BIOCHEMISTRY OF FOLATE

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BIOCHEMISTRY OF FOLATE AND VITAMIN B12 Sept 25,
2013 BASIC BIOCHEMICAL PATHWAYS
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IMPORTANCE OF FOLATE AND B12 IN NUTRITION KEY
FUNCTIONS Folate is essential for DNA
replication, because its needed for a critical
nucleic acid In addition, folate is needed for
most of the activities of B12 synthesis
of neurotransmitters, and lipids in the central
nervous system. Their biochemistry is closely
interrelated.
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RESOURCE THE
MEDICAL BIOCHEMISTRY PAGE http//themedicalbioc
hemistrypage.org/home.html
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KEY POINTS ABOUT FOLATE AND B12 They both
donate METHYL groups to other molecules. -METHYLE
NE-THF donates a CH2 group, that is needed for
DNA synthesis -METHYL-THFis a CH3 donor for
many biochemical steps that use vitamin
B12. These slides outline important
pathways that involve folate and B12.
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DIETARY FORMS, TRANSPORT FORMS, AND FOLATE INSIDE
THE CELL
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CLEAVAGE SITE, FOR ENYZME IN DUODENUM
TETRAHYDROFOLATE, WITH 3 GLUTAMIC ACID
SIDE CHAINS. THIS IS TYPICAL OF FOLATE IN
FOODS SUCH AS VEGETABLES. Only the monoglutamate
can be absorbed by humans. If a person has a
digestive disorder, the monoglutamate is
provided, since its absorbed more easily than
the form available naturally in foods. Vitamin
pills, flour, Wheaties, etc only have the
monoglutamate.
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Product of enzyme cleavage Folate-monoglutamate
This form is absorbed through the small
intestine, carried in plasma to the tissues, and
then taken up into cells. After it enters cells,
more glutamate residues are added, to remake the
POLYGLUTAMATE
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POLYGLUTAMATE RESYNTHESIZED IN CELLS. It does
not cross the cell membrane, so it remains inside
the cells as part of the folate pool.
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REDUCTIONS AND CARBON TRANSFERS The formation of
the active metabolites of folate
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FOLIC ACID, WITH A SINGLE GLUTAMATE Typical of
whats present in vitamins and supplements.
NADPH H
NADP
The first double bond is removed by NADPH this
is DIHYDROFOLATE.
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DIHYDROFOLATE (DHF) FIRST ACTIVE FORM IN
PATHWAYS IN THE CELL
TETRAHYDROFOLATE (THF) SECOND ACTIVE FORM IN
CELL METABOLISM
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-CH2-
C
ADDITION OF METHYLENE (CH2) TO
MAKE METHYLENE-TETRAHYDOFOLATE
(PLP)
H
H
OH-CH2-C-COOH
H-C-COOH
GLYCINE
SERINE
THE METHYLENE GROUP (CH2) COMES FROM SERINE.
THE REACTION IS B6-DEPENDENT
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NADHH NAD
CH3
METHYL TETRAHYDROFOLATE
METHYLENE TETRAHYDROFOLATE
The molecule on the left donates CH2 for DNA
synthesis the molecule on the donates CH3 in
many reactions. The reaction also needs
riboflavin (FAD). THE REDUCTION OF METHYLENE-THF
TO METHYL-THF IS NOT REVERSIBLE!
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The previous reaction, the conversion
of METHYLENE-THF to METHYL-THF, is done by an
important enzyme called Methylene-tetrahydrofolate
-reductase (MTHFR). There are major genetic
variants (polymorphisms) in humans, across
different populations, and these variants have
significant effects. We will discuss this
extensively on Oct 2.
