Lecture 22 - TCA Cycle

What are the three major parts of cellular respiration?

Pyruvate processing, the citric acid cycle, and the electron transport chain.

Where does the citric acid cycle take place?

In the mitochondrial matrix.

What are cristae in mitochondria?

Extensions of the inner membrane.

What is the function of cristae?

Increase surface area for ETC proteins.

How is pyruvate transported after glycolysis?

It is transported from cytosol into the mitochondria.

What is the net reaction of pyruvate processing?

Oxidative decarboxylation of pyruvate to acetyl-CoA.

During pyruvate processing, which carbon atoms of glucose are first fully oxidized?

The first carbons released as CO₂.

What enzyme complex catalyzes the conversion of pyruvate to acetyl-CoA?

Pyruvate dehydrogenase complex (PDC).

How many coenzymes does PDC require?

Five coenzymes.

What are the five coenzymes required by PDC?

TPP, lipoic acid (lipoate), FAD, NAD⁺, and CoA-SH.

What regulates PDC activity?

Feedback inhibition by NADH and acetyl-CoA.

What happens to pyruvate dehydrogenase when products are abundant?

It becomes phosphorylated and inhibited.

What happens during Step 1 of pyruvate decarboxylation?

CO₂ is released from pyruvate.

What cofactor is required for Step 1 of PDC?

Thiamine pyrophosphate (TPP).

What occurs during Step 2 of the PDC reaction?

Oxidation and transfer of electrons to lipoamide.

What high-energy intermediate forms during Step 2?

Acetyl-lipoamide (a thioester).

What enzyme performs Step 3 in the PDC mechanism?

E2, dihydrolipoyl transacetylase

What happens during Step 3 of PDC?

Transfer of the acetyl group to CoA, forming acetyl-CoA.

What enzyme performs Steps 4 and 5 of PDC?

E3, dihydrolipoyl dehydrogenase.

What happens during Step 4 of PDC?

Reoxidation of dihydrolipoamide using FAD.

What happens during Step 5 of PDC?

Regeneration of FAD by reducing NAD⁺ to NADH.

Is CoA a permanent part of enzyme structures?

No, it binds transiently.

What is the main functions of CoA?

To accept and carry acetyl groups.

When is CoA released in the CAC?

Immediately in Step 1 when citrate forms.

What molecule enters the citric acid cycle?

Acetyl-CoA.

What makes the citric acid cycle amphibolic?

It participates in both catabolic and anabolic pathways.

What two molecules react in CAC Step 1?

Acetyl-CoA and oxaloacetate.

What type of reaction is Step 1 of CAC?

Carbon–carbon bond formation.

What is the rate-limiting step of the TCA cycle?

Step 1 (citrate synthase).

What controls Step 1 of CAC?

[oxaloacetate] availability and product inhibition.

Why must citrate be isomerized?

Citrate (a tertiary alcohol) is a poor substrate for oxidation.

What is the product of citrate isomerization?

Isocitrate (a secondary alcohol).

Why is Step 2 thermodynamically unfavorable?

The equilibrium lies toward citrate.

What happens to isocitrate in Step 3?

It undergoes oxidative decarboxylation, releasing CO₂.

What high-energy molecule is produced in Step 3?

NADH

How is Step 3 regulated?

Inhibited by high ATP levels.

What does Step 4 produce?

NADH and CO₂.

Which enzyme complex resembles α-ketoglutarate dehydrogenase?

Pyruvate dehydrogenase complex.

What makes Step 4 highly favorable?

Large negative ΔG°′ (−33.5 kJ/mol).

What energy-rich bond drives Step 5?

A thioester bond in succinyl-CoA.

What nucleotide is produced in Step 5?

GTP.

What type of phosphorylation occurs in Step 5?

Substrate-level phosphorylation.

Where is succinate dehydrogenase located?

In the inner mitochondrial membrane.

What electron carrier is produced in Step 6?

FADH₂.

Why is FAD used instead of NAD⁺ in Step 6?

The oxidation potential of the C–H bond is too low for NAD⁺.

What type of stereochemistry does fumarase exhibit?

Trans addition of water to fumarate.

What product is formed in Step 7?

L-malate.

Why is Step 8 thermodynamically unfavorable?

ΔG°′ is highly positive.

How is Step 8 pulled forward?

Citrate synthase rapidly consumes oxaloacetate.

What is the net energy capture of one turn of CAC?

3 NADH, 1 FADH₂, 1 GTP.

How many turns of CAC occur per glucose molecule?

Two turns.

What does it mean that CAC intermediates are amphibolic?

They serve in both breakdown and biosynthesis.

Why are anaplerotic reactions necessary?

To replenish intermediates removed for biosynthesis.

How are 4-carbon intermediates replenished?

By carboxylation of 3-carbon precursors.

During one turn of TCA, is there net production of oxaloacetate?

No — oxaloacetate is regenerated, not produced.

What two mechanisms convert pyruvate to acetyl-CoA?

Decarboxylation and oxidation.

How many electrons are transferred per turn of CAC?

Eight electrons (6 to NAD⁺, 2 to FAD).

How many NADH form from complete glucose oxidation?

10 NADH total (2 glycolysis, 2 PDC, 6 TCA).

Which TCA enzyme is NOT irreversible?

Succinate dehydrogenase (near equilibrium).