The citric acid cycle and oxidative phosphorylation

cellular respiration: The stages and Location

Glycolysis (changes from glucose to pyruvate and occurs in the cytoplasm

Oxidative decarboxylation of pyruvate —> Acetyl CoA occurs in the mitochrondria

Citric acid cycle occurs in the mitochrondria

Oxidative phosphorylation occurs in the mitochrondria

Before the citric acid cycle: fates of pyruvate

Pyruvate is formed by glycolysis

Anaerobic (hypoxic) conditions (with no oxygen):Pyruvate—> lactate

Aerobic conditions (with oxygen): pyruvate—> acetyl CoA—> citric acid cycle

Pyruvate transport: cytoplasm to mitochrondria

Pyruvate is formed in the cytoplasm

Pyruvate—> acetyl CoA: In the mitochrondria

Pyruvate transported: cytoplasm—>mitochrondria

Porin is a prtein which form channels in mitochrondria outer membrane

What is the overall equation of the citric acid cycle?

Acetyl CoA + 3 NAD⁺ + FAD + ADP + Pᵢ + 2 H₂O → CoA + 2 CO₂ + 3 NADH + FADH₂ + ATP + 2 H⁺.

Use the mnemonic to list the steps of the citric acid cycle.

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1. Citrate

2. Isocitrate

3. α-Ketoglutarate

4. Succinyl-CoA

5. Succinate

6. Fumarate

7. Malate

8. Oxaloacetate

Match the enzyme to its step in the citric acid cycle.

1. Citrate synthase (Step 1: Acetyl CoA → Citrate).

2. Aconitase (Step 2: Citrate → Isocitrate).

3. Isocitrate dehydrogenase (Step 3: Isocitrate → α-Ketoglutarate).

4. α-Ketoglutarate dehydrogenase complex (Step 4: α-Ketoglutarate → Succinyl-CoA).

5. Succinyl CoA synthetase (Step 5: Succinyl-CoA → Succinate).

What is oxidative phosphorylation?

The process of ATP formation by transferring electrons from NADH/FADH₂ to O₂ via the electron transport chain (ETC) and chemiosmosis.

List the four complexes of the ETC and their roles

1. Complex I (NADH dehydrogenase): Transfers electrons from NADH to ubiquinone.

2. Complex II (Succinate dehydrogenase): Transfers electrons from FADH₂ to ubiquinone.

3. Complex III (Cytochrome bc₁): Transfers electrons from ubiquinone to cytochrome c.

4. Complex IV (Cytochrome oxidase): Transfers electrons from cytochrome c to O₂ (forms H₂O).

How does the proton-motive force drive ATP synthesis?

1. Protons pumped out by ETC create a gradient (high H⁺ in intermembrane space).

2. Protons flow back through ATP synthase (F₀ subunit), causing rotation.

3. Rotation induces conformational changes in F₁ subunit (β subunits), synthesizing ATP from ADP + Pᵢ.

What are anaplerotic reactions?

Reactions that replenish citric acid cycle intermediates (e.g., oxaloacetate) when they are used for biosynthesis (e.g., amino acid synthesis).

Why is the citric acid cycle amphibolic?

It serves both catabolic (oxidation of acetyl CoA to CO₂) and anabolic (providing precursors for biosynthesis, e.g., amino acids) roles.

Oxidative phosphorylation

The process of ATP formation by the transfer of electrons from NADH and FADH2 to O2 via a series of electron carriers

Chemiosmosis

The reduction of oxygen means when proton is pumped across the inner mitochondrial membrane

Where does oxidative phosphorylation occur

Occurs in the mitochondria

Electron transport chain enzymes are embedded in the mitochondrial inner membrane

The electron transport chain

F1 β Subunits: Binding-Change Mechanism Hypothesis​

Subunit β is one structure with 3 conformations during H+ ion transport across the membrane

L (loose) conformation binds ADP and P loosely: catalytically inactive

T (tight) conformation binds ADP and P tightly and synthesizes ATP which cannot release it