4. TCA Cycle (Mitochondria)

What is the main function of the TCA cycle?

The TCA cycle extracts high-energy electrons by reducing NAD⁺ and FAD to form NADH and FADH₂, which then donate electrons to the electron transport chain to drive ATP production. It also fully oxidizes acetyl-CoA into CO₂ and generates metabolic intermediates.

How does pyruvate enter the TCA cycle?

Pyruvate is transported into the mitochondria and converted into acetyl-CoA by the pyruvate dehydrogenase complex, producing NADH and releasing CO₂.

How is citrate synthase regulated?

Citrate synthase is inhibited by ATP, NADH, and Succinyl-CoA, which signal sufficient energy or abundant downstream products.

How is isocitrate dehydrogenase regulated?

Isocitrate dehydrogenase is inhibited by ATP and activated by ADP and NAD⁺, making it sensitive to the cell’s energy status.

What regulates α-ketoglutarate dehydrogenase?

It is inhibited by NADH and Succinyl-CoA and activated by AMP, linking its activity to cellular energy needs.

What drives the forward direction of the TCA cycle?

Strongly favorable steps — especially citrate synthase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase — pull the cycle forward

What is anaplerosis?

Anaplerosis consists of reactions that replenish TCA cycle intermediates, mainly through pyruvate carboxylase, PEP carboxylase, and malic enzyme.

What is the most important anaplerotic reaction?

Pyruvate carboxylase converting pyruvate into oxaloacetate to refill TCA intermediates.

What is cataplerosis?

Cataplerosis refers to the removal of TCA intermediates for biosynthesis, such as amino acid production.

What are the products of one turn of the TCA cycle per acetyl-CoA?

3 NADH, 1 FADH₂, 1 GTP (or ATP), and 2 CO₂

Step 1: What happens in the citrate formation step? describe regulation

Acetyl-CoA condenses with oxaloacetate to form citrate through citrate synthase– Inhibited by ATP, NADH & Succinyl-CoA; the reaction is driven by thioester hydrolysis.

Step 2: Why must citrate be converted to isocitrate?

Citrate is a poor substrate for oxidation (tertiary alcohol), so aconitase converts it to isocitrate (secondary alcohol), which can be oxidized.

Step 3: What occurs in the isocitrate dehydrogenase step? describe regulation

Isocitrate is oxidized to α-ketoglutarate, producing CO₂ and NADH in a classic NAD⁺-dependent decarboxylation. Isocitrate Dehydrogenase – Inhibited by ATP
Activated by ADP and NAD+

Step 4: What is special about the α-ketoglutarate dehydrogenase complex? describe regulation

It requires 5 cofactors (TPP, lipoate, CoA, FAD, NAD⁺) and produces NADH and CO₂, resembling the pyruvate dehydrogenase mechanism.a-Ketoglutarate Dehydrogenase – Inhibited by
NADH & Succinyl-CoA
Activated by AMP

Step 5: How is GTP (or ATP) formed in the TCA cycle?

Succinyl-CoA synthetase performs substrate-level phosphorylation by transferring the phosphate from succinyl-phosphate to GDP (or ADP).

Step 6: What makes the succinate dehydrogenase step unique?

It is located in the inner mitochondrial membrane and participates directly in the electron transport chain by passing electrons from FADH₂ to ubiquinone.

Step 7: What reaction does fumarase catalyze?

A stereospecific hydration of fumarate to malate via trans-addition of water across a double bond.

Step 8: Why does the malate dehydrogenase reaction proceed forward despite a high ΔG°?

Oxaloacetate is kept at a very low concentration because citrate synthase rapidly consumes it, pulling the reaction forward.

What activates pyruvate dehydrogenase kinase (PDH kinase)?

ATP, NADH, and acetyl-CoA activate PDH kinase, promoting phosphorylation and inactivation of PDH.

What inhibits pyruvate dehydrogenase kinase?

Pyruvate and ADP inhibit PDH kinase, preventing PDH phosphorylation and allowing PDH to stay active.

What is the effect of PDH kinase on pyruvate dehydrogenase?

PDH kinase phosphorylates PDH, which decreases PDH activity.

What activates pyruvate dehydrogenase phosphatase (PDH phosphatase)?

Insulin strongly activates PDH phosphatase, and Ca²⁺ also activates it (especially in muscle)

What does PDH phosphatase do to PDH activity?

PDH phosphatase dephosphorylates PDH, which increases PDH activity.

How do hormones regulate pyruvate dehydrogenase?

Insulin activates PDH phosphatase, increasing PDH activity; glucagon indirectly promotes PDH inhibition

Pyruvate dehydrogenase is the step between ___ and the TCA cycle

glycolysis

What are the TCA cycle products per glucose molecule?

Per glucose, the TCA cycle produces 6 NADH, 2 FADH₂, 2 GTP (or ATP), and 4 CO₂ because two turns of the cycle occur (one per acetyl-CoA).

How is pyruvate dehydrogenase regulated?

PDH is inhibited by ATP, NADH, and acetyl-CoA and activated by NAD⁺ and CoA.

What inhibits citrate synthase in the TCA cycle?

Citrate synthase is inhibited by ATP, NADH, and succinyl-CoA.

How is isocitrate dehydrogenase regulated?

It is inhibited by ATP and activated by ADP and NAD⁺.

What regulates α-ketoglutarate dehydrogenase?

It is inhibited by NADH and succinyl-CoA and activated by AMP.