3. TCA Cycle & Oxidative Phosphorylation

What reaction links glycolysis to the TCA cycle?

Pyruvate → Acetyl-CoA via the pyruvate dehydrogenase complex (PDH).

Where does the pyruvate dehydrogenase reaction occur?

Mitochondrial matrix (associated with the inner mitochondrial membrane).

What are the products of the PDH reaction?

Acetyl-CoA, NADH, CO₂.

What enzyme of PDH decarboxylates pyruvate?

Pyruvate decarboxylase (E1).

What happens to pyruvate when oxygen is limited (anaerobic metabolism)?

Pyruvate is reduced to lactate by lactate dehydrogenase (LDH). During this reaction NADH is oxidized to NAD⁺.

What is the purpose of converting pyruvate to lactate?

Regenerates NAD⁺ by reducing NADH, so glycolysis can continue producing ATP without oxygen.

What enzyme transfers the acetyl group to CoA in PDH?

Dihydrolipoamide acetyltransferase (E2

What enzyme regenerates oxidized lipoamide in PDH?

Dihydrolipoamide dehydrogenase (E3).

What mnemonic helps remember PDH cofactors?

TLCFN

What does TLCFN stand for?

T – Thiamine (B1 → TPP)
L – Lipoic acid
C – CoA (B5)
F – FAD (B2)
N – NAD⁺ (B3)

What cofactor forms a carbanion to attack pyruvate?

Thiamine pyrophosphate (TPP).

Thiamine correlates with vitamin ___

B1

Coa correlates with Pantothenate, vitamin ___

B5

Flavin correlates with vitamin __

B2

Niacin correlates with vitamin ___

B3

What form of PDH is active?

Dephosphorylated PDH (PDH-OH).

What form of PDH is inactive?

Phosphorylated PDH (PDH-P).

What enzyme inactivates PDH?

PDH kinase (phosphorylation)

What enzyme activates PDH?

PDH phosphatase (dephosphorylation).

What activates PDH phosphatase?

Ca²⁺

What molecules inhibit PDH (pyruvate dehydrogenase)?

ATP, NADH, Acetyl-CoA.

Why is Malate–aspartate shuttle needed for NADH from glycolysis?

NADH cannot cross the mitochondrial membrane.

What is the first step of the malate–aspartate shuttle in the cytosol?

Oxaloacetate is reduced to malate by malate dehydrogenase, using NADH → NAD⁺, which regenerates NAD⁺ for glycolysis.

How does malate enter the mitochondria during the shuttle?

enter: malate–α-ketoglutarate antiporter.

What happens to malate once it enters the mitochondrial matrix?

Malate is oxidized back to oxaloacetate, producing NADH inside the mitochondria, which then enters the electron transport chain.

Why must oxaloacetate be converted to aspartate in the malate–aspartate shuttle?

Oxaloacetate cannot cross the mitochondrial membrane, so it is converted to aspartate via transamination to allow transport out of the mitochondria.

How does aspartate complete the malate–aspartate shuttle?

Aspartate is transported to the cytosol, where it is converted back to oxaloacetate, completing the cycle.

Sources of acetyl CoA for TCA cycle?

pyruvate, fatty acids, ketone bodies, ketogenic amino acids, alcohol

What reaction converts isocitrate → α-ketoglutarate?

: Isocitrate dehydrogenase (produces NADH + CO₂).

What reaction converts α-ketoglutarate → succinyl-CoA?

α-ketoglutarate dehydrogenase (produces NADH + CO₂).

What enzyme produces GTP/ATP in the TCA cycle?

Succinyl-CoA synthetase. via SUBSTRATE LEVEL PHOSPHORYLATION

What reaction converts succinate → fumarate?

Succinate dehydrogenase (produces FADH₂).

What enzyme converts malate → oxaloacetate?

Malate dehydrogenase (produces NADH).

What coenzymes does a-Ketoglutarate require to add Coa

TLCFN

Why is succinate dehydrogenase unique?

