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Biochemistry II > Citric Acid Cycle > Flashcards

Citric Acid Cycle Flashcards

1
Q

The CAC is a circular pathway of ____ reactions that oxidize acetyl-CoA to carbon dioxide.

A

8

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2
Q

Where does the CAC take place?

A

The mitochondrial matrix

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3
Q

What are the 8 intermediates of the CAC (excluding acetyl-CoA)?

A
  1. Citrate
  2. Isocitrate
  3. Alpha-ketoglutarate
  4. Succinyl-CoA
  5. Succinate
  6. Fumarate
  7. Malate
  8. Oxaloacetate
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4
Q

What are the 8 enzymes of the CAC?

A
  1. Citrate synthetase
  2. Aconitase
  3. Isocitrate dehydrogenase
  4. Alpha-ketoglutarate dehydrogenase
  5. Succinyl-CoA synthetase
  6. Succinate dehydrogenase
  7. Fumarase
  8. Malate dehydrogenase
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5
Q

What is the structure of acetyl CoA?

A
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6
Q

What is the structure of pyruvate?

A
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7
Q

What is the structure of oxaloacetate?

A
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8
Q

Citrate synthase converts ________________ and __________________ to citrate via a Claisen condensation.

A

Acetyl-CoA

Oxaloacetate

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9
Q

Citrate synthase produces citrate and expels ______________.

A

CoASH

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10
Q

What is the mechanism of citrate synthase?

A

An irreversible Claisen condensation

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11
Q

What is the mechanism of action of citrate sythase?

A
  1. Acidic proton deprotonates and carbanion forms on acetyl-CoA
  2. Carbanion attacks C carbonyl and O protonates
  3. Water attacks chiral C and expels CoASH
  4. Citrate forms
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12
Q

Aconitase converts cistrate to isocitrate via an ___________________ and dehydration.

A

Isomerization

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13
Q

What is the structure of isocitrate?

A
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14
Q

The enzyme aconitase catalyzes the reversible transformation of citrate to isocitrate through the intermediary formation of the tricarboxylic acid ______________________, which normally does not dissociate from the active site.

A

Cis-aconitase

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15
Q

Aconitase contains an _________________________, which acts both in the binding of the substrate in the active site and in the catalytic addition and removal of water.

A

Iron-sulfur center

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16
Q

What is the mechanism of action for aconitase?

A
  1. Proton abstraction
  2. Formation of alkene
  3. Expulsion of hydroxyl
  4. Formation of cis-aconitase
  5. Water is added across alkene
  6. Formation of isocitrate
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17
Q

The iron-sulfur cluster of aconitase assists in what part of the enzyme’s mechanism?

A

The removal of water

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18
Q

The iron-sulfur cluster of aconitase contains a __________ residue that coordinates four iron artoms and four sulfur atoms.

A

Cysteine

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19
Q

Citrate synthase incorporates acetyl-CoA only into the pro-__ carboxymethyl group and that aconitase abstracts a proton only from the pro-__ carboxymethyl group of citrate.

A

pro-S

pro-R

20
Q

Isocitrate dehydrogenase catalyzes the formation of alpha-ketoglutarate from isocitrate via an ________________ _________________.

A

Oxidative decarboxylation

21
Q

Isocitrate dehydrogenase catalyzes an oxidative decarboxylation of a beta-keto acid just as ____________________ _________________ in the pentose phosphate pathway.

A

6-phosphogluconate dehydrogenase

22
Q

What is the structure of alpha-ketoglutarate?

A
23
Q

What is the mechanism of action for isocitrate dehydrogenase?

A
  1. Isocitrate binds enzyme and is oxidized via hydride transfer to NAD+ or NADP+
  2. Interaction of the carbonyl O with a bound Mg2+ or Mn2+ increases electron-withdrawing capacity of carbonyl group and facilitates decarboxylation
  3. Rearrangement of enol intermediate generates alpha-ketoglutarate
24
Q

What is the structure of succinyl-CoA?

A
25
Q

alpha-ketoglutarate dehydrogenase catalyzes the oxidative decarboxylation of an ____-keto acid.

A

Alpha keto acid

26
Q

Alpha-ketoglutarate dehydrogenase cataylzes the conversion of __________________________ to _______________________.

A

Alpha-ketoglutarate

Succinyl-CoA

27
Q

What is the mechanism of action for alpha-ketoglutarate dehydrogenase?

A
  1. TPP carbanion attacks alpha keto group
  2. Gamma O kicks back, expels carbon dioxide, and forms carbanionic intermediate
    3.
28
Q

Citrate synthase incorporates acetyl-coenzyme A only into the pro-___ carboxymethyl group and aconitase abstracts a protron from only the pro-___ carboxymethyl group of citrate.

A

Pro-S

Pro-R

29
Q

What coenzymes are required for alpha-ketoglutarate dehydrogenase to function?

A

TPP

Lipoic acid

CoASH

FAD

NAD+

30
Q

___________________ refers to an enzyme that catalyzes a condensation reaction that involves a nucleotide triphosphate while _______________ refers to an enzyme that catalyzes a condensation reaction that does not involve a nucleotide triphosphate.

A

Synthetase

Synthase

31
Q

What cosubstrate is required for the reaction catalyzed by succinyl-CoA synthetase?

A

Inorganic phosphate

32
Q

What amino acid residue assists in the succinyl-CoA synthetase reaction?

A

A deprotonated histidine

33
Q

What is the function of GDP in the succinyl-CoA synthetase mechanism?

A

To dephosphorylate the amino acid residue and to form GTP

34
Q

Succinyl-CoA synthetase catalyzes the conversion of _______________ to ______________.

A

Succinyl-CoA to succinate

35
Q

Which enzyme in the CAC reduces FAD?

A

Succinate dehydrogenase

36
Q

_____________________ _______________ catalyzes the conversion of succinate to fumarate.

A

Succinate dehydrogenase

37
Q

Which enzyme in the CAC catalyzes a hydration reaction?

A

Fumarase

38
Q

__________________ catalyzes the conversion of fumarate to malate via a hydration reaction.

A

Fumarase

39
Q

Isocitrate dehydrogenase, alpha-ketoglutarate dehydrogenase, and ___________________ dehydrogenase reduce NAD+ to NADH in the CAC.

A

Malate dehydrogenase

40
Q

_________________ _________________ catalyzes the conversion of malatae to oxaloacetate in the CAC>

A

Malate dehydrogenase

41
Q

___________________ ___________________ also known as Complex II of the electron transport chain; it is an integral membrane protein complex bound to the inner mitochondrial membrane that feeds electrons directly into the electron transport chain. It contains three different Fe-S clusters where electrons are passed from FADH2 to coenzuyme Q.

A

Succinate dehydrogenase

42
Q

Malate dehydrogenase is pro-____ specific.

A

Pro-R

43
Q

Why is the oxidation of malate to oxaloacetate spontaneous under cellular conditions despite the standard free energy change being close to 30 kJ?

A

Because OAA is constantly being converted to citrate; therefore, the product is very low, pushing the reaction forward despite such a positive standard free energy change

44
Q

What are the three irreversible steps of the CAC?

A
  1. Citrate synthase
  2. Isocitrate dehydrogenase
  3. Alpha-ketoglutarate dehydrogenase
45
Q

The CAC is controlled at the level of _________________ ____________ at three points: transport of pyruvate into the mitochondria by the mitochondrial pyruvate carrier, the pyruvate dehydrogenase complex, and citrate synthase.

A

Pyruvate entry

46
Q
A

Biochemistry II (19 decks)

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