Download Why should we study B12 and Folate? Deficiencies in both are still

B12 and Folate
10/17/2008 4:39:00 PM
Why should we study B12 and Folate?
 Deficiencies in both are still somewhat common.
 Deficiencies can cause major health problems and birth defects
 They present very similarly. Learning how to differentiate is very
important for treatment
Vitamin B12
 Cobalamin
o Corrin Ring with Cobalt in the center
o Dimethylbenzimidizole ribonucleotide (ribonucleotide-esque)
o R-Group
Cyano (-CN) is found in cyanocobalamin. Cyano- is the
most common form as it is the form that is marketed
 Hydroxyl (-OH) is found in meat and is the most
commonly ingested
 Methyl (-CH3) Methylcobalamin is the form found in
Methionine synthase
 5’ deoxyadenosyl is the coenzyme for methylmalonyl
CoA Mutase
o Cobalt interacts with 6 other atoms


 Four nitrogen from the corrin ring
 1 Nitrogen from the benzimidizole
 1 atom from the R group
Why is B12 required?
o Two enzymes
 Methionine Synthase
 Methylcobalamin acts a methyl donor
 Methyl-THF is required as it regenerates the
methylcobalamin


Methionine Cycle
 Methionine is converted to S adenosyl
Methionine, which is a methyl donor.
Product is Homocysteine
 Homocysteine is converted back to
methionine by methionine synthase
Deficiency leads
 Homocysteinuria/Homocysteinemia



o Independent risk factor for CAD
 Effective folate deficiency as Methyl-THF
cannot be converted to THF so it can be
used in other reactions
o Folate is transported as methyl-THF
and only methionine synthase can use
methyl-THF
Methylmalonyl CoA Mutase
 Converts Methylmalonyl CoA that is generated
during branched amino acid metabolism (among
other things)
B12 is stored in the liver
o Supply is sufficient for 3-5 years
o RDA of 2 µg per day
o Pernicious Anemia—Lack of B12 due to lack of intrinsic factor
(result of deteriorating stomach lining)
o B12 is only made by microorganisms, but is richest in meat.
 B12 travels up the food chain
B12 Uptake
o B12 is bound to its R group with the other proteins that it’s
related to.
o Stomach acid degrades the protein and releases B12. R
Binder protein made in the salivary glands will then bind to
the R Group and transport it to the small intestine.
o In the Small intestine, proteases chew up the R binder protein
and the B12 is released. It will then bind to intrinsic factor
and be marked for endocytosis.
o B12 is transported in the blood by Transcobalamin I and
stored in the liver as Transcobalamin II
Folate

Structure
o Three major parts
 Pterin Ring
 P-aminobenzoic acid (PABA)
 glutamate
o Three major variables



Polyglutaminylation
 Can have many glutamines added to the 
carboxyl of the glutamine
Different reduction states
 Different amounts of hydrogen
 Three reduction states: Folate, DHF, THF
Addition of 1 carbon units to the ring
 N5 and/or N10 involved in binding
 Allows folate to add 1 carbon units to compounds
 Carbon substituents on folate are interchangeable
with the exception of methyl


Uptake
o Folate is found in leafy vegetables
o Transported as methyl THF
o Cannot be used for other reactions until converted to THF by
methionine synthase
o RDA 200 µg per day (400-800 µg per day for pregnant
women)
o 5-10 mg stored in liver
 Sufficient for 3-6 months
Importance (if deficient this happens)
o Homocysteinemia/Homocysteinuria is caused by the inability
to regenerate methylcobalamin
o Neural Tube Defects in developing fetus due to lack of
nucleotide synthesis
o Megaloblastic Anemia due to lack of nucleotide synthesis
o Most symptoms of folate deficiency are seen in B12 deficiency
because if there is no action of methionine synthase, there
will be no available THF.
 Sulfa Drugs inhibit folate synthesis in bacteria by substituting
analogs of p-aminobenzanoic acid. They only affect bacteria
because we don’t synthesize our own folate.
Diagnosis
 Test for serum folate, serum B12 and methylmalonic acid.
o Methylmalonic acid will be high in B12 deficient persons but
normal in folate deficient persons.
Objectives
10/17/2008 4:39:00 PM
1. Describe the general structure of Vitamin B12, the forms in which it is
normally ingested and the active coenzyme forms.
a. Corrin Ring
b. Benzimidizole
c. R group
i. Ingested form is usually hydroxycobalamin
ii. Vitamin form is cyanocobalamin
iii. Methionine Synthase form is methylcobalamin
iv. Methylmalonyl CoA Mutase form is 5’ deoxyadenosylcobalamin
2. Describe the role of intrinsic factor and the transcobalamins in the uptake
and storage of Vitamin B12
a. Intrinsic factor is necessary to signal for B12 endocytosis.
Transcobalamins are necessary for transport in the blood
(transcobalamin I) and storage in the liver (transcobalamin II)
3. Name and write the two enzyme catalyzed reactions in humans that
require cobalamin
a. Methionine Synthase
b. Methylmalonyl CoA Mutase
4. Compare B12 and folate daily requirements, acquisition and storage in
the body.
a. B12—2 µg per day stored in the liver for 3-5 years before deficiency
5. Describe the metabolic effects of B12 deficiency.
a. B12 Deficiency will decrease nucleotide synthesis and amino acid
metabolism. The former of the two is way worse.
6. Explain how B12 and folate deficiencies can induce megaloblastic anemia,
and how to distinguish between the two.
a. Megaloblastic anemia is caused by deficiencies in DNA synthesis
needed or dividing cells. B12 and Folate deficiencies can be
differentiated by testing serum levels of both B12, Folate and
methylmalonyl CoA. If the last compound is elevated, you know
the problem is with B12. If it is normal, the problem is Folate
7. Recognize the chemical structures and different active forms of the folate
coenzymes
a. Pterin Ring
b. p-aminobenzoic acid (PABA)
c. Glutamic Acid
8. List 5 reactions that require folate coenzymes
a. Methylation of Cobalamin
b. Addition of the C8 carbon on purine synthesis
c. Addition of the C2 carbon in purine synthesis
d. Methylation of uracil to make thymidine
e. Serine-glycine hydroxymethyl transferase (it makes methyl THF in
reverse or uses methyl THF to make serine)
9. Describe the folate trap hypothesis and how it explains some of the
symptoms of B12 deficiency.
a. If there is a B12 deficiency, there will not be any methyl THF
converstion to THF so the cell cannot make any of the other one
carbon THF molecules. This is called functional THF deficiency.
Lack of B12 causes a lack of functional Folate
10. Explain the importance of folate polyglutamylation
a. Polyglutamylation causes increased retention of Folate in the cell.
11. Describe the mechanism of inhibition of bacterial growth by
sulfonamides.
a. Sulfonamides imitate the p-aminobenzanoic acid molecules
necessary for folate production. It mucks up the synthetic pathway
and causes folate deficiency.
10/17/2008 4:39:00 PM