Citric Acid Cycle (11)

What are the 3 main functions of the CAC?

1. Produce reducing equivalents (NADH and FADH₂)

2. Form organic intermediates

3. Release CO₂

Is CAC an aerobic or anaerobic process?

It runs under aerobic conditions but doesn’t consume O₂

Does the CAC generate energy?

It stores it in the form of high energy electrons by NAD+ and FAD+

First reaction carbon count

2C+4C → 6C

Last reaction carbon count

4C regenerated

From where does the CO₂ come from?

Following the addition of Acetyl-CoA to oxaloacetate, 2 CO₂ escape the cell

How are the high energy intermediates used?

To generate one C-C bond + 1 GTP

Net energy production

Total: 10 ATP

1. 3 NADH : 7.5 ATP

2. 1 FADH2 : 1.5 ATP

3. 1 GTP : 1 ATP

How is directionality provided to this cycle?

By the irreversible reactions (-ve delta G)

What are the 3 enzymes involved in the irreversible steps?

1. Citrate synthase (1)

2. Isocitrate dehydrogenase (3)

3. Ketoglutarate dehydrogenase (4)

Which steps are the high energy steps?

The first 5 (until formation of succinate)

What is the key point of step #1?

Only reaction where a C-C bond is formed

What is the key point of step #2?

The enzyme that perfoms this reaction is named after the high energy intermediate formed by it: aconitate

What makes a reaction irreversible?

The release of CO₂ out of the mitochondria

What is the key point of step #4?

The decarboxylation reaction provides the energy to generate the high energy intermediate: succinyl-CoA

What is special about Ketoglutarate dehydrogenase?

It is a multienzyme complex like PDC that shares the same last subunit (E3)

What is the key point of step #5?

Its enzyme uses the high energy succinyl-CoA to generate GTP

What are the last 3 steps called?

Recycling steps

What is the key point of step #6?

FAD+ is reduced to FADH2 through succinate dehydrogenase

What is specific about FAD+ compared to NAD+?

It is not labile so it has to be covalently bound to succinate dehydrogenase

What is special about succinate dehydrogenase?

It is an integral part of Complex II in the ETC

What is the key point of step #7?

Hydration of the double bond of fumarate

What is the key point of step #8?

Although it is at equilibrium, [malate] > > > > [oxaloacetatate] bc oxaloacetate is almost immediately consumed

ATP from glycolysis

2 ATP + 2 NADH → 7 ATP

ATP from PDH

1 NADH → 2.5 ATP

ATP from CAC

3 NADH + 1 FADH + 1 GTP → 10 ATP

Total amount of ATP

32 ATP

Are acetyl-CoA and oxaloacetate modulators of citrate synthase?

No, but the reaction rate varies with their concentration

Which molecule is a product inhibitor of citrate synthase?

Citrate competes with oxaloacetate

Which molecule competes with acetyl-CoA via competitive feedback?

Succinyl-CoA

What are the allosteric modulators of citrate synthase?

NADH (-) and ADP (+)

What molecule is a product inhibitor of isocitrate dehydrogenase?

NADH which displaces NAD+

Why is surplus citrate an indicator of high energy charge?

Bc it means that the enzyme was inhibited by NADH, leading to an accumulation of isocitrate ←> citrate

What are allosteric activators of isocitrate dehydrogenase?

ADP and Ca²⁺

What is a consequence of NADH surplus?

Low glycolytic activity (citrate inhibitor)

Which molecules are product inhibitors of ketoglutarate dehydrogenase?

NADH and succicyl-CoA

What molecule is an allosteric activator of ketoglutarate dehydrogenase?

Ca²⁺