Block 2 Lecture 3 -- One Carbon Pool Flashcards
What are the componants of folate?
1) pteridine ring
2) PABA
3) n-glutamate
What is the mechanism for sulfa drugs?
PABA analogs disrupting folate synthesis
What was the first sulfa discovered?
prontosil (R) – oxidized to sulphanilamide in vivo
How is folate absorbed?
1) synthesized by bacteria
2) brush-border enzymes cleave all but 1 Glu
How is folate reduced to THF?
dihydrofolate reductase
x2
What is the cofactor for dihydrofolate reductase?
NADPH
What is MTX mechanism of action?
THF analog that targets dihydrofolate reductase to inhibit conversion of FH2 –> FH4
How many additional glutamate residues are added in vivo?
4-5
What are the sources of one-carbon units for THF?
1) Serine
2) Gly
3) His –> FIGLU
4) Formaldehyde (from epinephrine)
5) formate (from Trp)
What is the most significant source of one-carbon units for THF?
serine
What are the DIRECT recipients of 1-carbon units from C.THF?
1) dUMP –> dTMP
2) Purine precursors (C2 & C8)
3) Glycine –> Serine
4) B12 –> B12.CH3
What are the recipients of 1-carbon units from B12.CH3?
1) Homocysteine –> Met –> SAM
What are the direct 1-carbon recipients from SAM?
1) Norepi –> Epi
2) Guanidinoacetate –> Creatine
3) Nucleotides –> methylated nucleotides
4) Phosphatidylethanolamine –> Phosphatidylcholine
5) Acetylserotonin –> Melatonin
NN-GAP
What enzyme converts dUMP –> dTMP?
thymidylate synthetase
What level of folate intake during first few weeks of pregnancy causes spina bifida?
< 0.4 mg/day…need more!
What are the symptoms of folate deficiency?
1) megaloblastic anemia
- – due to inhibition of dTMP synthesis
2) altered neuronal membrane
- – due to accumulation of methylmalonyl-CoA, precursor of malonyl-CoA
What is the most stable form of C.THF?
methyl-FH4
How does B12 differ from heme?
1) B12 has corrin ring instead of porphyrin ring
2) B12 has Co(III) instead of Fe(II)
How is B12 supplied in vitamins?
CYANOcobalamin
What are normal sources of B12?
1) meat
2) eggs
3) some from intestinal flora
What does “de novo” mean?
from the beginning
How does B12 exist in the body?
As methylcobalamin…
– B12 reacts with 5’carbon of deoxyadenosine
Describe B12 absorption.
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
What are the 2 B12 reactions in humans?
1) Homocysteine –> Methionine for SAM
2) Methylmalonyl CoA –> Succinyl CoA for brain cell membranes
How does a B12 deficiency present?
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
If folate is given for B12 deficiency, what is expected?
New FH4 allows for dTMP synthesis.
However, neuro deficits still exist (can’t make FAs from malonyl CoA)
What enzyme generates SAM?
methionine adenosyltransferase
What is the reaction for SAM generation?
Met + ATP —(Met Adenosyltransferase)—> SAM + 3Pi
What portion of Met is essential?
The homocysteine part
Why is homocysteine not an essential AA?
no good dietary source
Where is biotin a cofactor?
in carboxylation reactions
What causes biotin deficiency?
1) long antibiotic therapies
2) raw egg consumption = avidin protein = binds biotin to prevent absorption
What factors can cause decreased [SAM]?
1) defect in methylation
2) deficiency in Met, choline, or B-vitamin cofactors
Where does most TC2-B12 go?
to the liver
What causes B12 deficiency?
Rare…
1) gastric/ileal resection
2) decreased IF (elderly)
Where does Eniluracil act?
Inhibits dihydropyrimidine dehydrogenase (DPD) to prevent metabolism of 5-FU
(5-FU –> 5-FUH2)
What is the target of 5-FU?
thymidylate synthetase
How is glucose a source for the 1-carbon pool?
Ser is synthesized from glucose
What FH4-C state is irreversible?
methyl-FH4
What form of THF is used by thymidylate synthetase?
N5,N10-methylene-FH4
What is the form of THF-C that is the product of His donation?
N5-formimino-FH4
Describe the reaction when Ser donates a carbon unit to THF.
1) Enzyme is Serine hydroxymethyltransferase
2) PLP is a cofactor
3) products are N5,N10-methylene-FH4 + glycine
Describe C.THF from most to least oxidized.
Most... --- 10-formyl-FH4 --- 5,10-methENYL-FH4 --- 5,10-methyLENE-FH4 --- 5-methyl-FH4 ...least
