Vitamin+B12+and+Folate+Metabolism

=**Objectives**=


 * 1. Describe the general structure of vitamin B12, the forms in which it is normally ingested, and the active coenzyme forms.** (p.198-199)

Vitamin B12 contains a corrin ring coordinated with a cobalt and adimethylbenzimidazole ribonucleotide. The corrin ring is simular to the protoporphyrin ring in that both have tetrapyrrole structures that coordinate metals through 4 nitrogens. Different substituents coordinate to cobalt to give rise to the different forms of cobalamin:


 * -CN (Cyano) - cyanocobalamin; the most stable form, used commercially for vitamin form.
 * -OH (Hydroxyl) - hydroxycobalamin; the commonly injested form.
 * -CH3 (Methyl) - methylcobalamin; coenzymefor methionine synthase.
 * -5'-deoxyadenosyl - deoxyadenosylcobalamin; coenzymefor methylmalonyl-CoA Mutase.


 * 2. Describe the role of intrinsic factor and the transcobalamins in the uptake and storage of vitamin B12.** (p.204-205)

Intrinsic factor is a protein produced in the stomach that is required for upake of vitamin B12. Deficiency in intrinsic factor can be caued by deterioration of the stomach lining as is the case for chronic atrophic gastritis, an autoimmune disease. Inablility to produce intrinsic factor resulted in a deficiency of vitamin B12 uptake which can cause pernicious anemia.

Intrinsic factor binds to vitamin B12 to form a complex that binds to transcobalamin II for entry into circulation. Vitamin B12 is stored in the liver bound to transcobalamin I.


 * 3. Name and write the two enzyme-catalyzed reactions in humans that require cobalamin cofactors.** (p.200-203)

__Methionine Synthase__ Vitamin B12 is necessary for the conversion of Homocysteine --> Methionine. Methyl-THF gives a -CH3 to cobalamin which donates the -CH3 to convert homocysteine to methionine. This reaction occurs in the cytosol and uses methylcobalamin and methyl-THF as a required cofactor.

Methionine can be converted to S-adenosylmethionine (SAM) which can donate a -CH3 and be converted to S-adenosylhomocystine (SAH); SAH can be converted to homocysteine. Deficiency in B12 will result in the buildup of homocysteine which is a risk factor for CHD.

__Methylmalonyl-CoA Mutase__ Vitamin B12 is necessary to convert methylmalonyl-CoA to succinyl-CoA. This reaction is important for the metabolic pathway of of isoleucine, valine, methionine, odd chain fatty acids, and cholesterols into propionyl-CoA --> methylmalonyl-CoA --> Succinyl-CoA for use in the TCA cycle and for gluconeogenesis. This reaction occurs in the mitochondria and uses adenosylcobalamin.


 * 4. Compare B12 and folate daily requirements, acquisition, and storage in the body.** (p.205, 210-211)

__Vitamin B12__ Daily Requirements - Esssential vitamin; RDA3 micrograms/day.

Acquisition - B12 is made only by microorganisms; richest dietary source is in meat, making strict vegetarians especially at risk for deficiency Storage - Stored in large amounts in livier bound to transcobalamin I; supply is sufficient for 3-5 years; unusual for a water-soluable vitamin to be stored.

__Folate__ Daily Requirements - RDA 200 micrograms/day

Acquisition - Found in leafy vegetables, liver, and supplemented foods like bread; injested as folyl polyglutamates which must be cleaved off before absorption; transported in blood as 5-methyl THF; after uptake by cells, 5-methyl THF is converted to THF by methionine synthase and then polyglutamylated.

Storage - Stored to some extent in the liver (5-10 mg); sufficient for 3-6 months.


 * 5. Describe the metabolic effects of B12 deficiency.** (p.204, 212-213)

B12 deficiency can result in prenicious anemia and megaloblastic anemia when lack of intrinsic factor results in inability to absorb B12. Additionally, lack of B12 can result in buildup of homocysteine when it is unable to regenerate methionine. Homocysteinemia is recognized as a major risk factor for atherosclerotic cardiovascular disease. Neural tube defects can rise in early fetal development due to B12 deficiency due to insufficient nucleotide synthesis in rapidly dividing cells.


 * 6. Explain how B12 and folate deficiencies can induce megaloblastic anemia, and how to distinguish between the two.** (p.213-215)

Deficiency in B12 or folate can cause megaloblastic anemia as DNA synthesis becomes disrupted in rapidly developing cells in the bone marrow due to blocked nucleotide synthesis. Diagnosis should include measurement of serum folate, B12,and methylmalonic acid. Methylmalonic acid would be elevated only in B12 deficiency. Determination of which deficiency is important because B12 deficiency can cause neurological problems (demyelination) that is not found in folate deficiency.

Supplemental folate in breads can mask B12 deficiency, making it difficult to diagnosis because of the hidden anemia symptoms.


 * 7. Recognize the chemical structures and different active forms of the folate coenzymes.** (p.208-209)

Folate consists of a pteridine ring, p-Aminobenzoic acid, and glutamate. Folate exists in 3 oxidation states: folate, dihydrofolate (DHF), and tetrahydrofolate (THF). Reduction is catalyzed by dihydrofolate reductase and requires NADPH. The active form of the coenzyme is THF

Folate serves as a single-carbon donor with single carbon groups bound to the N5, N10, or both N5 and N10 positions.


 * 8. List 5 reactions that require folate coenzymes.** (p.209)


 * Coenzyme**||**Reaction**||**Product**||
 * 5-Formimino THF||N-Formiminoglutamate (FIGLU) -->||Glutamate||
 * 5-Methyl THF||Homocystein -->||Methionine||
 * 5,10-Methylene THF||Glycine -->||Serine||
 * 5,10-Methylene THF||dUMP -->||dTMP||
 * 10-Formyl THF||Glycineamide ribosyl-5P -->||Formylglycineamide ribosyl-5P||
 * 10-Formyl THF||Aminoimidazole carboxamide ribosyl-5P -->||Formimidoimidazole carboxamide ribosyl-5P||


 * 9. Describe the folate trap hypothesis and how it explains some of the symptoms of B12 deficiency.** (p.211-212)

Folate trap hypothesis may expain why B12 deficiency induces effective folate deficiency. B12 is required for methionine synthase reaction which converts 5-methyl THF to THF. THF, but not 5-methyl THF, can serve as the precursor for most of the important folates required by the cell. Accumulation in 5-methyl THF and deficit of THF effectively produces a folate deficiency.

As a result, B12 deficiency effectively is linked to folate deficiencies and exhibits many characteristics of folate deficincy including homocysteinemia, neural tube defects in fetus, and megaloblastic anemia.


 * 10. Explain the importance of folate polyglutamylation.** (p.207)

Folate in circulation contains a single glutamate residue. After it is taken up by cells, up to seven glutamate residues are added by an amide bond to the gamma-carboxyl group. Polyglutamylation serves two functions:


 * Greater retention within the cell
 * Increases affinity as coenzyme for enzymes


 * 11. Describe the mechanism of inhibition of bacterial growth by sulfonamides.** (p.206, 215-216)

Sulfa drugs act as analogs of the p-aminobenxoic acid, a component of folate. They inhibit endogenous folate sythesis which only occurs in bacteria (humans rely on ingested folate only). Bacteria DNA synthesis is blocked because of folate deficiency and is unable to synthesize nucleotides.

Bacterim is a first line antibiotic that combines sulfa drug sulfamethoxazole with trimethoprim, a dihydrofolate reductase (DHFR) inhibitor that prevents bacteria from recycling their existing folate.