Skildum- Tetrahydrofolate

Question 1

What is the most important source of single carbon groups?

Answer 1

Serine

Question 2

How does the one carbon pool interact with FH4?

Answer 2

1. Single C sources bind to FH4
2. C donated by precursors
3. FH4 regenerated and can do one c donation again

Question 3

What are the 1 C sources?

Answer 3

Serine
Glycine
HIstidine
Formaldehyde
Formate
choline

Question 4

What are the 1 c Products?

Answer 4

dTMP
Serine
Purines bases
B12-CH3
Methionine
S adenosyl methionine

Question 5

What are the structural features of folate inside and outside the body?

Answer 5

Polyglutamate tail, digested down to mono glutamate (N5-methyl FH4) in gut and poly-glutamated in liver

Question 6

What food is folate in?

Answer 6

Green leafy vegetables, liver, legumes, yeast, fortified flour

Question 7

What causes spina bifida?

Answer 7

1. Folate deficiency before and during pregnandcy

2. Can be associated with malnutrition/absorption

Question 8

What is hereditary folate malabsorption?

Answer 8

An inherited mutation in the proton coupled folate transporter (PCFT;gene SCL46A1). Causes functional folate deficiency despite adequate folate in diet.

Question 9

Why is DHFR important?

Answer 9

1. Metabolism of dietary folate
2. Recyciling of oxidized folate to FH4.
3. Drug target (Methotrexate–cancer, rheumatoid arthritis, Trimethoprim–antibacterial, Pyrimethamine–antimalarial)

Question 10

What is dihydrofolate reductase?

Answer 10

DHFR is the enzyme that reduces folate to FH2 and FH2 to FH4.

Question 11

What is FH4?

Answer 11

An active cofactor that can bind methyl groups

Question 12

What is a ketogenic AA that can be degraded?

Answer 12

Tryptophan to formate

Question 13

Where are methyl traps?

Answer 13

When reductions are irreversible. Allows form to build up to toxic levels if you have a B12 deficiency and you can’t regenerate THF.

Question 14

What happens when serine is turned to glycine ,the simplest AA?

Answer 14

Serine’s side chain is transferred to FH4

Question 15

What enzymes are involved in the simultaneous reactions of serine to glycine and FH4 to 5,10methyleneterfolate?

Answer 15

PLP

Serine hydroxymethyl transferase

Question 16

What does SAM do?

Answer 16

Donates methyl groups to substrates, including precursors to neurotransmitters.

Question 17

Why is the dTMP pathway important?

Answer 17

dTMP is a nucleotide that is needed to make DNA. If you have folate deficiency you can’t make DNA. Cells that are most affected have high turn over rate.

Question 18

What is the primary manifestation of folate deficiency?

Answer 18

Blood problems/ Megaloblastic anemia–Blood cells divide rapidly and have a high requirement for deoxynucleotides. In a dietary or functional folate deficiency, blood cells can not synthesize enough DNA to replicate their chromosomes. The cells grow large, because they can’t divide until they replicate their genomes. These megaloblastic cells are most apparent in the bone marrow, but they can also be seen in circulating blood.

Question 19

Why is the DNA synthesis arm of folate deficiency a drug target?

Answer 19

B/c cancer cells divide rapidly and have a high requirement for deoxlynucleotides.

Question 20

What are many cases of pernicious anemia caused by?

Answer 20

Autoimmune destruction of parietal cells.

Question 21

What are two cancer drugs that are used in folate deficiency?

Answer 21

1. Methotrexate–folate analog that inhibits DHFR
2. 5 Fluorouracil–uracil analog that inhibits thymidylate synthase
   * *both drugs also kill other rapidly dividing cells

Question 22

What happens when carbon is in N5 methyl FH4 form?

Answer 22

1. It is not donated directly
2. Methyl group is first donated to homocysteine to make methionine (B12 impt here!)
3. Methionine joinds adenosine base to form S-adenosyl methionine (SAM)
4. SAM provides methyl groups for biosynthetic rxns and can regenerate homocysteine

Question 23

Where is Vitamin B12?

Answer 23

Cobalamin, found in meat, eggs and diary. Cobalt at the center of hte ring can bind a methyl group or an adenine nucleotide.
Deficiency: GI tract injured/infected, or if accessory factors are abated through autoimmunity/hereditary

Question 24

How is cobalamin processed in the body?

Answer 24

1. B12 first binds to R binder proteins secreted in the stomach
2. R binders are digested, and B12 binds another protein, intrinsic factor.
3. B12-intrinsic factor complex is taken up by intestinal epithelial cells
4. It is transported in the blood as a complex iwth transcobalamin II protein.
5. Most is stored in liver in complex with cubillin

Question 25

What does B12 deficiency cause and what is it due to?

Answer 25

1. Pernicious anemia (megaloblastic anemia PLUS neuropathy) | 2. Dietary or the loss of fxn of intrinsic factor, transcobalamin II or cubullin.

Question 26

What is hyperhomocysteinemia?

Answer 26

Cardiovascular and neurological problems caused by mutations in methionine synthase.

Question 27

What causes hyperhomocysteinemia?

Answer 27

1. B12 deficiency 2. methionine synthase deficiency 3. B6 (PLP) deficiency

Question 28

Where does the S for cysteine come from?

Answer 28

Dietary methionine.

Question 29

How is Vit B12 linked to SAM (s-adenosylmethionine)?

Answer 29

Methionine can bind an adenosine nucleoside to SAM. SAm donates methyl groups to substrates, including precursors to NTs. 1. Norepinephrine >Sam> epinephrine 2. GUanidoacetate > creatine 3. Nucleotides > methylated nucleotides 4. phosphatidyethanolamine> phosphatidylcholine 5. acetylserotonin > melatonin

Question 30

What is the methyl trap hypothesis?

Answer 30

The only metabolic fate of N5methyl TH4 (the product created after the irreversible step) is to lose its methyl to cobalamin. In a dietary/fxnal deficiency of cobalamin, folate becomes "trapped" as N5methyl TH4, unable to participate in one C transfers. * *Methyl trap hypothesis - This is the reason you get symptoms of folate deficiency when have a B12 deficiency. The idea with the vitamin B12 deficiency is that all of your folate is locked up as the five methyl hydrofolate. Only one reaction that 5 methylTHF can participate in and that is the production of methylcobalamin. If you have a vitamin B12 deficiency this reaction doesn’t happen. Any time this reaction does happen you can’t go back. So all of the THF is in that form and can’t do the other reaction that involve donation of oxidized carbon. So this explains why get metaloblastic anemia with B12 deficiency despite having enough folate in diet. Because all of folate in diet get trapped in this form. But there is another part of vitamin B12 deficiency – pernicious anemia. Because the nucleotides can be neurotransmitters and those would be reduced. You don’t get neurological symptoms with a folate deficiency because you will have a little folate around and the one carbons get steared to this pathway rather than to the Thymatin acetylase reaction.