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Medical Biochemistry > Vitamin B12 and Folic Acid > Flashcards

Vitamin B12 and Folic Acid Flashcards

–Specify the one-carbon donors in metabolism and the groups they donate (SAM, THF, Cobalamin) –Explain the formation of SAM and the reactions requiring SAM –Analyze the metabolism of homocysteine, vitamins/ coenzymes required for metabolism & interpret the clinical significance of homocysteine –Explain the formation of THF from folate and specify the mechanism of action of their inhibitors –Summarize the formation of one carbon groups from amino acid metabolism and the utilization of 1-C gro

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USMLE® Step 1 flashcards
1
Q

What is another name for Vitamin B12?

A

Cobalamin

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2
Q

What are sources for Vitamin B12?

A

Meats
Shellfish
Poultry
Eggs
Diary products
(not present in plants)

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3
Q

What are the active forms of Vitamin B12?

A

5-deoxyadenosylcobalamin
methyl cobalamin

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4
Q

How does SAM act upon substrates?

A

methylation of substrates

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5
Q

How is SAM formed?

A

Made from adenosine triphosphate (ATP) and methionine by methionine adenosyltransferase.

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6
Q

What are the two major disposal pathways for homoscysteine?

A

-Conversion to methionine (Req: Folate & B12) - remethylation
-Conversion to Cysteine (Req: B6/Pyridoxine) - transsulfuration

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7
Q

Deficiency of B12 results in the accumulation of Homocysteine and the…

A

trapping of THF in the methyl-THF form

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8
Q

The trapping of THF in the methyl-THF form due to B12 deficiency impairs synthesis of what?

A

Nucleotide synthesis
DNA and RNA synthesis

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9
Q

trapped form of THF is known as

A

N5-methyl H4folate

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10
Q

THF is short for?

A

Tetrahydrofolate

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11
Q

Homocysteine + N5-Methyltetrahydrofolate undergo catalysis by which enzyme to form what?

A

Enzyme: Methionine synthase
Product: L-Methionine + THF

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12
Q

Oxidation of odd-chain fatty acids generate which three carbon fatty acid by-product?

A

Propionyl-CoA

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13
Q

Propionyl CoA carboxylase requires what?

A

Biotin

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14
Q

What is another name for methionine synthase?

A

Homocysteine N-Methyltransferase

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15
Q

Pernicious anemia associated with homocystinuria specifically results from?

A

No intrinsic factor because of autoimmune destruction of parietal cells (therefore can’t absorb B12)

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16
Q

Megaloblastic anemia must be treated with what?

A

BOTH folate and Vitamin B12

17
Q

Is there a vitamin B12 deficiency in pernicious anemia?

A

No. It is caused by a lack of the ability to absorb B12.

18
Q

Intrinsic factor, which carried B12 and helps for absorption into the muosa of the ileum, is produced by what cells?

A

Parietal cells of the Stomach

19
Q

Folate and Vitamin B12 deficiency BOTH lead to:

A

-Megaloblastic anemia
- Homocysteinemia, increased risk for cardiovascular disease

20
Q

THF is the active form of folic acid produced by which enzyme?

A

Dihydrofolate reductase

21
Q

Homocystinuria occurs due to deficiency of which enzyme?

A

Cystathionine synthase

22
Q

What are some other effects of homocysteine accumulation?

A

Lipid oxidation and platelet aggregation –> leads to fibrosis and calcification of atherosclerotic plaques

23
Q

Cysteine metabolized by cysteine dioxygenase produces what? Which is important for what?

A

-Taurine
-Important for bile synthesis

24
Q

Sulfate produced from cysteine catabolism is used as a precursor for what?

A

Formation of 3-Phosphoadenisine-5-Phosphate (PAPS; glycolipid ad GAG synthesis)

25
Q

Folic acid is which B vitamin?

A

b-9

26
Q

what are the active forms of Folic acid?

A

Tetrahydrofolate (THF) derivatives

27
Q

What are the sources of Folic acid?

A

Green leafy vegetables
Liver
Lima beans
whole grain cereals

28
Q

Folic acid is important in 1-C transfer reactions especially in what?

A

DNA and RNA synthesis (purines and thymidine), some amino acid metabolism (glycine serine, methionine from homocysteine, degradation of histidine)

29
Q

Dihydrofolate reductase requires which cofactor in order to form THF?

A

2 NADPH

30
Q

Which vitamin deficiency has been implicated in spina bifida?

A

Folic Acid (B-9)

31
Q

Serine to glycine interconversion is catalyzed by which enzyme? Requiring which cofactor?

A

Serine Hydroxymethyltransferase
THF

32
Q

FIGlu donates what group to what to produce what?

A

Donates: Formimino group
to: THF
produces: glutamate

33
Q

the degradation of Histidine requires what?

A

THF

34
Q

Histidine is eventually degraded to _______, which requires _____ to form _____.

A

FIGlu
THF
Glutamate

35
Q

Individuals who are deficient in folic acid show in increase in what in their urine?

A

FIGlu

36
Q

Histidinemia is due to what deficiency?

A

Histidinase.

37
Q

Folic acid deficiency results in what conditions?

A

Homocystinuria
Megaloblastic anemia

Medical Biochemistry (35 decks)

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