For CSIR-NET | GATE | IIT-JAM | UGC-NET | PG Entrance
1. Introduction
- Vitamin B12 is a water-soluble, cobalt-containing vitamin also known as cobalamin.
- Only vitamin with a metal ion (cobalt) in its structure → Corrin ring + Cobalt.
- Essential for DNA synthesis, RBC maturation, and myelin synthesis.
- Synthesized only by microorganisms (bacteria in animal gut); humans obtain from animal sources.
2. Chemical Structure
- Central corrin ring (similar to porphyrin) with Co³⁺ at center.
- Active coenzyme forms: 5'-deoxyadenosylcobalamin and methylcobalamin.
- Cyanocobalamin → synthetic, stable form used in supplements.
Structural Components
| Component | Description | Key Features |
|---|---|---|
| Corrin Ring | Tetrapyrrole ring system (similar to porphyrin but with one less methine bridge) | Contains 4 pyrrole rings (A, B, C, D); Ring D directly linked to A |
| Central Cobalt (Co³⁺) | Metal ion at the center of corrin ring | Octahedral coordination; 4 equatorial N from corrin, 2 axial ligands |
| Upper Axial Ligand (β-ligand) | Variable group above cobalt | Cyano (-CN), Methyl (-CH₃), 5'-deoxyadenosyl, Hydroxo (-OH), etc. |
| Lower Axial Ligand (α-ligand) | 5,6-Dimethylbenzimidazole (DMB) nucleotide | Links to cobalt via N-7; "nucleotide loop" attaches to corrin via phosphodiester |
| Side Chains | Acetamide and propionamide groups on corrin | 7 acetamide (a-g), 6 propionamide (except f-position) |
Key Difference from Heme: Corrin has one less carbon bridge between pyrrole rings and direct C-C bond between rings A and D.
3. Oxidation States & Electronic Configuration
Vitamin B12 is a coordination complex centered around a Cobalt ion. Its chemistry is dictated by three primary oxidation states:
| Form | Oxidation State | Electron Count | Geometry | Magnetic Nature |
|---|---|---|---|---|
| Common (B12a/b) | Co(III) | d6 | Octahedral | Diamagnetic |
| Reduced (B12r) | Co(II) | d7 | Square Pyramidal | Paramagnetic |
| Super-reduced (B12s) | Co(I) | d8 | Square Planar | Diamagnetic |
4. Crystal Field Splitting & Magnetism
Co(III) - The Resting State
In the d6 state, the strong-field corrin ring creates a large energy gap (Δo). All 6 electrons pair up in the lower-energy t2g orbitals. Because there are zero unpaired electrons, it is diamagnetic.
Co(II) - The Radical State
In the d7 state, the symmetry drops to square pyramidal. The first 6 electrons pair up, but the 7th electron remains alone in the dz2 orbital. This makes the molecule paramagnetic and measurable via EPR spectroscopy.
Co(I) - The Super-Nucleophile
In the d8 state, the geometry is square planar. The dx2-y2 orbital is pushed very high in energy. The 8 electrons fill the four lower orbitals in pairs, making it diamagnetic.
5. Biological Roles in Enzymes
Humans utilize Vitamin B12 in two critical enzymatic pathways:
A. Methionine Synthase (Cytoplasm)
- Cycle: Co(I) ↔ Co(III)
- Mechanism: Uses the Co(I) "super-nucleophile" to pick up a methyl group from folate and transfer it to homocysteine to create methionine.
- Importance: Essential for DNA synthesis. Failure leads to Megaloblastic Anemia.
B. Methylmalonyl-CoA Mutase (Mitochondria)
- Cycle: Co(II) ↔ Co(III)
- Mechanism: Uses a radical mechanism. The Co-C bond breaks homolytically to form a Co(II) center and an adenosyl radical, which rearranges carbon skeletons.
- Importance: Essential for energy production from fats and proteins.
6. Dietary Sources
| Source | Content (μg/100g) |
|---|---|
| Liver (beef) | 80–100 |
| Kidney | 30–50 |
| Fish (salmon, tuna) | 3–15 |
| Eggs | 1–2 |
| Milk | 0.3–0.5 |
| Meat | 1–3 |
| Plant sources | None (except fortified foods) |
High-yield: No natural plant source → Vegans at risk.
