzyme is responsible for the a-1,6-branching of the a-1,4-chain in the synthesis of glycogen. Branching enzyme enhances the rate of polysaccharide (endogenous glycogen) synthesis from glucose-1 -phosphate by phosphorylase. 

For the synthesis of glycogen-type polysaccharides from a-n-glucopyran-osyl phosphate, two enzymes are required. Phosphorylases, in presence of a suitable primer, synthesize linear chains of -( —> 4)-linked n-glucose residues these are then converted into a branched polysaccharide by a branching enzyme. ... 

The conversion of sucrose into a branched polysaccharide of the amylo-pectin-glycogen type by a bacterial-enzyme system was discovered by Hehre and his coworkers. Enzyme action involves the synthesis of polymeric chains of (1— 4)-linked a-D-glucose residues by a transglucosylase named amylosucrase, followed by the action of a branching enzyme. 

In the following section, enzymes in the EC 2.4 class are presented that catalyze the polymerization of polysaccharides. The Enzyme Commission classification scheme organizes enzymes according to their biochemical function in living systems. Enzymes can, however, also catalyze the reverse reaction which is very often used in biocatalytic synthesis. Therefore newer classification systems have been developed, based on the three-dimensional structure and function of the enzyme, the property of the enzyme, the biotransformation that the enzyme catalyzes etc. [10-15]. The Carbohydrate-Active enZYmes Database [13], which is currently the best database/classification system for carbohydrate active enzymes, uses an amino acid sequence-based classification and would classify some of the enzymes presented in the following as hydrolases rather than transferases (e.g., branching enzyme, sucrases and amylomaltase). Nevertheless we present these enzymes here because they are transferases according to the EC classification. 

Glycogen (or starch) synthesis represents a special case of synthesizing transglycosidation discussed above (section 3). For the formation of natural branched polysaccharides, a second special transglycosidase, the branching enzyme (Q-enzyme), is required. The enzyme removes one chain link from the 1 —> 4 linkage and reattaches it at the 6-hydroxyl group. A new branch point is thus created upon which phosphorylase can then act further. 

Incubation of D-[U-I4C]apiose with sterile Lemma minor (duckweed) produced less than 0.01% incorporation into the cell-wall polysaccharides.75 Most of the d-[U-i4C]apiose appeared as 14C02 some remained in solution in the medium and in the duckweed plants, primarily as degradation products of D-[U-14C]apiose, but not as the branched-chain sugar.75 There is an efficient synthesis of the [U-14C]apiose moiety of cell-wall polysaccharides from D-[U-14C]glucose under similar conditions.81 Of the plant tissues tested, only L. minor contained an enzyme system able to metabolize free apiose. Carrot, lettuce, and spinach tissues are unable to metabolize the free, branched-chain sugar.75... 

It appears that phosphorus is related to the formation of starch in the plant. Thus Hanes has synthesized a linear polysaccharide from a-D-glucopyranosc 1-phosphate (Cori ester) through the action of potato phosphorylase. Dunlap and Beckmann and likewise Cori have found that the B-fraction activates this enzymic synthesis, while the A-frac-tion is inactive. It has not been established whether this effect is due to the branched character of the B-fraction or to the presence of phosphate in its structure. 

---