Transcript LECTURE NOTES: Folate and B12, Part 1

BIOCHEMISTRY OF FOLATE
AND VITAMIN B12:
Oct 1, 2014
BASIC BIOCHEMICAL
PATHWAYS
IMPORTANCE OF FOLATE AND B12
IN NUTRITION: KEY FUNCTIONS
Folate is essential for DNA replication, because
it’s 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.
RESOURCE:
THE MEDICAL BIOCHEMISTRY PAGE
http://themedicalbiochemistrypage.org/home.html
KEY POINTS ABOUT FOLATE AND B12:
They both donate METHYL groups to
other molecules.
-METHYLENE-THF: donates a CH2 group,
that is needed for DNA synthesis
-METHYL-THF:is a CH3 donor for many
biochemical steps that use vitamin B12.
These slides outline important pathways
that involve folate and B12.
DIETARY FORMS, TRANSPORT FORMS,
AND FOLATE INSIDE THE CELL
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 it’s
absorbed more easily than the form available naturally in foods.
Vitamin pills, flour, Wheaties, etc only have the monoglutamate.
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
POLYGLUTAMATE: RESYNTHESIZED IN CELLS.
It does not cross the cell membrane, so it remains
inside the cells as part of the folate pool.
REDUCTIONS AND CARBON TRANSFERS:
The formation of the active metabolites
of folate
FOLIC ACID, WITH A SINGLE GLUTAMATE:
Typical of what’s present in vitamins and supplements.
NADPH + H+
NADP+
H
H
The first double bond is removed by NADPH:
this is DIHYDROFOLATE.
DIHYDROFOLATE (DHF):
FIRST ACTIVE FORM
IN PATHWAYS IN THE CELL
TETRAHYDROFOLATE (THF):
SECOND ACTIVE FORM
IN CELL METABOLISM
+ -CH2C
ADDITION OF METHYLENE (CH2): TO MAKE
METHYLENE-TETRAHYDOFOLATE
H
OH-CH2-C-COOH
N
H2
SERINE
(PLP)
H
H-C-COOH
N
H2
GLYCINE
THE METHYLENE GROUP (CH2) COMES FROM
SERINE. THE REACTION IS B6-DEPENDENT
NADH+H+
C
METHYLENE
TETRAHYDROFOLATE
NAD+
CH3
METHYL
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!
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.
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)
FOLATE PROVIDES METHYL GROUPS
BY TWO DIFFERENT CYCLES
DNA
synthesis
DIETARY
FOLATE
REDUCTION BY
NAPDH
DIHYDROFOLATE
dTMP
(methylated)
dUMP
REDUCTION BY
NAPDH
CYCLE
#1
TETRAHYDROFOLATE
SERINE
METHYL GROUP
ADDITION
GLYCINE
CYCLE
#2
METHYLENE
TETRAHYDROFOLATE
REQUIRES
Vit B12
REDUCTION
BY NADH
METHYL
TETRAHYDROFOLATE
HCys
S-adenosylHCys
METHYLATED
SUBSTRATE
Met
S-adenosylMet
SUBSTRATE
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
METHYL-THF: Transfers a CH3 group (which requires B12)
to make S-adenosyl-methionine (SAM)
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.
From
S-adenosyl
methionine
There are many reactions like this!
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.
The complex stucture
of vitamin B12.
There is an atom
of cobalt in each
B12 molecule.
An additional form of folate, FORMINOTETRAHYDROFOLATE, 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.
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
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.
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?
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).
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.
There is evidence now that 1 mg/day is
protective. We will discuss optimal
doses of folate on Oct 2.