Vitamin B 12

1. Vitamin B12 Julie Roche 12/7/04

2. Vitamin B12 • Cobalamin • Porphyrin ring • Cobalt • Nucleotide • Net charge = +1

3. Vitamin B12 Derivatives • Cyanocobalamin (digested form) • Hydroxycobalamin • Chlorocobalamin • Methylcobalamin • Adenosylcobalamin • (5’-deoxyadenosylcobalamin) physiological useful forms

5. Biosynthesis of Vitamin B12 • One of the most complicated biosynthetic pathways • Involves over 30 enzymes • Two pathways • Aerobic • anaerobic • 16 intermediates between uroporphyrinogen III and adenosylcobalamin (aerobic pathway)

7. Difference between pathways • Need for molecular O2 • Aerobic: between precorrin-3a and b • Point of Cobalt addition • Anaerobic: added early, between uroporphyrinogen III and precorrin-2 • Aerobic: added late, between hydrogenobyrinic acid a,c-diamide and cob(II)lyrinic acid a,c-diamide *Anaerobic synthesis is more difficult

8. 13C labeling (1990) NMR spectroscopy Enzymology Molecular genetics Developments Leading to the Discovery of the Biosynthetic Pathway of Vitamin B12

9. Vitamin B12 • Produced on an industrial level • Fermentation • Methanosarcina • Butribacterium • Acetobacterium • Propionibacterium • Produces largest amt of cobalamin • Secretes propionic & acetic acid • Propionic acid inhibits cobalamin production

10. Homocysteine Methionine Methylmalonyl CoA Succinyl CoA Vitamin B12 affects two Major Pathways

11. The effects of Vitamin B12 on the conversion of homocysteine to methionine Homocysteine Methionine Synthase Methylcobalamin Methionine 5-methyltetrahydrofolate tetrahydrofolate Tetrahydrofolate methyltransferase

12. The effects of Vitamin B12 on the conversion of Methylmalonyl CoA to Succinyl CoA Methylmalonyl Co A Methylmalonyl CoA mutase Adenosylcobalamin Succinyl CoA Kreb's cycle

13. Vitamin B12 Deficiency • homocysteine and methylmalonyl CoA • Increase in methylmalonyl CoA • Increased enzyme activity in fatty acid synthesis • Build up of odd fatty acids around peripheral nerves • Increase in homocysteine • Vascular/nervous problems

14. Vitamin B12 Deficiency • Excess homocysteine & MMA excreted in urine • Diagnosis for cobalamin deficiency • Methylmalonyl CoA mutase & Methionine synthase affect amino acid metabolism • Amino acid metabolism inhibited by deficiency

15. Vitamin B12 Binding Proteins Transcobalamin I Transcobalamin II Transcobalamin III

16. Transcobalamin I • R-type binding protein • 33% is carbohydrate • Molecular weight = 125,000-150,000 • Beta globulin • Contains more sialic acid than transcobalamin III • Carries ~80% of Vitamin B12 in blood • Vitamin B12 has half-life of 10-12 days when bound to it

17. Transcobalamin II • Molecular weight = 38,000 • Alpha globulin • NOT a glycoprotein • Carries less than 25% of Vitamin B12 in blood • Vitamin B12 has half-life of under 1 ½ hours when bound to it • Encourages absorption in a number of tissues • Degenerates once B12 is released • B12 then recirculates • Transcobalamin II deficiency results in pernicious anemia

19. Transcobalamin III • R-type binding protein • 33% is carbohydrate • Molecular weight = 125,000-150,000 • Alpha globulin • Released from granulocytes • Contains more fucose than transcobalamin I

20. Antibacterial Roles of Transcobalamin I & III • Binds to large amounts of vitamin B12 and carries it to liver • Excreted in bile • Prevents bacteria from using the vitamin for growth

21. Vitamin B12 separated from Foods in stomach Binds to Transcobalamin I and III Binds to receptors on Liver cells Pinocytosis Binding Proteins Degraded Within 1 ½ hrs Cobalamin 20% excreted in bile 80% binds to BP & reenters blood BP degenerates, IF binds to cobalamin Carried to Ileum and absorbed in by endocytosis Released and bound to Transcobalamin II Returns to liver or Carried to other tissues

23. Sources of Vitamin B12 • Fish • Eggs • Meat • Dairy Products 2.4

24. Vitamin B12 deficiency • Cobalamin level in blood = below 200 pg/ mL • Common in elderly

25. Causes of Vitamin B12 Deficiency Most Common • Malabsorption (inability to absorb food containing cobalamin) • Inability to separate cobalamin from food in stomach • Lack of recommended intake • Inability to use/store cobalamin • Proton pump inhibitors • Gastritis • Stomach/bowel resection • Chron’s disease • Pancreatitis • Gastric lymphoma • Myeloma • HIV • Antibiotics • Anticonvulsants • Excess Vitamin C • Nitric Oxide

26. Symptoms/Effects of Vitamin B12 Deficiency • Pernicious Anemia (Vitamin B12 is necessary for RBC production) • Lethargy • Weight loss • Weakness • Dementia • Leucopenia • Thrombocytopenia • Axonal degeneration • Demyelination • Urethral Sphincter problems • Depression • Alzheimer’s Disease • Increased liver weight • Fat accumulation around heart, liver, peripheral nerves

27. Increase Homocysteine MMA Bilirubin excretion LDH Liver glycogen Mitochondrial cristae in liver Hepatic citrate synthase Propionic acid Succinate dehydrogenase Cytochrome c activity Propionyl CoA Amino Acids Cell metabolism Protein synthesis Fatty acid synthesis enzymes ATP citrate lyase Causes rise in Kreb’s cycle Decrease Transcobalamin II Intrinsic factor Symptoms/Effects of Vitamin B12 Deficiency

28. Treatment • Supplements • Oral pills • Intramuscular • Intranasal • Sublingual 1% 500-2000 3-7 days... monthly... 4x year