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USMLE Biochemistry > TCA Cycle > Flashcards

TCA Cycle Flashcards

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USMLE® Step 1 flashcards
1
Q

Where does the TCA cycle occur?

A

All reactions occur in mitochondria

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2
Q

What are the products of the TCA cycle?

A
  • NADH
  • FADH2 -> ETC (ATP)
  • GTP
  • CO2
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3
Q

What is Citrate synthesised from?

A
  • Acetyl CoA (2C)

- Oxaloacetate (4C)

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4
Q

What enzyme synthesises citrate?

A

Citrate Synthase

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5
Q

What can inhibit citrate synthase?

A

ATP

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6
Q

What does citrate inhibit?

A

PFK1

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7
Q

What does citrate activate?

A

ACoA carboxylase (fatty acid synthesis)

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8
Q

What is used for gluconeogenesis in the fasting state?

A

Oxaloacetate

this lowers the amount able to be used for TCA cycle

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9
Q

How is ketone synthesis triggered in the fasting state?

A

Oxaloacetate used for gluconeogenesis

  • Less acetyl coA used for TCA cycle, build up
  • Excess acetyl coA converted into ketones
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10
Q

What is isocitrate to citrate?

A

An isomer

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11
Q

What enzyme converts citrate to isocitrate?

A

Aconitase

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12
Q

What does fluoroacetate (rat poison) inhibit?

A

Aconitase

- Citrate -> Isocitrate

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13
Q

What is the rate-limiting step of the TCA cycle?

A
  • Isocitrate
    -> Co2 NADH
    -> Alpha-ketoglutarate
    (Enzyme: Isocitrate Dehydrogenase)
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14
Q

What enzyme converts Isocitrate to alpha-ketoglutarate?

A

Isocitrate Dehydrogenase

- Rate-limiting step

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15
Q

What inhibits alpha-Ketoglutarate?

A
  • ATP

- NADH

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16
Q

What activates alpha-Ketoglutarate?

A
  • ADP

- Ca2+

17
Q

What are the irreversible steps of the TCA cycle?

A

Citrate synthase (oxaloacetate -> Citrate)

Isocitrate dehyrogenase
(Isocitrate -> alpha-ketoglutinate)

alpha ketoglutinate dehydrogenase
(a-KG -> Succinyl CoA)

18
Q

What enzyme converts alpha ketoglutarate to Succinyl-CoA?

A

alpha-KetoGlutinate Dehydrogenase

19
Q

What inhibits alpha-KetoGlutinate Dehydrogenase ?

A
  • Succinyl-CoA (product)

- NADH

20
Q

What stimulates alpha-KetoGlutinate Dehydrogenase ?

A

Ca2+

21
Q

What is taken off of alpha ketoglutarate and added to it to make Succinyl-CoA?

A
  • CoA added

- CO2 and NADH taken off

22
Q

What is taken off of Succinyl-CoA to make Succinate?

A
  • CoA

- GTP

23
Q

What enzyme converts succinyl-CoA into Succinate?

A

Succinyl-CoA synthase

24
Q

What enzyme converts Succinate to fumarate?

What is significant?

A

Succinate dehydrogenase

  • Embedded in mitchondrial membrane
  • Function as complex II electron transport
25
Q

What molecule is also produced from Succinate dehydrogenase?

A

FADH2 (FAD+ the other way)

26
Q

What other pathways (other than TCA) can produce fumarate?

A
  • Urea cycle
  • Purine synthesis (formation of IMP)
  • Amino acid breakdown (phenylalanine, tyrosine)
27
Q

What converts malate into fumarate?

A

Fumarase

28
Q

What converts Malate into NADH and Oxaloacetate?

A

Malate dehydrogenase

29
Q

What are the 2 major functions of malate/

A
  • Transfer of NADH into mitochondria

- Transfer of oxaloacetate OUT of mitochondria

30
Q

How does malate transfer NADH?

A

Oxaloacetate -> Malate
NADH-> NAD+ (electrons donated to malate)

  • Malate then transported into mitochondria
  • Malate converted to oxaloacetate making a molecule of NADH from NAD+
31
Q

How is oxaloacetate replinshed in the cytosol when used in malate shuttle?

A

Oxaloacetate converted to aspartate (Glut-> a-KG)

  • Aspartate then shuttled across
  • Aspartate trnasfered to OAA (glut-> a-KG)
32
Q

Describe how oxaloacetate can be transfered out of the mitochondria to participate in gluconeogensis?

A

OAA -> Malate (NADH -> NAD+)

  • Malate transfered out of mitochondria
  • Malate converted to OAA (NAD+ -> NADH)
33
Q

What is citrate also used for?

A

Fatty acid synthesis

34
Q

What is alpha-Ketoglutarate also used for?

A

Amino acid synthesis

35
Q

What is Oxaloacetate also used for?

A
  • Amino acid synthesis

- Glucose

36
Q

What can succinyl coA be used for other than TCA cycle?

A

Heme synthesis

37
Q

What can Succinyl-CoA be synthesised from?

A
  • Methylmalonyl coA (from odd chain fatty acids and branched chain amino acids)
  • a-KG (TCA cycle)
38
Q

What are all the inhibitors of the TCA cycle?

A
  • ATP
  • NADH
  • Acetyl-CoA
  • Citrate
  • Succinyl CoA
    Signs of high energy
39
Q

What is the TCA cycle activated by?

A

Ca2+ and ADP

USMLE Biochemistry (44 decks)

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