Block 2 Lecture 3 -- One Carbon Pool

Question 1

What are the componants of folate?

Answer 1

1) pteridine ring
2) PABA
3) n-glutamate

Question 2

What is the mechanism for sulfa drugs?

Answer 2

PABA analogs disrupting folate synthesis

Question 3

What was the first sulfa discovered?

Answer 3

prontosil (R) – oxidized to sulphanilamide in vivo

Question 4

How is folate absorbed?

Answer 4

1) synthesized by bacteria

2) brush-border enzymes cleave all but 1 Glu

Question 5

How is folate reduced to THF?

Answer 5

dihydrofolate reductase

x2

Question 6

What is the cofactor for dihydrofolate reductase?

Answer 6

NADPH

Question 7

What is MTX mechanism of action?

Answer 7

THF analog that targets dihydrofolate reductase to inhibit conversion of FH2 –> FH4

Question 8

How many additional glutamate residues are added in vivo?

Answer 8

4-5

Question 9

What are the sources of one-carbon units for THF?

Answer 9

1) Serine
2) Gly
3) His –> FIGLU
4) Formaldehyde (from epinephrine)
5) formate (from Trp)

Question 10

What is the most significant source of one-carbon units for THF?

Answer 10

serine

Question 11

What are the DIRECT recipients of 1-carbon units from C.THF?

Answer 11

1) dUMP –> dTMP
2) Purine precursors (C2 & C8)
3) Glycine –> Serine
4) B12 –> B12.CH3

Question 12

What are the recipients of 1-carbon units from B12.CH3?

Answer 12

1) Homocysteine –> Met –> SAM

Question 13

What are the direct 1-carbon recipients from SAM?

Answer 13

1) Norepi –> Epi
2) Guanidinoacetate –> Creatine
3) Nucleotides –> methylated nucleotides
4) Phosphatidylethanolamine –> Phosphatidylcholine
5) Acetylserotonin –> Melatonin

NN-GAP

Question 14

What enzyme converts dUMP –> dTMP?

Answer 14

thymidylate synthetase

Question 15

What level of folate intake during first few weeks of pregnancy causes spina bifida?

Answer 15

< 0.4 mg/day…need more!

Question 16

What are the symptoms of folate deficiency?

Answer 16

1) megaloblastic anemia
- – due to inhibition of dTMP synthesis
2) altered neuronal membrane
- – due to accumulation of methylmalonyl-CoA, precursor of malonyl-CoA

Question 17

What is the most stable form of C.THF?

Answer 17

methyl-FH4

Question 18

How does B12 differ from heme?

Answer 18

1) B12 has corrin ring instead of porphyrin ring

2) B12 has Co(III) instead of Fe(II)

Question 19

How is B12 supplied in vitamins?

Answer 19

CYANOcobalamin

Question 20

What are normal sources of B12?

Answer 20

1) meat
2) eggs
3) some from intestinal flora

Question 21

What does “de novo” mean?

Answer 21

from the beginning

Question 22

How does B12 exist in the body?

Answer 22

As methylcobalamin…

– B12 reacts with 5’carbon of deoxyadenosine

Question 23

Describe B12 absorption.

Answer 23

1) R-binders/haptocorrins/TC1 in stomach
2) pancreatic proteases free B12
3) B12 binds to IF (from parietal cells)
4) cubilin receptor
5) carried to liver via transcobalamin2

Question 24

What are the 2 B12 reactions in humans?

Answer 24

1) Homocysteine –> Methionine for SAM

2) Methylmalonyl CoA –> Succinyl CoA for brain cell membranes

Question 25

How does a B12 deficiency present?

Answer 25

1) hematopoietic
- - functional folate deficiency
- - megaloblastic anemia
- - leukocyte polysegmentation
2) neurological
- - accumulation of methylmalonyl CoA, substitute for malonyl-CoA in FA synthesis for neuronal membranes

Question 26

If folate is given for B12 deficiency, what is expected?

Answer 26

New FH4 allows for dTMP synthesis.

However, neuro deficits still exist (can’t make FAs from malonyl CoA)

Question 27

What enzyme generates SAM?

Answer 27

methionine adenosyltransferase

Question 28

What is the reaction for SAM generation?

Answer 28

Met + ATP —(Met Adenosyltransferase)—> SAM + 3Pi

Question 29

What portion of Met is essential?

Answer 29

The homocysteine part

Question 30

Why is homocysteine not an essential AA?

Answer 30

no good dietary source

Question 31

Where is biotin a cofactor?

Answer 31

in carboxylation reactions

Question 32

What causes biotin deficiency?

Answer 32

1) long antibiotic therapies

2) raw egg consumption = avidin protein = binds biotin to prevent absorption

Question 33

What factors can cause decreased [SAM]?

Answer 33

1) defect in methylation

2) deficiency in Met, choline, or B-vitamin cofactors

Question 34

Where does most TC2-B12 go?

Answer 34

to the liver

Question 35

What causes B12 deficiency?

Answer 35

Rare…

1) gastric/ileal resection
2) decreased IF (elderly)

Question 36

Where does Eniluracil act?

Answer 36

Inhibits dihydropyrimidine dehydrogenase (DPD) to prevent metabolism of 5-FU
(5-FU –> 5-FUH2)

Question 37

What is the target of 5-FU?

Answer 37

thymidylate synthetase

Question 38

How is glucose a source for the 1-carbon pool?

Answer 38

Ser is synthesized from glucose

Question 39

What FH4-C state is irreversible?

Answer 39

methyl-FH4

Question 40

What form of THF is used by thymidylate synthetase?

Answer 40

N5,N10-methylene-FH4

Question 41

What is the form of THF-C that is the product of His donation?

Answer 41

N5-formimino-FH4

Question 42

Describe the reaction when Ser donates a carbon unit to THF.

Answer 42

1) Enzyme is Serine hydroxymethyltransferase
2) PLP is a cofactor
3) products are N5,N10-methylene-FH4 + glycine

Question 43

Describe C.THF from most to least oxidized.

Answer 43

Most...
--- 10-formyl-FH4
--- 5,10-methENYL-FH4
--- 5,10-methyLENE-FH4
--- 5-methyl-FH4
...least