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Biochem > TCA Cycle > Flashcards

TCA Cycle Flashcards

1
Q

Main function of TCA cycle

A

Capture 8 electrons from Acetyl CoA

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2
Q

Acetyl CoA gets completely _____

A

Oxidized

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3
Q

TCA cycle chemistry

A

4 oxidation reduction reactions that transfer electrons

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4
Q

What type of bonds are important in the TCA cycle? Why?

A

Thioester bonds because they are high energy

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5
Q

Key regulatory enzyme are regulated by ?

A

ALL are regulated allosterically by energy charge

NONE are regulated hormonally

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6
Q

What can enter the TCA cycle?

A

All types of fuels- CHO, FA, amino acids, ethanol

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7
Q

Intermediates from the TCA cycle are also used for?

A

Biosynthesis

Ex. Making amino acids, substrates for purines and pyrimidines, protoporphyrin ring in heme

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8
Q

Total yield per 1 Acetyl CoA

A 
2 CO2
3 NADH
1 FADH2
1 ATP (or GTP)
8 electrons
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9
Q

Catabolic role of TCA cycle? (4)

A
  1. Oxidizes acetyl CoA —> releases CO2
  2. Produces and delivers reducing equivalents (NADH and FADH2) to ETC
  3. Regenerates oxaloacetate (OAA)
  4. Generates a high energy molecule (ATP or GTP)
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10
Q

2 phases of the TCA cycle

A

Phase 1 = 4 reactions
- Oxidation of Acetyl CoA to yield 8 electrons and release 2 CO2

Phase 2 = 4 reactions
- Regenerate OAA

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11
Q

What needs to happen before acetyl CoA can be oxidized and why?

A

Have to combine acetyl CoA (2-C) with oxaloacetate (4-C) and carry out a beta cleavage

B/c acetyl CoA cannot be oxidized directly

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12
Q

BASIC summary of TCA cycle

A

Oxidize acetyl CoA —> release CO2 —> regenerate OAA —> capture all the energy as NADH and ATP

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13
Q

Step 1

A

Condensation reaction

Oxaloacetate + Acetyl CoA —> Citrate + CoA + H+

Enzyme: Citrate synthase

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14
Q

Ordered binding

  • What step?
  • What happens and why?
A

Step 1

OAA has to bind to citrate synthase first —> induce a conformational change to create a binding site for acetyl CoA

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15
Q

When is the thioester bond in acetyl CoA hydrolyzed?

Importance?

A

Step 1

Nrg release - ensures that reaction proceeds irreversibly in forward direction because OAA as a substrate in citrate synthase reaction is not sufficient enough to drive the cycle forward

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16
Q

Step 1 is a __________ reaction

Explain

A

First and last reaction of cycle are COUPLED

OAA + Acetyl CoA —> Citrate + CoA + H+ (first- favorable)
Enzyme: Citrate synthase
L-maleate + NAD+ —> OAA + NADH + H+ (last- unfavorable)
Enzyme: Malate dehydrogenase

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17
Q

How does step 1 proceed in the forward direction?

A

Combo of OAA being used as substrate in the reaction (product of last reaction) AND thioester bond in acetyl CoA being hydrolyzed

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18
Q

Step 2

A

Isomerization

Citrate Isocitrate

Importance:
Citrate is a poor substrate for oxidation
Isocitrate has a secondary -OH, which can be oxidized
Allows for oxidative decarboxylation in next step

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19
Q

Step 3

A

Oxidative Decarboxylation of Isocitrate

Isocitrate —> Oxalosuccinate —> alpha-ketoglutarate

RATE LIMITING STEP

Enzyme: Isocitrate dehydrogenase

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20
Q

Rate limiting step of TCA cycle

Why?

A

Step 3- Oxidative decarboxylation of Isocitrate

Why?
Because this is the first step in the cycle that produces NADH which a has to be regenerated to NAD+ to continue cycle

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21
Q

TCA cycle is dependent on the rate of ________ —> __________ is dependent on the rate of _______________

A

TCA cycle is dependent on the rate of the electron transport chain —> electron transport chain is dependent on the rate of ATP utilization

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22
Q

Use more ATP —> _______ electron transport chain

A

Faster

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23
Q

Faster regenerate NAD+ —> ____________ TCA Cycle

A

Faster

24
Q

If ATP levels build up, how does this affect the TCA cycle and ETC?

A

If ATP builds up —> ETC slows down —> NADH builds up —> feeds back and inhibits isocitrate dehydrogenase

25
Q

By what are ETC and TCA cycle chemically linked?

