Folate

Question 1

How many modes of action does folate have

Answer 1

One
It only acts as a coenzyme in one-carbon transfer reaction

Question 2

What reaction have we lost that prevents us from synthesizing folate

Answer 2

linking p-aminobenzoic acid to the pteridine ring

Question 3

How many forms of folate are there? Which two positions are important

Answer 3

six different one-carbon conjugated forms of folate
N5 and N10

Question 4

What are the two modification require to activate folate

Answer 4

1. Fully reduced in order to have carbon units attached
2. Active form of folate is conjugated with up to six glutamates

Question 5

How is folate fully reduced? How many steps and what enzymes

Answer 5

2 sequential steps
both using Dihydrofolate reductase
Folate –> 7,8 dihydrofolate –> 5,6,7,8 tetrahydrofolate

Question 6

what is the predominant form of folate in cells

Answer 6

pentaglutamate

Question 7

What level of glutamylation is needed to be taken into cell? Retained?

Answer 7

First glutamate is added to pABA, it can then be taken into cells, and glutamated further
Converted to diglutamate in order to be retained in cells
The rate of glutamylation slows as the glutamate tail gets longer

Question 8

What is the major form of folate that is taken into tissues

Answer 8

methyl-THF, once in the cell it is demethylated then glutamated

Question 9

Most dietary folate is bound to _____

Answer 9

albumin

Question 10

What are the three classes of folate-binding proteins

Answer 10

High affinity binding proteins: Can be membrane associated or soluble

other membrane associated proteins: Lower affinity then previous group

Enzyme: cytosolic enzymes that require folate (lower affinity)

Question 11

How is folate excreted

Answer 11

very little excreted in urine
most of the folate bound to proteins is reabsorbed by the kidneys

Question 12

Describe the catabolism of folate

Answer 12

Folate polyglutamate that is not enzyme-bound can be degraded by having the polyglutamate tail cleaved off followed by a cleavage in the molecule
the latter cleavage is catalyzed by ferritin

Iron dependent because ferritin is the storage form of iron

Question 13

Why are folate requirements higher during pregnancy

Answer 13

Ferritin levels rise during pregnancy and ferritin is involved in the catabolism of folate

Question 14

Biochemical function of folate

Answer 14

participate in catabolic or biosynthetic reactions as a coenzyme

Question 15

What is the primary source of the one carbon unit?

Answer 15

Side-chain carbon of serine

Serine becomes glycine

Question 16

What are the five major one-carbon transfer reactions

Answer 16

Serine to glycine
catabolism of histidine
synthesis of thymidylate
methionine, and purines

Question 17

What is the most important step of the one-carbon interconversions of folate

Answer 17

The most important step is the conversion from THF to N5N10 methylene-THF because all one-carbon conjugate forms can be obtained from N5N10methylene THF

Catalyzed by serine hydroxymethyltransferase which is a PLP enzyme

This step uses the methylene group donated from the conversion of serine to glycine

Question 18

What is the single carbon-folate that cannot be interconverted to any of the other forms

Answer 18

5-methyl-THF

Question 19

What are the three interconvertible one-carbon conjugate forms? why are they freely interconvertible

Answer 19

methenyl THF, methylene THF, N10-formyl THF

Are freely interconvertible due to the three enzymes involved forming a trifunctional enzyme

Question 20

Why is the conversion of methylene-THF to methyl-THF important? What process is it involved in? What vitamin is that process dependent on

Answer 20

Decreases levels of homocysteine which are a CV risk factor
Replenishes free THF that can be re-conjugated with one carbon units
It is irreversible and methyl-THF is involved in methionine synthesis which is dependent on vitamin B12

Question 21

What is the methyl-folate trap in B12 deficiency?

Answer 21

In b12 deficiency, methyl-THF is not consumed and thus traps THF in an unusable form

There is another mechanism for freeing up THF, by oxidation of N10-formyl-THF which releases THF, however it is inhibited by low methionine which occurs when there is a B12 deficiency exacerbating the usable folate deficiency

Question 22

What are the three metabolic processes where folate acts as a coenzyme

Answer 22

purine biosynthesis
Pyrimidine biosynthesis
Methionine biosynthesis

Question 23

Which steps in purine biosynthesis use folate? Which form of folate?

Answer 23

N10-formyl-THF is used in steps 3 and steps 9

Question 24

Which step in pyrimidine biosynthesis is folate used as a coenzyme? What form?

Answer 24

N5N10-methylene THF
Converts dUMP to dTMP using thymidylate synthetase
Makes thiamine containing nucleotides important for DNA

Question 25

What needs to happen to Dihydrofolate to salvage it

Answer 25

Needs to be reduced to THF using DHFR

Question 26

Why is DHFR a common anti-cancer target

Answer 26

By inhibiting DHFR we cannot get the active form of folate slowing down purine and pyrimidine synthesis

Question 27

What are the two enzymes that can synthesize methionine? Describe them

Answer 27

MetH and MetE

MetH is B12 dependent and is also much more active than MetE

Question 28

How do folate deficiencies result in neural tube defects

Answer 28

Rapid cell division and differentiation requires DNA which requires folate fast

Question 29

Folate requirement

Answer 29

RDI 200 ug (double during pregnancy)

Hard to achieve toxicity need above 5000

Question 30

What are the problems with excessive folate supplementation

Answer 30

Interferes with anticonvulsants
Can mask the development of anemia