Vitamin B12 (Cobalamin) Biochemistry

Summary

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:
    • methionine synthase (in Homocysteine Metabolism)
      • Homocysteine elevated in deficiency
    • methylmalonyl-CoA mutase (in Propionic Acid Pathway)
      • MMA elevated in deficiency
  • 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)

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