Vitamin B 12 Julie Roche

Vitamin B 12  Cobalamin  Porphyrin ring  Cobalt  Nucleotide  Net charge = +1

Vitamin B 12 Derivatives  Cyanocobalamin (digested form)  Hydroxycobalamin  Chlorocobalamin  Methylcobalamin  Adenosylcobalamin  (5’-deoxyadenosylcobalamin)

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Biosynthesis of Vitamin B 12  One of the most complicated biosynthetic pathways  Involves over 30 enzymes  Two pathways  Aerobic  anaerobic  16 intermediates between uroporphyrinogen III and adenosylcobalamin (aerobic pathway)

Difference between pathways  Need for molecular O 2  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

Developments Leading to the Discovery of the Biosynthetic Pathway of Vitamin B C labeling (1990) NMR spectroscopy Enzymology Molecular genetics

Vitamin B 12 Produced on an industrial level Fermentation –Methanosarcina –Butribacterium –Acetobacterium –Propionibacterium Produces largest amt of cobalamin Secretes propionic & acetic acid –Propionic acid inhibits cobalamin production

Vitamin B 12 affects two Major Pathways Homocysteine Methionine Methylmalonyl CoA Succinyl CoA

The effects of Vitamin B 12 on the conversion of homocysteine to methionine Homocysteine Methionine Methylcobalamin Methionine Synthase 5-methyltetrahydrofolate tetrahydrofolate Tetrahydrofolate methyltransferase

The effects of Vitamin B 12 on the conversion of Methylmalonyl CoA to Succinyl CoA Methylmalonyl Co A Succinyl CoA Adenosylcobalamin Methylmalonyl CoA mutase

Vitamin B 12 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

Vitamin B 12 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

Vitamin B 12 Binding Proteins

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

Transcobalamin II  Molecular weight = 38,000  Alpha globulin  NOT a glycoprotein  Carries less than 25% of Vitamin B 12 in blood  Vitamin B 12 has half-life of under 1 ½ hours when bound to it  Encourages absorption in a number of tissues  Degenerates once B 12 is released  B 12 then recirculates  Transcobalamin II deficiency results in pernicious anemia

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

Antibacterial Roles of Transcobalamin I & III BBinds to large amounts of vitamin B 12 and carries it to liver EExcreted in bile PPrevents bacteria from using the vitamin for growth

Vitamin B 12 separated from Foods in stomach Binds to Transcobalamin I and III Binds to receptors on Liver cells 20% excreted in bile 80% binds to BP & reenters blood Cobalamin Binding Proteins Degraded Within 1 ½ hrs Pinocytosis Returns to liver or Carried to other tissues Released and bound to Transcobalamin II Carried to Ileum and absorbed in by endocytosis BP degenerates, IF binds to cobalamin

Sources of Vitamin B 12 Fish Eggs Meat Dairy Products

Vitamin B 12 deficiency  Cobalamin level in blood = below 200 pg/ mL  Common in elderly

Causes of Vitamin B 12 Deficiency  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

Symptoms/Effects of Vitamin B 12 Deficiency  Pernicious Anemia (Vitamin B 12 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

Symptoms/Effects of Vitamin B 12 Deficiency  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

Treatment Supplements –Oral pills –Intramuscular –Intranasal –Sublingual