Medicine & USMLE

Vitamin B12 (Cobalamin) Biochemistry

  1. Vitamin B1 (Thiamine) Biochemistry
  2. Vitamin B1 (Thiamine) Deficiency
  3. Vitamin B2 (Riboflavin)
  4. Vitamin B3 (Niacin) Biochemistry
  5. Vitamin B3 (Niacin) Deficiency and Excess
  6. Hartnup Disease
  7. Vitamin B5 (Pantothenic Acid)
  8. Vitamin B6 (Pyridoxine)
  9. Vitamin B7 (Biotin)
  10. Vitamin B9 (Folate)
  11. Vitamin B12 (Cobalamin) Biochemistry
  12. Vitamins B9 and B12 Deficiencies
  13. Vitamin A (Retinol) Biochemistry
  14. Vitamin A (Retinol) Deficiency and Excess
  15. Vitamin C (Ascorbic Acid) Biochemistry
  16. Vitamin C (Ascorbic Acid) Deficiency and Excess
  17. Vitamin D Biochemistry
  18. Vitamin D Deficiency and Excess
  19. Vitamin E (Tocopherol/Tocotrienol)
  20. Vitamin K Biochemistry
  21. Vitamin K Deficiency
  22. Zinc
  23. Kwashiorkor and Marasmus


Vitamin B12 (Cobalamin) is a water-soluble vitamin important in many metabolic reactions. Cobalamin plays roles as a cofactor for the enzymes methionine synthase and methylmalonyl-CoA mutase. It is important in the nervous system for its role in myelin synthesis, as well as in the bone marrow for the maturation of red blood cells. Vitamin B12 is primarily obtained from the diet in meat products, as it is synthesized by the gut bacteria in animals. Before cobalamin can be absorbed, however, parietal cells in the stomach must first secrete intrinsic factor. Intrinsic factor then binds to cobalamin, after which it can be absorbed in the terminal ileum.

Key Points

  • Cobalamin (Vitamin B12)
    • Cofactor for several enzymatic reactions, including:
    • Found in animal/meat products
      • synthesized by colonic flora (no absorption from colon in humans)
      • deficiency in strict vegetarian diets
    • Absorbed in terminal ileum when coupled to intrinsic factor (IF)
      • deficiency in Crohn’s disease (disrupted absorption at terminal ileum)
      • deficiency in pernicious anemia or gastric bypass (lack of IF)

Find Vitamin B12 Biochemistry and other Vitamins among Pixorize's visual mnemonics for the USMLE Step 1 and NBME shelf exams.