Epimer mbbs biochemistry question answers
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Epimers - MBBS Biochemistry: Questions & Answers
DEFINITION
Q1. What is an epimer?
A: Epimers are monosaccharides that differ from each other in the configuration of a single asymmetric (chiral) carbon atom - i.e., the position of the -OH group is reversed at only ONE carbon, while all other carbons remain identical.
- They are a subclass of diastereomers (stereoisomers that are NOT mirror images of each other).
- The enzyme that interconverts epimers is called an epimerase.
KEY EPIMER PAIRS (Most Exam-Tested)
| Epimer Pair | Carbon Differing | Enzyme Involved |
|---|---|---|
| Glucose ↔ Galactose | C-4 | UDP-hexose 4-epimerase |
| Glucose ↔ Mannose | C-2 | Phosphomannose isomerase |
| UDP-glucose ↔ UDP-galactose | C-4 | UDP-hexose 4-epimerase |
| D-glucuronic acid ↔ L-iduronic acid | C-5 | Epimerase (in GAG synthesis) |
The classic diagram (Lippincott Biochemistry, Fig. 7.4):
Glucose and Galactose are C-4 epimers (OH at C-4 flipped). Glucose and Mannose are C-2 epimers (OH at C-2 flipped). Fructose is a structural isomer (not an epimer) of glucose.
MCQs WITH ANSWERS
Q2. Glucose and galactose are epimers differing at which carbon?
- a) C-1
- b) C-2
- c) C-3
- d) C-4 ✓
Explanation: In galactose, the -OH at C-4 is on the opposite side compared to glucose. All other carbons are identical. They are C-4 epimers. - Lippincott Biochemistry, 8th ed.
Q3. Glucose and mannose are epimers differing at which carbon?
- a) C-1
- b) C-3
- c) C-2 ✓
- d) C-4
Explanation: Mannose is the C-2 epimer of glucose. Mannose is phosphorylated by hexokinase to mannose 6-phosphate, then converted to fructose 6-phosphate by phosphomannose isomerase.
Q4. Which of the following is an example of epimers?
- a) Glucose & fructose
- b) D-glucose & L-glucose
- c) Glucose & galactose ✓
- d) Glucose & ribose
Explanation:
- Glucose & fructose = structural isomers (keto vs. aldose)
- D-glucose & L-glucose = enantiomers (mirror images)
- Glucose & galactose = epimers (differ at C-4 only) ✓
- Glucose & ribose = different number of carbons (not isomers at all in the classical sense)
Q5. The enzyme UDP-hexose 4-epimerase catalyzes:
- a) Glucose → Mannose
- b) Mannose 6-P → Fructose 6-P
- c) UDP-galactose → UDP-glucose ✓
- d) Galactose → Galactitol
Explanation: This reaction is critical in galactose metabolism. UDP-galactose must be converted to UDP-glucose (its C-4 epimer) to enter mainstream glucose metabolism. This enzyme is also used in biosynthesis of UDP-galactose from UDP-glucose when dietary galactose is absent (e.g., lactose intolerance). - Lippincott, p.415
Q6. What type of isomerism is seen between α-D-glucose and β-D-glucose?
- a) Epimerism
- b) Enantiomerism
- c) Anomerism ✓
- d) Structural isomerism
Explanation: Anomers differ only at the anomeric carbon (C-1 for aldoses). This is NOT epimerism. The process of interconversion between α and β anomers is called mutarotation. Do NOT confuse anomers with epimers!
Q7. In glycosaminoglycan (GAG) synthesis, which pair represents C-5 epimers?
- a) Glucose & Galactose
- b) Glucose & Mannose
- c) D-glucuronic acid & L-iduronic acid ✓
- d) Glucose & Fructose
Explanation: D-glucuronic acid and L-iduronic acid are C-5 epimers found in GAGs such as heparan sulfate and dermatan sulfate. L-iduronic acid is formed from D-glucuronic acid by epimerization at C-5. - Lippincott, p. carbohydrate section
Q8. Mannose enters glycolysis via:
- a) Direct phosphorylation to glucose 6-phosphate
- b) Hexokinase → Mannose 6-P → Fructose 6-P ✓
- c) Glucokinase → Glucose 1-P
- d) UDP-hexose epimerase reaction
Explanation: Hexokinase phosphorylates mannose to mannose 6-phosphate; phosphomannose isomerase then converts it to fructose 6-phosphate, which enters glycolysis. - Lippincott, p.409
HIGH-YIELD SUMMARY TABLE
| Concept | Detail |
|---|---|
| Definition | Monosaccharides differing at only ONE asymmetric carbon |
| Enzyme | Epimerase (interconverts epimers) |
| Glucose & Galactose | C-4 epimers |
| Glucose & Mannose | C-2 epimers |
| Glucuronic acid & Iduronic acid | C-5 epimers (in GAGs) |
| NOT an epimer | Anomers (differ at anomeric C-1 only) |
| NOT an epimer | Enantiomers (mirror images of the whole molecule) |
| NOT an epimer | Fructose (structural isomer of glucose, not epimer) |
| Clinical significance | Galactosemia: defect in galactose→glucose pathway; UDP-hexose 4-epimerase deficiency |
TRICKY DISTINGUISHING POINTS (Exam Traps)
| Comparison | Epimers | Anomers | Enantiomers |
|---|---|---|---|
| Differ at | One specific C (not C-1) | Anomeric C-1 (or C-2) | ALL asymmetric carbons |
| Mirror image? | No | No | Yes |
| Example | Glu & Gal (C-4) | α-Glucose & β-Glucose | D-Glucose & L-Glucose |
| Interconversion | Epimerases | Mutarotation | Specific enzymes |
Remember: All epimers are diastereomers, but not all diastereomers are epimers. Anomers are a special subtype of diastereomers, NOT epimers.
Sources: Biochemistry, 8th ed. Lippincott Illustrated Reviews, pp. 258-261, 409, 415; Basic Medical Biochemistry, 6e.
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