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3. Carbohydrates — Biochemistry MCQs

KEMU · 53 MCQs with clinical pearls · Prof Jariya

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Sample MCQs with clinical pearls

Mutarotation is a property of reducing sugars. Which of the following best describes mutarotation?
A) Aldose to ketose conversion
B) Enantiomer formation
C) Interconversion of alpha and beta anomers ✔
D) Isomerisation at non-anomeric carbon
E) Rotation of polarisation plane only
MUTAROTATION = spontaneous INTERCONVERSION between ALPHA and BETA ANOMERS of a reducing sugar in solution, passing through the open-chain form. Alpha-D-glucose (specific rotation +112°) ⇌ open chain ⇌ beta-D-glucose (specific rotation +18.7°) → equilibrium mixture (specific rotation +52.7°). ANOMERS differ only at the anomeric carbon (C1 in aldoses): alpha = OH axial (below ring in Haworth), beta = OH equatorial (above ring). EPIMERISM = differing at any non-anomeric carbon. 📹 [VIDEO LINK]
Glucose and galactose have the same molecular formula (C₆H₁₂O₆) but differ only in the configuration at carbon-4. They are therefore classified as which of the following?
A) Anomers
B) Diastereomers
C) Enantiomers
D) Epimers ✔
E) Isomers
EPIMERS = two sugars that differ in configuration at ONE specific non-anomeric carbon. Glucose and galactose = C4 EPIMERS (differ only at C4). Glucose and mannose = C2 EPIMERS. Compare: ANOMERS = differ at the anomeric carbon (C1) only. ENANTIOMERS = mirror images (D vs L — all carbons inverted). Clinical: GALACTOSAEMIA — GALT enzyme deficiency → galactose-1-phosphate accumulates → liver damage, cataracts, intellectual disability. [Diagram: Haworth projections of glucose and galactose side by side. Glucose: ring structure with OH at C4 pointing DOWN (equatorial). Galactose: identical except OH at C4 pointing UP (axial). Red circle highlighting C4 in both. Arrow between them labelled: C4 EPIMERS — differ ONLY at C4. Interconversion by UDP-galactose-4-epimerase. Clinical box: Galactosaemia = deficient GALT → galactose toxic → cataracts + liver damage + intellectual disability.] 📹 [VIDEO LINK]
Glucuronic acid is converted to iduronic acid in proteoglycan synthesis by the action of which of the following enzymes?
A) Epimerase ✔
B) Isomerase
C) Kinase
D) Mutase
E) Racemase
EPIMERASE converts glucuronic acid → IDURONIC ACID by inverting the configuration at C5 → occurs in the GOLGI during proteoglycan processing. Iduronic acid is found in: heparin, heparan sulfate, and DERMATAN SULFATE. Distinction: EPIMERASE (changes configuration at one carbon), ISOMERASE (converts aldose ↔ ketose), MUTASE (moves phosphate group), RACEMASE (inverts all stereocentres). 📹 [VIDEO LINK]

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