7. Recommended Dietary Allowance (RDA)
| Group | RDA (μg/day) |
|---|---|
| Adults | 2.4 |
| Pregnancy | 2.6 |
| Lactation | 2.8 |
| Infants (0–6 mo) | 0.4 |
| Children (1–3 yr) | 0.9 |
Body stores: 2–5 mg (mainly in liver) → lasts 3–5 years even if intake stops.
8. Absorption, Transport & Storage
Step-by-Step Absorption (Complex Process)
- Stomach: Food-bound B12 released by pepsin + HCl.
- Binds to R-protein (haptocorrin) from saliva.
- Duodenum: Pancreatic proteases degrade R-protein → B12 binds to Intrinsic Factor (IF) secreted by parietal cells.
- Ileum: IF-B12 complex binds to Cubilin receptor → endocytosis.
- Inside enterocyte: B12 released, binds to transcobalamin II (TCII) → enters blood.
Transport: 80% bound to transcobalamin I & III (storage), 20% to TCII (delivery).
Storage: Liver (2–5 mg), muscle, kidney.
Most important: Intrinsic Factor essential for ileal absorption.
9. Biochemical Functions (Coenzyme Roles)
| Reaction | Coenzyme Form | Enzyme |
|---|---|---|
| Isomerization of methylmalonyl-CoA → succinyl-CoA | 5'-deoxyadenosylcobalamin | Methylmalonyl-CoA mutase |
| Remethylation of homocysteine → methionine | Methylcobalamin | Methionine synthase |
Clinical relevance:
- Deficiency → ↑ methylmalonic acid & homocysteine.
- Links folate trap: B12 needed to release folate from 5-methyl THF.
10. Deficiency: Causes
| Cause | Mechanism | Examples |
|---|---|---|
| Pernicious anemia | Autoimmune destruction of parietal cells → no IF | Most common in elderly |
| Gastrectomy / Bariatric surgery | Loss of parietal cells | Post-surgery |
| Pancreatic insufficiency | No protease to release B12 from R-protein | Chronic pancreatitis |
| Ileal disease/resection | Loss of absorption site | Crohn’s, celiac |
| Strict vegan diet | No dietary B12 | Long-term vegans |
| Drugs | ↓ absorption | Metformin, PPIs, colchicine |
| Nitrous oxide abuse | Inactivates B12 | Dentists, recreational |
11. Clinical Manifestations
Hematological
- Megaloblastic anemia: Macrocytic (MCV >100), hypersegmented neutrophils.
- Pancytopenia (late).
Neurological (Subacute Combined Degeneration)
- Symmetric paresthesia, loss of vibration & position sense.
- Ataxia, spasticity, demyelination of dorsal & lateral columns.
- Irreversible if untreated >6 months.
Others
- Glossitis, fatigue, infertility, hyperpigmentation.
Classic triad: Anemia + Neuropathy + Glossitis.
12. Diagnosis
| Test | Finding in B12 Deficiency |
|---|---|
| Serum B12 | <150 pg/mL (definite), 150–300 (borderline) |
| Methylmalonic acid (MMA) | Elevated (specific) |
| Homocysteine | Elevated (also ↑ in folate def.) |
| Anti-IF antibody | Positive in pernicious anemia |
| Schilling test | Obsolete; used radiolabeled B12 |
13. Treatment
- Intramuscular cyanocobalamin: 1000 μg weekly × 8 weeks → then monthly lifelong (if malabsorption).
- Oral high-dose: 1000–2000 μg/day (1–2% absorbed passively).
- Dietary correction if cause is intake.
- Neurological symptoms respond within weeks; anemia in days.
