Step by Step
Str
Cobalamin structure — the most complex vitamin
Vitamin B12 (cobalamin) is the largest and most structurally complex vitamin — a cobalt-containing corrin ring. It exists in several forms: cyanocobalamin (supplement), methylcobalamin (methionine synthase reaction), adenosylcobalamin (methylmalonyl-CoA mutase reaction). The cobalt atom in the center is essential for its enzymatic function.
Memory trick: B12 = cobalt core. Complex molecule, complex absorption, complex deficiency.
Abs2
Unique absorption mechanism
B12 absorption is unique: Stomach parietal cells secrete intrinsic factor (IF). B12 binds IF in the stomach → B12-IF complex → absorbed by specific receptors in terminal ileum → transported to blood bound to transcobalamin II. Causes of malabsorption: pernicious anemia (autoimmune anti-parietal cell or anti-IF antibodies), gastrectomy, terminal ileal disease (Crohn's), surgical resection. Bacteria compete for B12 in 'blind loop syndrome'.
Enz
Two enzymatic roles
(1) Methylcobalamin + methionine synthase: homocysteine + methyl-THF → methionine + THF. Regenerates THF (active folate) AND makes methionine (needed for SAM — the universal methyl donor). (2) Adenosylcobalamin + methylmalonyl-CoA mutase: methylmalonyl-CoA → succinyl-CoA. Essential for metabolism of odd-chain fatty acids, valine, isoleucine, threonine, methionine.
Memory trick: B12 has Two jobs: Methionine (methyl-cobalamin) and Methylmalonyl mutase (adenosyl-cobalamin).
Neuro
Neurological damage — the distinguishing feature
B12 deficiency causes subacute combined degeneration (SCD) of the spinal cord: dorsal columns (vibration, proprioception) + lateral corticospinal tracts (upper motor neuron signs). Mechanism: defective myelin synthesis (impaired methionine → SAM → methylation reactions). Lab: elevated MMA (methylmalonyl-CoA mutase deficiency) AND elevated homocysteine. Schilling test (now rarely done) identified the level of B12 absorption defect.
Applied Walkthrough
1
A strict vegan (no animal products) has adequate dietary folate but no B12 intake. After 5 years, liver B12 stores are depleted.
2
Labs: megaloblastic anemia (large RBCs, hypersegmented neutrophils). Elevated MMA and homocysteine. B12 level low.
3
Neurological exam: loss of vibration sense at ankles (dorsal column), brisk reflexes with upgoing plantar response (lateral column — UMN signs) = subacute combined degeneration.
4
Treatment: IM B12 injections (bypasses absorption problem). Folate supplementation alone MUST be avoided — it corrects the anemia but allows neurological damage to progress.
Exam Application
Exams test the IF requirement, causes of deficiency (pernicious anemia, gastrectomy, terminal ileum disease, veganism), the two enzymatic roles (methionine synthase + methylmalonyl-CoA mutase), SCD as the neurological manifestation, and why MMA is elevated in B12 (not folate) deficiency. The folate trap and the danger of folate-only treatment are repeated from the Folate and B12 lesson.
⚠ Common Trap
Students mix up B12 with folate in the 'megaloblastic anemia' category — both cause it, but ONLY B12 causes neurological damage. The distinguishing tests: MMA elevated = B12 deficiency. Homocysteine elevated = both B12 and folate deficiency. Anti-intrinsic factor antibodies = pernicious anemia specifically.
✓ Quick Self-Check
1. What two coenzyme forms does B12 exist in?
Methylcobalamin (for methionine synthase) and adenosylcobalamin (for methylmalonyl-CoA mutase).
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2. What is pernicious anemia?
Autoimmune destruction of gastric parietal cells (or anti-IF antibodies) → no intrinsic factor → B12 malabsorption → megaloblastic anemia + neurological damage.
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3. Why is MMA elevated in B12 deficiency?
Methylmalonyl-CoA mutase requires adenosylcobalamin — without B12, methylmalonyl-CoA cannot be converted to succinyl-CoA → MMA accumulates.
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4. What spinal cord tracts are damaged in subacute combined degeneration?
Dorsal columns (vibration, proprioception) and lateral corticospinal tracts (UMN signs) — hence 'combined.'
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5. How is B12 deficiency treated when caused by pernicious anemia?
IM (intramuscular) B12 injections — bypasses the IF-dependent intestinal absorption that is defective.
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