A

Energy Charge

26
Q

Step 3 “Oxidative”

A

NAD+ —> NADH

Oxidation of alcohol group to keto group (C=O)

27
Q

Step 3 “Decarboxylation”

A

Get rid of a CO2

28
Q

Step 4

A

Second Oxidative Decarboxylation

Alpha-ketoglutarate + NAD+ + CoA —> Succinylcholine CoA + CO2 + NADH

Enzyme: alpha-ketoglutarate dehydrogenase

  • Produces a thioester bond in succinyl CoA
29
Q

The enzyme alpha-ketoglutarate dehydrogenase is identically to _____ mechanistically and structurally

A

Pyruvate dehydrogenase complex

30
Q

Importance of succinyl CoA

A

Has a thioester bond

31
Q

Step 5

A

Substrate level Phosphorylation

Succinyl CoA + Pi + GDP (or ADP) —> Succinate + CoA + GTP (or ATP)

32
Q

Explain substrate level phosphorylation

A

Transfer an inorganic phosphate to either GDP or ADP using energy from the hydrolysis of a CoA thioester bond

33
Q

Step 6

A

Dehydrogenation

Succinate + FAD —> fumerate + FADH2

Enzyme: Succinate dehydrogenase

34
Q

Importance of Succinate dehydrogenase

A

Bound to the inner mitochondrial membrane

Part of complex II

35
Q

______ is covalently bound to Succinate dehydrogenase.

Why?

A

FAD

To reduce the activity of the molecule because the FADH radical can damage. So ensure the full reduction to FADH2 by accepting 1 e-/1H+ and 1e-/1H+ in succession

36
Q

Step 7

A

Hydration

Fumerate —> Malate

Add a water molecule across the double bond

37
Q

Step 8

A

NAD+ Dependent Oxidation

L-Malate + NAD+ —> OAA + NADH + H+

Enzyme: Malate dehydrogenase

38
Q

3 enzymes that are regulated

A
  1. Citrate synthase
  2. Isocitrate dehydrogenase
  3. Alpha-keto dehydrogenase
40
Q

Regulation of citrate synthase

A
  1. [OAA] - very limiting

2. Feedback inhibited by product, citrate

41
Q

concentration of ___ and ___ fluctuate a lot in cell depending on metabolic needs

A

OAA and acetyl CoA

42
Q

Regulation of alpha-ketoglutarate dehydrogenase

A

ATP, NADH Inhibit

Succinyl CoA feedback Inhibit

43
Q

ATP plays 2 roles

A

Allosteric modifier of many key enzymes and indirect feedback inhibits

44
Q

Usually…..

ATP and _____ go together
ADP and _____ go together

A

ATP & NADH

ADP & NAD+

45
Q

Is molecular oxygen consumed in TCA cycle?

A

NO!

46
Q

Why is TCA cycle still considered part of aerobic respiration?

A

Even though molecular oxygen is not consumed, oxygen is required as the final electron acceptor in the ETC. It is needed for the regeneration of NAD+ and FAD

47
Q

ATP controls its ________.

How?

A

Own rate of synthesis

By acting as negative allosteric modifier on enzymes in glycolysis, on PDC, and TCA cycle

48
Q

Why do you need hormonal regulation at PDC and pyruvate kinase?

A

Critical when pathways share reactions

Ex. Glycolysis and gluconeogenesis

49
Q

Cells in resting metabolic state

A

High ATP/ADP

Hight NADH/NAD+

50
Q

Cells in highly active metabolic state

A

Low ATP/ADP

Low NADH/NAD+

51
Q

3 molecules that DO NOT leave the mitochondria directly

A
  1. Acetyl CoA
  2. OAA
  3. NADH
52
Q

Citrate in other pathways

A
  • Exits TCA cycle in fed state
  • Used as carrier for 2-C of acetyl CoA
  • Synthesis of fatty acids/sterols
53
Q

Alpha-ketoglutarate in other pathways

A
  • Glutamic acid synthesis by adding amino group
  • Make other amino acids and purines
  • Reaction can go both ways
54
Q

Succinyl CoA in other pathways

A
  • Building block for porphyrin ring structure

Ex. Home

55
Q

Oxaloacetate in other pathways

A
  • keep TCA cycle going
  • gluconeogenesis
  • asparatic acid synthesis by adding amino acid to make other amino acids, purines, pyrimidines (goes both ways)
56
Q

Pyruvate carboxylase function

Type of reaction? How?

A

Can generate OAA when TCA cycle intermediates are used in other reactions

“Anapleurotic reaction”

Use pyruvate as a substrate —> Add CO2 —> yield 4-C OAA

57
Q

Why is pyruvate carboxylase important (2)

A
  1. Maintains TCA cycle in fed AND fasted states

2. Supplies OAA in liver for gluconeogenesis

70
Q

Regulation of Isocitrate dehydrogenase

A

RATE LIMITING STEP - 1st reaction that produces NADH —> which must be oxidized to NAD+ in ETC for TCA cycle to continue in this step

ADP and NAD+ = positive allosteric modifier (activate)
(Low energy charge stimulates)

NADH and ATP = negative allosteric modifier (inhibit)

Biochem (15 decks)

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