TCA Cycle Flashcards by Lindsay Roethke

Main function of TCA cycle

Capture 8 electrons from Acetyl CoA

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Acetyl CoA gets completely _____

Oxidized

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TCA cycle chemistry

4 oxidation reduction reactions that transfer electrons

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

Thioester bonds because they are high energy

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Key regulatory enzyme are regulated by ?

ALL are regulated allosterically by energy charge

NONE are regulated hormonally

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What can enter the TCA cycle?

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

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Intermediates from the TCA cycle are also used for?

Biosynthesis

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

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Total yield per 1 Acetyl CoA

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

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Catabolic role of TCA cycle? (4)

Oxidizes acetyl CoA —> releases CO2
Produces and delivers reducing equivalents (NADH and FADH2) to ETC
Regenerates oxaloacetate (OAA)
Generates a high energy molecule (ATP or GTP)

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2 phases of the TCA cycle

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

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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BASIC summary of TCA cycle

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

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Step 1

Condensation reaction

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

Enzyme: Citrate synthase

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Ordered binding

What step?
What happens and why?

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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When is the thioester bond in acetyl CoA hydrolyzed?

Importance?

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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Step 1 is a __________ reaction

Explain

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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How does step 1 proceed in the forward direction?

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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Step 2

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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Step 3

Oxidative Decarboxylation of Isocitrate

Isocitrate —> Oxalosuccinate —> alpha-ketoglutarate

RATE LIMITING STEP

Enzyme: Isocitrate dehydrogenase

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Rate limiting step of TCA cycle

Why?

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

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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Use more ATP —> _______ electron transport chain

Faster

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Faster regenerate NAD+ —> ____________ TCA Cycle

Faster

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

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

By what are ETC and TCA cycle chemically linked?

Energy Charge

Step 3 “Oxidative”

NAD+ —> NADH Oxidation of alcohol group to keto group (C=O)

Step 3 “Decarboxylation”

Get rid of a CO2

Step 4

Second Oxidative Decarboxylation Alpha-ketoglutarate + NAD+ + CoA —> Succinylcholine CoA + CO2 + NADH Enzyme: alpha-ketoglutarate dehydrogenase * Produces a thioester bond in succinyl CoA

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

Pyruvate dehydrogenase complex

Importance of succinyl CoA

Has a thioester bond

Step 5

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

Explain substrate level phosphorylation

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

Step 6

Dehydrogenation Succinate + FAD —> fumerate + FADH2 Enzyme: Succinate dehydrogenase

Importance of Succinate dehydrogenase

Bound to the inner mitochondrial membrane Part of complex II

______ is covalently bound to Succinate dehydrogenase. Why?

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

Step 7

Hydration Fumerate —> Malate Add a water molecule across the double bond

Step 8

NAD+ Dependent Oxidation L-Malate + NAD+ —> OAA + NADH + H+ Enzyme: Malate dehydrogenase

3 enzymes that are regulated

1. Citrate synthase 2. Isocitrate dehydrogenase 3. Alpha-keto dehydrogenase

Regulation of citrate synthase

1. [OAA] - very limiting | 2. Feedback inhibited by product, citrate

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

OAA and acetyl CoA

Regulation of alpha-ketoglutarate dehydrogenase

ATP, NADH Inhibit Succinyl CoA feedback Inhibit

ATP plays 2 roles

Allosteric modifier of many key enzymes and indirect feedback inhibits

Usually..... ATP and _____ go together ADP and _____ go together

ATP & NADH ADP & NAD+

Is molecular oxygen consumed in TCA cycle?

NO!

Why is TCA cycle still considered part of aerobic respiration?

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

ATP controls its ________. How?

Own rate of synthesis By acting as negative allosteric modifier on enzymes in glycolysis, on PDC, and TCA cycle

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

Critical when pathways share reactions Ex. Glycolysis and gluconeogenesis

Cells in resting metabolic state

High ATP/ADP | Hight NADH/NAD+

Cells in highly active metabolic state

Low ATP/ADP | Low NADH/NAD+

3 molecules that DO NOT leave the mitochondria directly

1. Acetyl CoA 2. OAA 3. NADH

Citrate in other pathways

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

Alpha-ketoglutarate in other pathways

- Glutamic acid synthesis by adding amino group - Make other amino acids and purines - Reaction can go both ways

Succinyl CoA in other pathways

- Building block for porphyrin ring structure Ex. Home

Oxaloacetate in other pathways

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

Pyruvate carboxylase function Type of reaction? How?

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

Why is pyruvate carboxylase important (2)

1. Maintains TCA cycle in fed AND fasted states | 2. Supplies OAA in liver for gluconeogenesis

Regulation of Isocitrate dehydrogenase

*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)