Q49.Dextrans are:
(1) Polymeric glucose
(2) Polymeric galactose
(3) Polymeric fructose
(4) Polymeric lactose
Dextrans are bacterial polysaccharides composed of glucose monomers linked primarily by α-1,6 glycosidic bonds. The correct answer for Q49 is (1) Polymeric glucose.
Option Breakdown
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(1) Polymeric glucose: Correct. Dextrans are homopolysaccharides of D-glucose units (~95% α-D-1,6 linkages, 5% α-1,3 branches), produced by Leuconostoc mesenteroides from sucrose.
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(2) Polymeric galactose: Incorrect. Galactans (e.g., agar, gum arabic) are galactose polymers; dextrans lack galactose.
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(3) Polymeric fructose: Incorrect. Fructans like inulin or levan are fructose-based; dextran is exclusively glucose.
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(4) Polymeric lactose: Incorrect. Lactose (Gal-β1,4-Glc) is a disaccharide, not a polymer; no such “polymeric lactose” exists.
Why Glucose Only?
Dextransucrase enzyme synthesizes α-1,6-glucan backbone from sucrose, distinguishing from other microbial EPS like xanthan (GlcUA-Man-Glc-Rha).
Understand dextrans polymeric glucose for CSIR NET Life Sciences and GATE Biotech: Q49 confirms dextrans as polymeric glucose (α-1,6-D-glucan from lactic acid bacteria), not galactose/fructose/lactose. Master structure, biosynthesis, and why distractors fail.
Dextran Structure
Complex branched glucan: linear α-1,6-D-glucose backbone (95%) with α-1,3 branches (~5%, 1-2 Glc units).
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MW: 1 kDa–40 MDa; water-soluble, viscous.
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Producer: Leuconostoc mesenteroides B-512(F).
Correct Answer: (1) Polymeric Glucose
Universal definition—no galactose/fructose in dextran composition.
Why Options Fail
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Galactose: Agarose (agar), galactomannan.
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Fructose: Bacterial levan (β-2,6), plant inulin (β-2,1).
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Lactose: Milk disaccharide; doesn’t polymerize.
Exam Applications
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Uses: Plasma expander, chromatography matrix (Sephadex), drug delivery.
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CSIR trick: Microbial EPS monomers (dextran=Glc, gellan=GlcUA-Rha-Glc).
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Biosynthesis: Sucrose → dextran + fructose (dextransucrase).
