Vitamin B12

Julie Roche

12/7/04

Vitamin B12

CobalaminPorphyrin ringCobaltNucleotideNet charge = +1                                               

                              

                                                     

                                    

Vitamin B12 Derivatives

Cyanocobalamin (digested form)HydroxycobalaminChlorocobalaminMethylcobalaminAdenosylcobalamin

(5’-deoxyadenosylcobalamin)

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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)

http://www.jbc.org/content/vol275/issue51/images/large/bc5209456002.jpeg

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

Developments Leading to the Discovery of the Biosynthetic

Pathway of Vitamin B12

• 13C labeling (1990)• NMR spectroscopy

• Enzymology• Molecular genetics

                                                                        

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

Vitamin B12 affects two Major Pathways

• Homocysteine

• Methionine

• Methylmalonyl CoA

• Succinyl CoA

The effects of Vitamin B12

on the conversion of homocysteine to methionine

Homocysteine

Methionine

Methylcobalamin

Methionine Synthase

5-methyltetrahydrofolate tetrahydrofolateTetrahydrofolate methyltransferase

The effects of Vitamin B12

on the conversion of Methylmalonyl CoA to Succinyl CoA

Methylmalonyl Co A

Succinyl CoA

Adenosylcobalamin

Methylmalonyl CoA mutase

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

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

Vitamin B12 Binding Proteins

Transcobalamin IR-type binding protein33% is carbohydrateMolecular weight = 125,000-150,000Beta globulinContains more sialic acid than

transcobalamin IIICarries ~80% of Vitamin B12 in blood

Vitamin B12 has half-life of 10-12 days when bound to it

Transcobalamin IIMolecular weight = 38,000Alpha globulinNOT a glycoprotein Carries less than 25% of Vitamin B12 in blood Vitamin B12 has half-life of under 1 ½ hours

when bound to itEncourages absorption in a number of tissuesDegenerates once B12 is released

B12 then recirculatesTranscobalamin II deficiency results in

pernicious anemia

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Amt of B12Carried (%)

TC I

TC II

Transcobalamin IIIR-type binding protein33% is carbohydrateMolecular weight = 125,000-150,000Alpha globulinReleased from granulocytesContains more fucose than

transcobalamin I

Antibacterial Roles of Transcobalamin I & III

Binds to large amounts of vitamin B12

and carries it to liverExcreted in bile

Prevents bacteria from using the vitamin for growth

Vitamin B12 separated from Foods in stomach

Binds to Transcobalamin Iand III

Binds to receptors on Liver cells

20% excreted in bile

80% binds to BP & reenters bloodCobalamin

Binding Proteins DegradedWithin 1 ½ hrs

Pinocytosis

Returns to liver or Carried to other tissues

Released and bound to Transcobalamin II

Carried to Ileum and absorbedin by endocytosis

BP degenerates, IF binds to cobalamin

http://faculty.etsu.edu/currie/New_Folder/endocytosis.JPG

Sources of Vitamin B12

• Fish• Eggs• Meat • Dairy Products                                                                          

Vitamin B12 deficiency

Cobalamin level in blood = below 200 pg/ mL

Common in elderly

Causes of Vitamin B12

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 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

Symptoms/Effects of Vitamin B12

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