BIOCHEMISTRY OF FOLATEAND VITAMIN B12:

Oct 1, 2014

BASIC BIOCHEMICALPATHWAYS

IMPORTANCE OF FOLATE AND B12 IN NUTRITION: KEY FUNCTIONS

Folate is essential for DNA replication, becauseit’s needed for a critical nucleic acid

In addition, folate is needed for most of the activities of B12: synthesis ofneurotransmitters, and lipids in thecentral 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 toother 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 pathwaysthat involve folate and B12.

DIETARY FORMS, TRANSPORT FORMS,AND FOLATE INSIDE THE CELL

TETRAHYDROFOLATE, WITH 3 GLUTAMIC ACID SIDECHAINS. 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.

CLEAVAGE SITE, FOR ENYZME

IN DUODENUM

Product of enzyme cleavage: Folate-monoglutamate This form is absorbed through the small intestine,carried in plasma to the tissues, and then taken up intocells. After it enters cells, more glutamate residuesare added, to remake the POLYGLUTAMATE

POLYGLUTAMATE: RESYNTHESIZED IN CELLS.It does not cross the cell membrane, so it remainsinside 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.

HH

The first double bond is removed by NADPH: this is DIHYDROFOLATE.

NADPH + H+

NADP+

DIHYDROFOLATE (DHF):FIRST ACTIVE FORMIN PATHWAYS IN THE CELL

TETRAHYDROFOLATE (THF):SECOND ACTIVE FORMIN CELL METABOLISM

+ -CH2-

C

ADDITION OF METHYLENE (CH2): TO MAKEMETHYLENE-TETRAHYDOFOLATE

OH-CH2-C-COOHNH2

SERINE

H-C-COOHNH2

H H

GLYCINE

THE METHYLENE GROUP (CH2) COMES FROM

SERINE. THE REACTION IS B6-DEPENDENT

(PLP)

C CH3

NADH+H+ NAD+

METHYLENETETRAHYDROFOLATE

METHYLTETRAHYDROFOLATE

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 TOMETHYL-THF IS NOT REVERSIBLE!

The previous reaction, the conversion ofMETHYLENE-THF to METHYL-THF,is done by an important enzyme calledMethylene-tetrahydrofolate-reductase (MTHFR).

There are major genetic variants (polymorphisms)in humans, across different populations, and thesevariants have significant effects.

We will discuss this extensively on Oct 2.

The previous slides illustrate all theforms of folate that will be shown in the next slides.

These slides show how folate pathwaysare inter-related, with transfer of

CH2 (for DNA synthesis) and for CH3

(methylation pathways using B12)

dUMP

DIETARYFOLATE

REDUCTION BYNAPDH

DIHYDROFOLATE

TETRAHYDROFOLATE

REDUCTION BYNAPDH

METHYLENETETRAHYDROFOLATE

SERINE

GLYCINE

METHYL GROUPADDITION

METHYLTETRAHYDROFOLATE

REDUCTIONBY NADH

HCys Met

SUBSTRATE

S-adenosyl-Met

S-adenosyl-HCys

METHYLATEDSUBSTRATE

dTMP(methylated)

DNAsynthesis

REQUIRESVit B12

CYCLE#1

CYCLE#2

FOLATE PROVIDES METHYL GROUPSBY TWO DIFFERENT CYCLES

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)

S-adenosyl-methionine (often called SAM) isa very important CH3 donor. It participatesin hundreds of pathways.

The function of the methyl-THF/B12 pathwayis to make SAM. The intermediates were shownon a previous slide.

This methyl came from methyl-THF, and willnext be donated to a recipient molecule.

FromS-adenosylmethionine

There are many reactions like this!

This is a VERY complicated slide. It outlines theREQUIREMENT for B12, as intermediate to make SAM.

Study of B12 function is a very advanced topic.

The complex stuctureof vitamin B12.

There is an atom of cobalt in each

B12 molecule.

An additional form of folate, FORMINOTETRAHYDROFOLATE, isnecessary 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 camefrom theamino acid

Folate deficiency causes a change in histidine metabolism. This isthe 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 makedeoxythymidine, needed for DNA synthesis andcell division

-Another form (methyl-THF) transfers CH3, tomake S-adenosyl-methionine (SAM), a step thatrequires 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 protectiveagainst birth defects, a highly reliable observation.

Ten years ago, the decision was made to add folateto flour, breakfast cereal, bakery products, andmany other consumer goods.

As a result, folate deficiency is now very rarein 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 thethermolabile 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 hadpreviously 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 isprotective. We will discuss optimal

doses of folate on Oct 2.