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The previous slides illustrate all the forms of
folate that will be shown in the next
slides. These slides show how folate
pathways are inter-related, with transfer of CH2
(for DNA synthesis) and for CH3 (methylation
pathways using B12)
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FOLATE PROVIDES METHYL GROUPS BY TWO DIFFERENT
CYCLES
DNA synthesis
DIETARY FOLATE
REDUCTION BY NAPDH
DIHYDROFOLATE
dTMP (methylated)
REDUCTION BY NAPDH
CYCLE 1
TETRAHYDROFOLATE
SERINE
dUMP
METHYL GROUP ADDITION
CYCLE 2
GLYCINE
METHYLENE TETRAHYDROFOLATE
REQUIRES Vit B12
REDUCTION BY NADH
METHYL TETRAHYDROFOLATE
HCys
Met
S-adenosyl- HCys
S-adenosyl- Met
SUBSTRATE
METHYLATED SUBSTRATE
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METHYLENE-THF DONATES CH2 FOR DNA SYNTHESIS
DEOXYTHYMIDINE (dTMP) REQUIRED FOR DNA
SYNTHESIS, AND FOR CELL DIVISON
THIS PATHWAY IS VERY ACTIVE IN RAPIDLY-DIVIDING
CELLS
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METHYL-THF Transfers a CH3 group (which requires
B12) to make S-adenosyl-methionine (SAM)
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This methyl came from methyl-THF, and will next
be donated to a recipient molecule.
S-adenosyl-methionine (often called SAM) is a
very important CH3 donor. It participates in
hundreds of pathways. The function of the
methyl-THF/B12 pathway is to make SAM. The
intermediates were shown on a previous slide.
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From S-adenosyl methionine
There are many reactions like this!
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This is a VERY complicated slide. It outlines
the REQUIREMENT for B12, as intermediate to make
SAM. Study of B12 function is a very advanced
topic.
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The complex stucture of vitamin B12. There is
an atom of cobalt in each B12 molecule.
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An additional form of folate, FORMINOTETRAHYDROFO
LATE, is necessary for the catabolism of the
amino acid HISTIDINE. The folate functions as the
ACCEPTOR for the amino group, and the histidine
carbon skeleton is finally converted to
alpha-keto-glutarate.
The NH came from the amino acid
Folate deficiency causes a change in histidine
metabolism. This is the basis of a useful test
that examine metabolites in urine. This will be
examined in the lecture on October 2.
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SUMMARY OF BASIC FOLATE/B12 PATHWAYS -Dietary
folate is absorbed, transported to cells, and
converted back to a polyglutamate -A complex
series of reductions and methyl transfers occurs,
which involves serine -One form (methylene-THF)
transfers CH2 to make deoxythymidine, needed for
DNA synthesis and cell division -Another form
(methyl-THF) transfers CH3, to make
S-adenosyl-methionine (SAM), a step that requires
B12. The SAM molecule is a key methyl donor for
many pathways in the cell
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• Megadose supplements and health benefits
• In the last 50 years, 3 separate vitamins
• have been promoted as having miraculous
• benefits, when consumed high levels
• Vitamin C 1-5 grams/day, for cold prevention
• Vitamin E 0.5-1 gram/day, to prevent CHD
• Folate (5 mgs/day)/B12 (100 ugs/day), also
• to decrease CHD
• Benefits are questionable, but folate is
  protective
• against birth defects, a highly reliable
  observation.

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Ten years ago, the decision was made to add
folate to flour, breakfast cereal, bakery
products, and many other consumer goods. As a
result, folate deficiency is now very rare in
many parts of the world. Why was this decision
taken?
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SPINA BIFIDA Can occur early in the first
trimester. Incidence reduced 50-75 by folate
supplementation! The risk may be higher if the
embryo has the thermolabile variant of MTHFR (to
be discussed shortly).
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Summary of a study done between 1985-1990 The
dose was 4 mg/day, given to women who
had previously had a child with an
NTD. ASSIGNMENT Review the study (posted) that
describes prevention of NTD with supplements of
dietary folate.
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There is evidence now that 1 mg/day
is protective. We will discuss optimal doses of
folate on Oct 2.