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The Citric Acid Cycle Flashcards

1
Q

Anaplerotic Reaction

A

A reaction that replenishes the citric acid cycle’s intermediates.

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2
Q

Example of an Anaplerotic Reaction

A

Synthesis of succinyl CoA from Ile, Val, Met

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3
Q

Cataplerotic Reaction

A

A reaction that removes intermediates from the citric acid cycle.

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4
Q

Example of a Cataplerotic Reaction

A

OAA entering gluconeogenesis

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5
Q

What is produced by glycolysis?

A

2 pyruvate, 2 NADH, 2 ATP

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6
Q

What are the benefits of multienzyme complexes?

A
  • Reduce the distance that substrates have to diffuse
  • Reduces risk of side reactions
  • Allows coordinate control of reactions
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7
Q

What does pyruvate dehydrogenase do?

A

Form acetyl CoA from pyruvate.

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8
Q

E1

A

Pyruvate dehydrogenase

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9
Q

E2

A

Dihydrolypoyl transacetylase

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10
Q

E3

A

Dihydrolypoyl dehydrogenase

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11
Q

What are the 5 coenzymes required by PDH?

A
  • TPP
  • Lipoamide
  • CoA
  • FAD (Bound)
  • NAD+
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12
Q

Which of the PDH complex’s coenzymes are prosthetic?

A
  • TPP
  • Lipoamide
  • FAD (Bound)
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13
Q

Prosthetic group

A

A nonprotein group that is tightly linked to the protein.

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14
Q

What does TPP-E1 do (first reaction of PDH complex)?

A

It decarboxylates the pyruvate, forming the hydroxyethyl group and CO2.

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15
Q

What is the second reaction of the PDH complex?

A

The hydroxyethyl group attacks the disulfide component of E2, forming dihydrolipoamide and regenerating E1.

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16
Q

How does arsenic poisoning occur?

A

Arsenite binds to the thiol group on E2, preventing hydroxyethyl from binding.

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17
Q

What does E2 catalyze (third reaction of the PDH complex)?

A

Transesterification where acetyl group is transferred to CoA, forming acetyl CoA and dihydrolipoamide-E2.

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18
Q

What type of reaction does E2 catalyze?

A

Transesterification

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19
Q

What type of reaction does E1 catalyze?

A

Decarboxylation

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20
Q

What type of reaction does E3 catalyze (fourth reaction of PDH complex)?

A

It reoxidizes the dihydrolipoamide (thiol back to disulfide), regenerating the original E2 structure.

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21
Q

What is the fifth reaction of the PDH complex?

A

E3 transfers its protons and electrons to FAD, then to FADH. Forms NADH and regenerates E3.

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22
Q

Which component of a PDH subunit acts as a swinging arm to transfer intermediates between the complexes?

A

The lipoamide group on E2

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23
Q

Citrate Synthase

A

Condensation of oxaloacetate and acetyl CoA

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24
Q

What type of reaction does citrate synthase catalyze?

