Q.25 Which is not the heteropolysaccharide of extra cellular matrix in multicellular animals?
Keratin is Not a Heteropolysaccharide of Animal ECM
Keratin, a fibrous scleroprotein, is absent from the extracellular matrix (ECM) of multicellular animals, making option (3) correct. The ECM contains glycosaminoglycans (GAGs) like hyaluronan, keratan sulfate, and heparin as heteropolysaccharides.
Introduction
The heteropolysaccharide extra cellular matrix multicellular animals primarily consists of GAGs providing hydration, elasticity, and signaling. Keratin, being a protein, does not qualify as this type of ECM component. This guide clarifies each option for biochemistry exam precision.
Option Analysis
Animal ECM heteropolysaccharides: Unbranched/repeating disaccharides (hexosamine + uronic/hexose), often sulfated, highly negatively charged.
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(1) Hyaluronan: ECM heteropolysaccharide. Non-sulfated GAG: D-glucuronic acid-β1,3-N-acetylglucosamine repeating. Major synovial fluid/cartilage component; synthesized at plasma membrane.
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(2) Keratan sulfate: ECM heteropolysaccharide. Galactose-β1,4-N-acetylglucosamine-6-sulfate; cornea, cartilage proteoglycan (lumican, keratocan). Variable sulfation.
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(3) Keratin: Not a heteropolysaccharide. α/β fibrous protein (keratinocytes, hair, nails); intracellular cytoskeletal/epidermal barrier. No carbohydrate component—pure polypeptide.
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(4) Heparin: ECM heteropolysaccharide (intracellular in mast cells). Highly sulfated GAG: glucosamine-Iduronic acid; anticoagulant (antithrombin III activator).
ECM Composition Table
| Component | Type | Location | Function |
|---|---|---|---|
| Hyaluronan | Heteropolysaccharide (GAG) | Synovium, cartilage | Shock absorption |
| Keratan sulfate | Heteropolysaccharide (GAG) | Cornea, intervertebral disc | Hydration |
| Keratin | Protein | Epidermis, appendages | Mechanical strength |
| Heparin | Heteropolysaccharide (GAG) | Mast cells | Coagulation control |
Key Distinctions
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GAG definition: ≥95% iduronic/glucuronic + hexosamines
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Keratin biochemistry: Cysteine-rich α-helix coils (disulfide bonds)
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Exam trap: “Keratan” (GAG) vs. “Keratin” (protein)—similar spelling!
Exam Relevance
GATE Life Sciences: ECM = GAGs + proteoglycans + proteins (collagen, elastin). Mnemonic: “HYALuronan, KERATan, HEParin hydrate ECM; KERATin keeps KERATinocytes cohesive.”