Summary Table
| Aspect | Key Points |
|---|---|
| Sources | Animal products only |
| RDA | 2.4 μg/day |
| Absorption | IF-dependent in ileum |
| Functions | DNA synth, myelin, odd-chain FA metabolism |
| Deficiency | Pernicious anemia, SCD, megaloblastic anemia |
| Diagnosis | ↓B12, ↑MMA, ↑Homocysteine |
| Treatment | IM 1000 μg → lifelong if malabsorption |
50 Vitamin B12 MCQs – PYQ-Based for CSIR-NET | GATE | IIT-JAM
CSIR-NET Life Sciences | GATE Biotechnology | IIT-JAM Biotechnology
1. Which of the following vitamins contains a corrin ring with cobalt at its center?
Viwe Answer
Answer: c) Vitamin B12
Vitamin B12 (cobalamin) is the only vitamin with a corrin ring and cobalt (Co³⁺). CSIR-NET 2018
Level: Easy
2. Intrinsic factor required for vitamin B12 absorption is secreted by:
Viwe Answer
Answer: b) Parietal cells
Intrinsic factor (IF) is a glycoprotein secreted by gastric parietal cells. Essential for B12 absorption in ileum. GATE BT 2020
Level: Easy
3. The coenzyme form of vitamin B12 involved in the conversion of methylmalonyl-CoA to succinyl-CoA is:
Viwe Answer
Answer: b) 5'-Deoxyadenosylcobalamin
Methylmalonyl-CoA mutase requires adenosylcobalamin. CSIR-NET 2019
Level: Medium
4. Pernicious anemia is primarily caused by:
Viwe Answer
Answer: c) Autoimmune destruction of gastric parietal cells
Leads to lack of intrinsic factor → B12 malabsorption. IIT-JAM BT 2021
Level: Medium
5. Which of the following is a specific biochemical marker for vitamin B12 deficiency?
Viwe Answer
Answer: c) Methylmalonic acid
MMA accumulates only in B12 deficiency (not in folate def.). CSIR-NET 2022
Level: Hard
6. The transport protein that delivers vitamin B12 to tissues is:
Viwe Answer
Answer: b) Transcobalamin II
TCII-B12 complex is taken up by receptor-mediated endocytosis. GATE BT 2019
Level: Medium
7. Subacute combined degeneration in B12 deficiency affects primarily:
Viwe Answer
Answer: b) Dorsal and lateral columns
Demyelination → loss of vibration, position sense, spastic paraparesis. CSIR-NET 2020
Level: Medium
8. Which drug is known to cause functional vitamin B12 deficiency by inactivating methionine synthase?
Viwe Answer
Answer: b) Nitrous oxide
N₂O oxidizes Co⁺ to Co³⁺ → inactivates methylcobalamin. IIT-JAM BT 2022
Level: Hard
9. The enzyme requiring methylcobalamin as a coenzyme is:
Viwe Answer
Answer: b) Methionine synthase
Converts homocysteine → methionine; traps folate as 5-methyl THF in deficiency. CSIR-NET 2017