A

Condensation

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25
Aconitase
Reversible isomerization of citrate, cis-aconitate, and isocitrate.
26
What type of reaction does aconitase catalyze?
Reversible isomerization
27
What is notable about the structure of aconitase?
It has an iron-sulfur complex that helps coordinate the reaction
28
Isocitrate Dehydrogenase
Oxidative decarboxylation of isocitrate to alpha-ketoglutarate.
29
What type of reaction does isocitrate dehydrogenase catalyze?
Oxidative decarboxylation
30
Which enzyme produces the first CO2 and NADH of the CAC?
Isocitrate dehydrogenase (isocitrate to alpha-ketoglutarate)
31
Alpha-Ketoglutarate Dehydrogenase
Oxidative decarboxylation of alpha-ketoglutarate to succinyl CoA.
32
What type of reaction does alpha-ketoglutarate dehydrogenase catalyze?
Oxidative decarboxylation (alpha-ketoglutarate to succinyl CoA)
33
Which enzyme produces the second NADH and CO2 of the CAC?
Alpha-ketoglutarate Dehydrogenase (alpha-ketoglutarate to succinyl CoA)
34
Which enzyme of the CAC resembles PDH's reactions?
Alpha Ketoglutarate Dehydrogenase (high-energy thioester)
35
Succinyl CoA Synthetase
Succinyl CoA to succinate; substrate-level phosphorylation of GDP
36
What type of reaction does succinyl CoA synthetase catalyze?
Substrate-level phosphorylation (succinyl-CoA to succinate, GTP produced)
37
Which enzyme catalyzes the creation of GTP in the CAC?
Succinyl-CoA Synthetase
38
Succinate Dehydrogenase
Stereospecific dehydrogenation of succinate to fumarate
39
What type of reaction does succinate dehydrogenase catalyze?
Stereospecific dehydrogenation (succinate to fumarate), producing FADH2.
39
What is unusual about the FAD in the succinate dehydrogenase reaction?
It is covalently linked to the enzyme
40
What is the one membrane-bound enzyme in the CAC?
Succinate dehydrogenase (also complex II in ETS)
41
Which enzyme in the CAC produces FADH2?
Succinate dehydrogenase (complex II in ETS)
42
Fumarase
Hydrogenation of fumarate to malate.
43
What type of reaction does fumarase catalyze?
Hydrogenation (fumarate to malate)
44
Malate Dehydrogenase
Malate to oxaloacetate, producing NADH.
45
Which enzyme of the CAC catalyzes an endergonic reaction?
Malate dehydrogenase (malate to OAA)
46
What does the conversion of malate to OAA rely on?
High concentrationsof malate
47
Which enzyme creates the third NADH of the CAC?
Malate dehydrogenase
48
How many electron pairs are transferred during the CAC?
4 pairs (3 to NADH, 1 to FADH2)
49
What are the products of the CAC (from one glucose molecule)?
6 NADH, 2 FADH2, 4 CO2, 2 GTP
50
What are the products of the conversion of pyruvate to acetyl CoA (from one glucose molecule)?
2 acetyl CoA, 2 CO2, 2 NADH
51
How many ATP generated per NADH?
2.5
52
How many ATP generated per FADH2?
1.5
53
How many protons pumped per NADH?
10
54
How many protons pumped per FADH2?
6
55
Covalent modification of PDH
NADH and Acetyl CoA produce pyruvate dehydrogenase kinase; insulin activates pyruvate dehydrogenase phosphatase.
56
Equation of conversion of alanine to pyruvate
Alanine + Alpha-Ketoglutarate --> Pyruvate + Glutamate
57
Allosteric inhibition of the citric acid cycle
NADH and acetyl CoA compete for binding spots on the enzymes; high rates of NADH and acetyl CoA keep E2 unable to accept hydroxyethyl.
58
What are the three rate-determining enzymes of the CAC?
- Citrate Synthase - Isocitrate Dehydrogenase - Alpha-ketoglutarate dehydrogenase
59
Why is citrate synthase a rate-determining enzyme?
Relies on quantities of OAA (relies on malate) and acetyl CoA
60
Why is isocitrate dehydrogenase a rate-determining enzyme?
Activated by ADP, inhibited by NADH and ATP, NAD+ dependent
61
Why is alpha-ketoglutarate dehydrogenase a rate-determining enzyme?
Activated by ADP, inhibited by NADH and ATP, NAD+ dependent
62
What are the three primary mechanisms of flux control in the CAC?
- Substrate availability (i.e. OAA and acetyl CoA) - Product inhibition (e.g. succinyl CoA and NADH) - Competitive feedback inhibition (NADH and ATP allosteric inhibitors)
63
Why is the citric acid cycle amphibolic?
The intermediates are used differently (e.g. OAA used for gluconeogenesis, amino acids form succinyl CoA)
64
Glutamate dehydrogenase
Alpha-ketoglutarate converted to glutamate.
65
What type of reaction does glutamate dehydrogenase catalyze?
Reductive amination (glutamate biosynthesis)
66
What other molecule does glutamate biosynthesis require?
NADH or NADPH
67
Equation for aspartate biosynthesis
OAA + Alanine --> Aspartate + Pyruvate
68
What three enzymes convert pyruvate to molecules for the CAC?
- Pyruvate dehydrogenase - Transaminase - Pyruvate carboxylase
69
Transaminase
Converts pyruvate --> glutamate --> alpha-ketoglutarate
70
Pyruvate carboxylase
Converts pyruvate to OAA
71
Glyoxylate Cycle
Converts acetyl CoA (usually from fatty acids) to OAA, bypassing the CO2-generating steps. Can be used to synthesize sugars (hence need to conserve carbon).
72
Which organisms use the glyoxylate cycle?
Plants, bacteria, and fungi
73
Where does the glyoxylate cycle occur?
Acetyl CoA molecules are turned into succinate in the glyoxysome. Succinate goes to mitochondria or cytosol for conversion to OAA.
74
Which component of the glyoxylate cycle occurs in the glyosysome?
Conversion of acetyl CoA to succinate
75
When does succinate produced in the glyoxysome go to the cytosol?
When OAA is needed for glucoeneogenesis
76
When does succinate produced in the glyoxosome go to the mitochondrion?
When OAA is needed for the CAC
77
What are the two new enzymes in the glyoxylate cycle?
Isocitrate lyase, malate synthase
78
Isocitrate lyase
Converts isocitrate to succinate and glyoxylate
79
Malate synthase
Combines glyoxylate and acetyl CoA to form malate

BB 491 (7 decks)

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