Biochemistry - The Chemical Reactions of Living Cells, Volume 2 - D. Metzler 1980

Biosynthesis: How New Molecules Are Formed
Polysaccharide Synthesis
Chain Elongation via Insertion

In some cases, a completely distinct mechanism of polysaccharide chain elongation is observed. For instance, in Leuconostoc or Streptococcus, the action of dextransucrase catalyzes The addition of glucosyl residues to the reducing ends of dextran polysaccharide chains (Ch. 2, Sec. B, 3)¹⁾. In this case, sucrose serves as the direct donor of glucosyl groups; the initial stage of the enzymatic reaction apparently involves The formation of a glucosylated enzyme (with the glucosyl residue attached to a nucleophilic group Y' in accordance with equation (12-13), reaction ^a).

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During growth, the polysaccharide chain remains attached to the enzyme. Each successive glucose residue is inserted between the enzyme and the polysaccharide linked to it. The Mechanism of such insertion is easily visualized by assuming that the enzyme possesses two binding sites for activated glycosyl groups: one site binds the extending polysaccharide chain, while the other binds the activated glucosyl residue [Equation (12-13)]. The enzyme catalyzes a transfer reaction in which the growing polysaccharide is shifted from the nucleophilic group Y of the enzyme to the free 6-hydroxyl group of the incoming glucosyl residue [11].

¹⁾ If inhibitors of this enzyme could be discovered, they would be well worth adding to toothpaste to prevent dental plaque formation [suggested by J. Robyt].

These observations shed light on a long-standing puzzle concerning starch Biosynthesis, which can be outlined as follows. The branched component of starch, amylopectin, appears to be synthesized via essentially the same pathway as Glycogen. The only difference is that the outer chains of amylopectin are elongated prior to the formation of new branches. A specific branching enzyme (the Q-enzyme), analogous to the corresponding enzyme in glycogen synthesis, transfers a segment of a chain to an OH group of a glucose residue located within an adjacent, parallel polysaccharide chain. Within starch granules, amylose and amylopectin are intimately intertwined; how is it, then, that the branching enzyme never attaches side chains to the unbranched amylose chains? One possible explanation is that the linear amylose chains are oriented in the opposite direction relative to the amylopectin chains. The non-reducing ends of the amylose molecules might face toward the center of the starch granule, whereas elongation via the insertion mechanism would proceed from the reducing ends. Naturally, as the granule grows, these ends must continually be displaced toward the periphery [12]. We present this highly speculative hypothesis solely to illustrate that many unresolved questions remain in the field of Polysaccharide Biosynthesis.