Level: Medium
10. The site of vitamin B12 absorption in the gastrointestinal tract is:
Viwe Answer
Answer: d) Terminal ileum
IF-B12 complex binds to cubam receptor in ileum. GATE BT 2021
Level: Easy
11. In vitamin B12 deficiency, the 'folate trap' hypothesis explains:
Viwe Answer
Answer: b) Accumulation of 5-methyl tetrahydrofolate
B12 needed to regenerate THF from 5-methyl THF. CSIR-NET 2021
Level: Hard
12. Which of the following conditions does NOT cause vitamin B12 deficiency?
Viwe Answer
Answer: a) Celiac disease affecting duodenum
B12 absorption occurs in ileum, not duodenum. IIT-JAM BT 2020
Level: Medium
13. The earliest neurological symptom in vitamin B12 deficiency is usually:
Viwe Answer
Answer: c) Loss of vibration and position sense
Due to dorsal column involvement. CSIR-NET 2019
Level: Medium
14. The Schilling test is used to diagnose:
Viwe Answer
Answer: b) Pernicious anemia
Measures absorption of radiolabeled B12 with and without IF. GATE BT 2018
Level: Medium
15. Vitamin B12 is NOT naturally present in:
Viwe Answer
Answer: c) Wheat
No plant source contains B12 naturally. IIT-JAM BT 2019
Level: Easy
16. The RDA of vitamin B12 for a normal adult is:
Viwe Answer
Answer: b) 2.4 μg/day
ICMR-NIN recommendation. CSIR-NET 2020
Level: Easy
17. Hypersegmented neutrophils are seen in peripheral smear of:
Viwe Answer
Answer: b) Megaloblastic anemia
Due to B12 or folate deficiency. GATE BT 2022
Level: Easy
18. The synthetic form of vitamin B12 used in injections is:
Viwe Answer
Answer: c) Cyanocobalamin
Stable, converted to active forms in body. CSIR-NET 2016
Level: Medium
19. In B12 deficiency, serum homocysteine is elevated because:
Viwe Answer
Answer: a) Methionine synthase is inhibited
Requires methylcobalamin. IIT-JAM BT 2023
Level: Hard
20. Long-term metformin use can cause B12 deficiency by:
Viwe Answer
Answer: c) Reducing calcium-dependent ileal absorption
Metformin interferes with Ca²⁺-mediated B12-IF uptake. CSIR-NET 2023
Level: Hard
21. The receptor involved in the endocytosis of Intrinsic Factor–Vitamin B12 complex in the ileum is:
Viwe Answer
Answer: b) Cubilin
Cubilin (with amnionless) forms the cubam receptor for IF-B12 uptake. CSIR-NET 2021
Level: Hard
22. Which of the following is NOT a clinical feature of vitamin B12 deficiency?
Viwe Answer
Answer: c) Koilonychia
Koilonychia is seen in iron deficiency, not B12 deficiency. GATE BT 2020
Level: Medium
23. The protein that binds vitamin B12 in saliva and protects it in the stomach is:
Viwe Answer
Answer: b) Haptocorrin (R-protein)
Later degraded by pancreatic proteases to release B12 for IF binding. CSIR-NET 2019
Level: Medium
24. In vitamin B12 deficiency, megaloblastic anemia occurs due to:
Viwe Answer
Answer: b) Defective DNA synthesis
Due to folate trap and impaired thymidylate synthesis. IIT-JAM BT 2022
Level: Medium
25. The form of vitamin B12 stored in the liver is primarily:
Viwe Answer
Answer: c) 5'-Deoxyadenosylcobalamin
Major storage form; used in methylmalonyl-CoA mutase reaction. CSIR-NET 2020
Level: Hard
26. Which of the following conditions is associated with anti-intrinsic factor antibodies?
Viwe Answer
Answer: b) Pernicious anemia
Autoimmune gastritis → anti-IF and anti-parietal cell antibodies. GATE BT 2021
Level: Medium
27. The daily requirement of vitamin B12 is met by biliary excretion and enterohepatic circulation in approximately:
Viwe Answer
Answer: b) 2–5 μg
Liver secretes 2–5 μg/day into bile; reabsorbed in ileum. CSIR-NET 2022
Level: Hard
28. Neurological symptoms in B12 deficiency are due to:
Viwe Answer
Answer: d) Both a and b
Methylmalonic acid is neurotoxic; B12 needed for myelin maintenance. IIT-JAM BT 2021
