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USMLE Biochemistry > B12 and Folate > Flashcards

B12 and Folate Flashcards

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USMLE® Step 1 flashcards
1
Q

What nucleoside are both B9 and B12 used in the synthesis of?

A

Thymidine

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2
Q

Homocysteine levels are increased in what vitamin deficiencies?

A

B9 and B12

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3
Q

What enzyme converts dUridine-MP to Thymidine-MP?

A

Thymidylate Synthase

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4
Q

What is the difference between uracil and thymine?

A

Thymine has an extra carbon

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5
Q

Where does the carbon come from to make Thymidine-MP from dU-MP?

A

N5, N10 Tetrahydrofolate

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6
Q

What does N5, N10 Tetrahydrofolate become after the transfer of its carbon to make Thymidine-MP?

A

DHF

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7
Q

What enzyme converts DHF to THF?

A

Dihydrofolate reductase

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8
Q

What is Folate converted to and how is it then eventually converted to N5, N10 Tetrahydrofolate?

A

DHF (
tetrahydrofolate)
- THF
-> N5, N10 Tetrahydrofolate

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9
Q

What is the main function of N5, N10 Tetrahydrofolate?

A

Donates a Carbon to dUridine-MP to then become Thymidine-MP

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10
Q

What converts N5 Methyl THF into THF?

A

B12

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11
Q

What does S-Adenosyl Methionine (SAM) donate?

A

Methyl groups

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12
Q

What is S-Adenosyl Methionine (SAM) synthesised from?

A

ATP and methionine

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13
Q

SAM has a methyl group and Adenosine removed what does it become?

A

Homocysteine

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14
Q

B12 and Folate convert Homocysteine into what?

A

Methionine

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15
Q

Methionine + ATP yeilds what?

A

SAM

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16
Q

Methionine Synthase converts Homocysteine into what?

A

Methionine and Tetrahydrofolate

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17
Q

What are the causes of Megaloblastic anemia? (outside of B9 and B12 deficiency)

A
  • Orotic aciduria
  • Drugs (MTX, 5-FU, Hydroxyurea)
  • Zidovudine (HIV, NRTIs)
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18
Q

What is the difference between Tetrahydrofolate and N5, N10 THF?

A

Addition of a carbon

- Used in thymidine synthesis

19
Q

Why do alcoholics often have a folate deficiency?

A
  • Decreased intake (nutrition)

- Poor absorption

20
Q

What are the 3 drugs which interfere with folate deficiency?

A
  • Phenytoin
  • Trimethoprim
  • Methotrexate
21
Q

How does phenytoin cause folate deficiency?

A

Affects folate uptake

22
Q

How can trimethoprim and Methtrexate cause folate deficiency?

A

Inhibiting Dihydrofolate reductase

DHF -> THF reaction

23
Q

What element does cobalamin contain?

A

Cobalt

- Large, complex structure (corrin ring)

24
Q

Where is B12 found?

A

Meats

- Only synthesised in bacteria

25
Q

Outside of folate metabolism what is B12 also involved in?

A

Odd chain fatty acid metabolism

- Into Succinyl CoA

26
Q

How can B12 deficiency cause peripheral neuropathy?

A
  • Increased Methylmalonic acid

- Myelin synthesis affected

27
Q

What is B12 Neuropathy called?

A

Subacute combined degeneration (SCD)

28
Q

What part of the spinal cord is affected in B12 neuropathy?

A

Dorsal spinal columns

29
Q

What are the symptoms of Subacute combined degeneration (SCD)?

A
  • Bilateral
  • Legs&raquo_space; arms
  • Paresthesia
  • Ataxia
  • Loss of vibration and position sense
  • Severe weakness and paraplegia if severe
30
Q

What are odd chain fatty acids converted into?

A

Propionyl coA

31
Q

Propionyl coA is converted to Methylmalonyl CoA thru what cofactor?

A

Biotin

32
Q

Methylmalonyl-CoA is converted to Succinyl CoA via what enzyme and cofactor?

A

Methylmalonyl-CoA mutase

- B12

33
Q

Methylmalonyl-CoA and Succinyl CoA are similar in what way?

A

isomers

34
Q

What substance enters the TCA cycle?

- From AAs, cholesterol,

A

Succinyl CoA

35
Q

What vitamin is stored in the liver for years?

A

B12

36
Q

Pernicious anemia causes the destruction of what?

A

Gastric parietal cells

- gastric body

37
Q

What kind of HS reaction is involved in Pernicious anemia?

A

Type 2

38
Q

What cancer is pernicious anemia associated with?

A

Gastric adenocarcinoma

39
Q

What a HLA group is associated with Pernicious nemia?

A

HLA-DR antigens

40
Q

What are other causes of B12 deficiency?

A
  • ileum resection/dysfunction - Crohn’s
  • Loss of IF from gastric bypass
  • Diphyllobothrium latum
41
Q

What is Diphyllobothrium latum?
What type of parasite?
How is it transmitted?
What does it destroy/consume?

A
  • Helminth (tapeworm)
  • Transmission from infected fish
  • Consumes B12
42
Q

What is the diagnostic test for pernicious anemia?

What are the steps involved?

A

Schilling test

  • Oral radiolabeled B12
  • IM B12 to saturate liver receptors
  • Radiolabeled B12 should be detectable in urine normally
  • Can repeat with oral IF
43
Q

Radiolabeled B12 is detactable in the urine from the Schilling test

Does this individual have Pernicious anemia?

A

No

44
Q

What cells are increased when given B12 injection?

A

Reticulocytes

- Immature RBCs

USMLE Biochemistry (44 decks)

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