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CHEM 441A+B > Chapter 10: The Citrate Cycle > Flashcards

Chapter 10: The Citrate Cycle Flashcards

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1
Q

How does the citrate cycle capture energy

A

Redox reactions

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2
Q

What are two biomolecules that the citrate cycle generates?

A
  • NADH and FADH2
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3
Q

Why is the Citrate cycle so important?

A

It is central to aerobic metabolism and ATP production

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4
Q

What is the primary function of Acetyl CoA? What does each Citrate cycle yield.

A
  • The primary function is to oxidize acetyl CoA

Each cycle yields:
- Transfer of 8 electrons
- Generates 3 NADH
- Generates 1 FADH2
- 1 GTP (ATP equivalent)
- Each cycle results in production of 10 ATP
- Mostly via oxidative phosphorylation of 3NADH and 1 FADH2

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5
Q

What converts Pyruvate into acetyl-CoA?

A

Pyruvate dehydrogenase

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6
Q

What five coenzymes are required for the pyruvate dehydrogenase complex?

A
  • NAD+
  • FAD
  • CoA
  • Thiamine pyrophosphate (TPP)
  • Alpha-lipoic acid
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7
Q

What part of NAD+ and NADP+ is reduced?

A

Nicotinamide

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8
Q

What is NAD+ derived from?

A

Niacin (vitamin B3)

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9
Q

What is FAD derived from?

A

Riboflavin (vitamin B5)

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10
Q

What is CoA derived from?

A

Vitamin B5
- Generated by Pantothenic acid

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11
Q

What is Thiamine pyrophosphate (TPP) derived from?

A

Vitamin B1
- Required for pyruvate dehydrogenase

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12
Q

What does α-lipoic acid do?

A
  • Provides reactive disulfide that participates in redox reactions
  • Accepts and transfers acetyl group to CoA
  • Not a vitamin
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13
Q

What are the five steps in pyruvate dehydrogenase?

A

1.Pyruvate binds and is decarboxylated causing the formation of hydroxyethyl-TPP
2. Acetyl group transferred to lipoamide
3. Acetyl group transfered to CoA
4. Dihydroxylipoamide oxidized to lipoamide
5. FAD oxidized forming FADH2 which can reduce NAD+ to NADH

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14
Q

Describe the general structure of PDH protein complex

A
  • 22 E1 subunits
  • 60 E2 subunits
  • 6 E3 subunits
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15
Q

How does Arsenic affect Pyruvate dehydrogenase?

A
  • Irreversibly blocks catalytic activity of lipoamide-containing enzymes
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16
Q

Describe the steps of E1, E2, and E3 in the ball and chain PDH

A
  1. E2 ball and chain move lipoamide domain between E1 and E2 catalytic site
  2. Acetyl group from TPP in E1 to the CoA substrate in the E2 catalytic site
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17
Q

What are negative and positive regulators of PDH?

A

Positive:
- NAD+
- CoA
- ADP

Negative:
- NADH
- Acetyl- CoA
- ATP

18
Q

How many reactions are in the citrate cycle?

A

8

19
Q

How favorable or unfavorable are the equations in the citrate cycle?

A
  • 3 reactions have a large -ΔG°’ and thus are key regulators
  • 4 reactions have a ΔG°’ close to 0
  • 1 reaction has a large +ΔG°’
20
Q

What happens in step 1 of the citrate cycle?

A

Reaction: Condensation
Enzyme: Citrate synthase
Reactants: Oxaloacetate, Acetyl-CoA, and H2O
Product: Citrate

(Highly Favorable)

21
Q

What is the mechanism behind citrate synthase?

A
  1. Asp375 deptrotenates acetyl-CoA -> enolate intermediate (stabilized by H bond of His274)
  2. Nu attack by enolate in on oxaloacetate -> citryl-CoA + deprotonation of His320
  3. Water is added which helps hydrolyze citryl-CoA thioester
22
Q

How does conformation affect binding in citrate synthase?