Level: Medium
29. The vitamin B12 coenzyme involved in odd-chain fatty acid metabolism is:
Viwe Answer
Answer: b) Adenosylcobalamin
Required for methylmalonyl-CoA → succinyl-CoA (TCA cycle entry). CSIR-NET 2018
Level: Medium
30. In a patient with total gastrectomy, vitamin B12 deficiency develops after approximately:
Viwe Answer
Answer: c) 3–5 years
Due to depletion of liver stores (2–5 mg). GATE BT 2019
Level: Medium
31. The Schilling test becomes normal when intrinsic factor is administered in:
Viwe Answer
Answer: c) Pernicious anemia
Corrects IF deficiency; no effect in absorption site defects. CSIR-NET 2017
Level: Hard
32. Which of the following is a rich source of vitamin B12?
Viwe Answer
Answer: d) Clams
Clams, liver, and fish are richest sources (~80–100 μg/100g). IIT-JAM BT 2020
Level: Easy
33. The enzyme methionine synthase is dependent on:
Viwe Answer
Answer: b) Methylcobalamin and folate
Transfers methyl group from 5-methyl THF to homocysteine. CSIR-NET 2023
Level: Hard
34. In B12 deficiency, the MCV (mean corpuscular volume) is typically:
Viwe Answer
Answer: b) >100 fL
Macrocytic anemia due to impaired DNA synthesis. GATE BT 2022
Level: Easy
35. The body's total vitamin B12 store is approximately:
Viwe Answer
Answer: b) 2–5 mg
Mostly in liver; lasts 3–5 years. CSIR-NET 2019
Level: Medium
36. Which of the following is a cause of functional B12 deficiency?
Viwe Answer
Answer: c) Nitrous oxide anesthesia
Inactivates B12 by oxidizing cobalt; normal absorption. IIT-JAM BT 2023
Level: Hard
37. The treatment of choice for pernicious anemia is:
Viwe Answer
Answer: b) IM cyanocobalamin...
Lifelong parenteral therapy due to permanent IF loss. CSIR-NET 2020
Level: Medium
38. The vitamin B12–intrinsic factor complex is absorbed via:
Viwe Answer
Answer: c) Receptor-mediated endocytosis
Via cubam receptor in ileum. GATE BT 2021
Level: Medium
39. Which of the following is elevated in both B12 and folate deficiency?
Viwe Answer
Answer: b) Homocysteine
Due to impaired methionine synthase. CSIR-NET 2022
Level: Medium
40. The corrin ring in vitamin B12 is structurally similar to:
Viwe Answer
Answer: c) Porphyrin ring
Corrin is a contracted porphyrin-like structure with cobalt. IIT-JAM BT 2019
Level: Hard
41. The only vitamin synthesized exclusively by microorganisms is:
Viwe Answer
Answer: c) Vitamin B12
Humans cannot synthesize B12; obtained from animal foods or gut bacteria (not absorbed). CSIR-NET 2021
Level: Easy
42. Fish tapeworm (Diphyllobothrium latum) infestation causes B12 deficiency by:
Viwe Answer
Answer: b) Competing for luminal B12
Worm absorbs large amounts of B12 → host deficiency. IIT-JAM BT 2020
Level: Medium
43. The diagnostic test that distinguishes B12 deficiency from folate deficiency is:
Viwe Answer
Answer: b) Urinary methylmalonic acid
MMA ↑ only in B12 def.; both ↑ homocysteine. CSIR-NET 2023
Level: Hard
44. In B12 deficiency, neurological damage is thought to be mediated by:
Viwe Answer
Answer: c) Accumulation of methylmalonic acid...
MMA is neurotoxic; B12 needed for myelin lipid synthesis. GATE BT 2022
Level: Hard
45. The active transport of vitamin B12 into enterocytes requires:
Viwe Answer
Answer: c) Calcium ions
Metformin reduces Ca²⁺ → ↓ B12 absorption. CSIR-NET 2022
Level: Hard
46. The enzyme that converts cyanocobalamin to active coenzyme forms in the body is:
Viwe Answer
Answer: c) Cobalamin reductase
Removes cyanide group and reduces Co³⁺ → Co⁺ for coenzyme attachment. IIT-JAM BT 2021
Level: Hard
47. A vegan individual is most likely to develop B12 deficiency after how many years of strict veganism?
Viwe Answer
Answer: c) 3–5 years
Depletion time of liver stores (2–5 mg). CSIR-NET 2020
Level: Medium
48. The peripheral blood smear in B12 deficiency classically shows:
Viwe Answer
Answer: c) Macro-ovalocytes and hypersegmented neutrophils
Hallmark of megaloblastic anemia. GATE BT 2021
Level: Easy
49. High-dose oral B12 therapy (1000–2000 μg/day) works in malabsorption because:
Viwe Answer
Answer: b) 1–2% is absorbed by passive diffusion
Sufficient to meet daily needs (2.4 μg). CSIR-NET 2023
Level: Medium
50. The cobalt in vitamin B12 is present in which oxidation state in the active coenzyme?
Viwe Answer
Answer: b) Co⁺
Reduced from Co³⁺ (cyanocobalamin) to Co⁺ for nucleophilic attack in reactions. IIT-JAM BT 2022
Level: Hard