It is both a TCA enzyme (succinate → fumarate) and ETC Complex II, producing FADH₂ that transfers electrons to CoQ.

What is the glycerol-3-phosphate shuttle?

Transfers electrons from cytosolic NADH → mitochondrial FADH₂, allowing them to enter the ETC at Complex II.

Why is the TCA cycle called amphibolic?

It functions in both catabolism and anabolism.

Why is the TCA cycle called amphibolic?

Because TCA intermediates can be removed to make other molecules (e.g., amino acids, glucose, heme) while the cycle also breaks down fuels to produce energy.

How does the TCA cycle supply electrons to the electron transport chain?

The TCA cycle produces NADH and FADH₂, which carry high-energy electrons to the electron transport chain where they are used to generate ATP.

Where does the electron transport chain occur?

Inner mitochondrial membrane.

What is the final electron acceptor in the ETC?

Oxygen (O₂).

What molecule is formed when oxygen accepts electrons?

Water (H₂O).

What are the energy yields of NADH and FADH₂ in the ETC in kcal?

NADH oxidation ≈ −56 kcal/mol and FADH₂ oxidation ≈ −42 kcal/mol.

What enters Complex I?

NADH.

What enters Complex II?

FADH₂.

What molecule transfers electrons between complexes?

Coenzyme Q (CoQ).

What carries electrons from Complex III to IV?

Cytochrome C.

What complex produces ATP?

ATP synthase (Complex V).

What inhibits Complex I?

Rotenone and barbiturates.

What inhibits Complex III?

Antimycin.

What inhibits Complex IV?

Cyanide (CN) and carbon monoxide (CO).

What inhibits ATP synthase at F0?

Oligomycin

What do uncouplers do in oxidative phosphorylation? and what are the 2?

Destroy the proton gradient.

2,4-Dinitrophenol (DNP) and Thermogenin (UCP1)

What happens to ATP production with uncouplers?

ATP decreases.

What happens to oxygen consumption with uncouplers?

O₂ consumption increases, oxidaion of NADH inc, ATP synthesis dec.

What is an exogenous uncoupler?

2,4-Dinitrophenol (DNP).

What endogenous uncoupler generates heat in brown fat?

Thermogenin (UCP1).

What causes myocardial infarction–related cell damage at the metabolic level?

Hypoxia (low O₂) disrupts oxidative phosphorylation, leading to ATP depletion and membrane damage.

What metabolic pathway becomes dominant in cardiac cells during hypoxia?

Anaerobic glycolysis, which increases lactate production.which results in lactic acidosis

What happens to cardiac cell membranes during myocardial infarction?

Membranes become leaky due to ATP depletion, leading to cell injury and necrosis.

Which enzymes increase in the blood during myocardial infarction due to cell damage?

Creatine kinase (CK) and lactate dehydrogenase (LDH) leak into the bloodstream.

How does cyanide (CN⁻) poison the electron transport chain?

Binds irreversibly to Complex IV (cytochrome a/a₃), blocking oxygen use and stopping ATP production.

ow is cyanide poisoning treated?

: Nitrites + thiosulfate. Nitrites form methemoglobin, which binds cyanide and prevents ETC inhibition.

How does carbon monoxide (CO) cause toxicity?

Binds hemoglobin and cytochrome oxidase, reducing oxygen delivery and ETC function.

How does doxorubicin affect the electron transport chain?

Inhibits Coenzyme Q (CoQ) in the ETC, disrupting electron transport and ATP production.

What enzymes detoxify ROS?

Superoxide dismutase and catalase.

What antioxidants neutralize ROS?

Vitamin E and Vitamin C.

What causes reperfusion injury?

after tissue damage Sudden oxygen return → large ROS production.

What is respiratory control of the TCA cycle and oxidative phosphorylation?

O₂ and ADP levels regulate ATP production: low O₂ ↑ NADH and slows the TCA cycle, while high ADP activates the TCA cycle and oxidative phosphorylation to make more ATP.