A
  • Open conformation promotes oxo binding
  • Closed conformation promotes acetyl-CoA binding
  • Prevents acetyl-CoA thioester hydrolysis
23
Q

What is step 2 in the citrate cycle?

A

Reaction: dehydration-hydration
Enzyme: Aconitase
Reactants: Citrate and H2O
Product: (intermediate cis-aconitate) Isocitrate

24
Q

What is the function of the Fe-S cluster enzyme?

A
  • Facilitates the removal of OH group from citrate
25
Q

What is fluoroacetate and what does it do?

A
  • Converted to Fluoroacetyl-CoA then Fluorocitrate
  • Potent inhibitor of Aconitase
26
Q

What is step 3 in the citrate cycle?

A

Reaction: Oxidative decarboxylation
Enzyme: Isocitrate dehydrogenase
Reactants: Isocitrate, NADP+, and H+
Product: NADPH, (intermediate oxalosuccinate), CO2, and alpha ketoglutarate

27
Q

Which step is the rate limiting reaction in the citrate cycle?

A

Step 3 or isocitrate dehydrogenase

28
Q

What are positive and negative effectors of step 3 in the Citrate cycle?

A

Positive:
- ADP and CA2+

Negative:
- ATP and NADH

29
Q

What is step 4 in the citrate cycle?

A

Reaction: Decarboxylation
Enzyme: α-Ketoglutarate Dehydrogenase (E1, E2, E3)
Reactants: α-Ketoglutarate, CoA, and NAD+
Product: CO2, NADH +H+, and Succinyl-CoA

(Same reaction as Pyruvate dehydrogenase)
(Energetically favorable)

30
Q

What is step 5 in the citrate cycle?

A

Reaction: Substrate level phosphorylation
Enzyme: Succinyl-CoA synthetase
Reactants: Succinyl-CoA, Pi, and GDP
Product: CoA, GTP, and Succinate

31
Q

What is step 6 in the citrate cycle?

A

Reaction: Oxidation
Enzyme: Succinate dehydrogenase
Reactants: Succinate and FAD
Product: FADH2 and Fumarate

(part of ETC)

32
Q

What is step 7 in the citrate cycle?

A

Reaction: Hydration
Enzyme: Fumarase
Reactants: Fumarate, OH-, and H+
Product: (carbanion intermediate) and Malate

(stereospecific L-isomer)

33
Q

What is step 8 in the citrate cycle?

A

Reaction: Oxidation
Enzyme: Malate dehydrogenase
Reactants: Malate and NAD+
Product: NADH + H+ and Oxaloacetate

34
Q

What does one citrate cycle produce

A
  • One cycle:
    • 3 NADH
    • 1 FADH2
    • 1 GTP
  • Equals 10 ATP
  • 2 carbons enter as Acetyl-CoA
  • Leave cycle via decarboxylation as CO2
    • Not same carbons as Carbons eliminated in step 1
35
Q

What are the three main regulatory regulatory enzymes in the citrate cycle?

A
  • Citrate Synthase
  • Isocitrate synthase
  • α-ketoglutarate
    dehydrogenase
36
Q

How can culture conditions affect the citrate cycle? How can citrate export increase?

A
  • Citrate cycle can be inhibited by culture conditions
  • Inhibitions of Citrate lyase results in increase citrate export
37
Q

The citrate cycle is an ________ pathway.

A

Amphibolic (both anabolic and catabolic

38
Q

What is an anaplerotic reaction?

A

A reaction that replenishes Citrate cycle intermediates from other metabolic pathways

Examples:
Pyruvate carboxylase
Requires biotin

39
Q

What is biotin?

A

A CO2 carrier

40
Q

What does phosphoenolpyruvate carboxylase do?

A

Replenish Oxaloacetate

41
Q

What are the Two main functions of the citrate cycle?

A

Function 1
- Oxidize acetyl-CoA
- Yields 3 NADH, 1 FADH2, 1 GTP
- Can produce 10 ATP

Function 2
- Provides metabolites for other pathways
- Amino acids
- Heme
- Fatty acids

CHEM 441A+B (22 decks)

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