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Microbial gums are produced in two different ways: (i) extracellularly by means of glycosyltransferase enzymes secreted from the microbial cells, or (ii) intracellularly, where a cascade of enzymes produced inside the cells is responsible for precursor synthesis, glycosyltransferase, polymerization, and export of the microbial biopolymer (Figure 3.1). HoPS, such as dextran, alternan, and levan, are produced by extracellular glycosylsucrase enzymes that hydrolyses sucrose to produce long‐chain biopolymers. Well‐known microbial gums such as pullulan, a HoPS of α‐(1 → 6)‐linked polymer of maltotriose subunits, is produced by fungus Aureobasidium pullulans using enzyme glucosyltransferase. UDP‐glucose acts as a precursor for pullulan synthesis. The exact biosynthesis pathway for pullulan is still not known [5]. Curdlan, a (1→3)‐β‐glucan produced by Alcaligenes faecalis, is synthesized by the action of an operon containing four genes, namely crdA, crdS, crdC, and crdR. crdS is curdlan synthase, a glucosyltransferase enzyme, which uses UDP‐glucose as a precursor. The crdC gene is responsible for polymerization of curdlan and subsequent translocation of the biopolymer across the cytoplasmic membrane [6].

Figure 3.1 Schematics of biosynthesis of microbial gums.

Source: Based on [3].

HePS are synthesized intracellularly and subsequently exported out from the cells. HePS shows a similar biosynthesis pathway to HoPS‐like pullulan. This pathway is controlled by a collection of genes called EPS gene cluster, which is responsible for regulation, chain‐length determination, polymerization, and export of HePS. The precursors for HePS biosynthesis are UDP and dTDP sugars, and in rare occurrences, non‐sugar groups like acetyl group or pyruvate group could be attached to the biopolymer chain [7, 8].

Biosynthesis of microbial gums is highly strain‐dependent, and these variability can be seen as differences in biopolymer yield, linkages present between moieties and molecular weight. These differences also can change the functional properties of these microbial gums. Modification of genes involved in the biosynthesis of microbial gums through metabolic engineering of the strains can yield modified biopolymers or produce a higher quantity of microbial gums [9].