Reactions of the Citric Acid Cycle

With acetyl-CoA formed, it's ready to enter the citric acid cycle — also called the tricarboxylic acid (TCA) cycle. This is where most of the electron carriers that power ATP production are made.

Key Takeaways

• The citric acid cycle's main job is oxidizing carbon and capturing energy as NADH/FADH₂ — not directly making large amounts of ATP.

• Eight steps, each turn: citrate synthase → aconitase → isocitrate dehydrogenase (rate-limiting) → alpha-ketoglutarate dehydrogenase → succinyl-CoA synthetase → succinate dehydrogenase → fumarase → malate dehydrogenase.

• Net yield per turn: 3 NADH, 1 FADH₂, 1 GTP, 2 CO₂.

• Combined with PDC, one pyruvate yields 4 NADH + 1 FADH₂ + 1 GTP = 12.5 ATP; one glucose (2 pyruvate) yields 25 ATP from this stage alone.

• Three regulatory control points — citrate synthase, isocitrate dehydrogenase, alpha-ketoglutarate dehydrogenase — are all inhibited by high ATP/NADH and activated by low-energy signals like ADP/NAD⁺ (plus calcium for alpha-ketoglutarate dehydrogenase).

What the Citric Acid Cycle Actually Does

The citric acid cycle takes place in the mitochondrial matrix. Its main role is not to directly make large amounts of ATP — it's to oxidize carbon atoms. The carbons that enter as acetyl-CoA move through a series of reactions and are gradually released as CO₂.

As these oxidation reactions occur, the released energy is captured in the form of high-energy electron carriers: NADH and FADH₂. The cycle also produces a small amount of GTP through substrate-level phosphorylation, which readily converts to ATP — but this isn't the cycle's main energy payoff. The real value lies in NADH and FADH₂, which later deliver their electrons to the electron transport chain, where the majority of ATP is ultimately produced.

The Eight Steps of the Citric Acid Cycle

Each turn of the cycle starts when the two-carbon acetyl group from acetyl-CoA attaches to the four-carbon acceptor molecule oxaloacetate. By the end of the cycle, oxaloacetate is regenerated so the process can repeat.

Step 1: Citrate Synthase

Citrate synthase catalyzes the condensation of acetyl-CoA with oxaloacetate to form citrate, a six-carbon molecule. Coenzyme A is released with a free thiol group, and hydrolyzing acetyl-CoA's high-energy thioester bond releases a large amount of energy — making this first step strongly favorable and helping pull the whole cycle forward.

Step 2: Aconitase

Aconitase converts citrate into isocitrate, an isomerization reaction that passes through the intermediate cis-aconitate. Citrate itself isn't positioned for efficient oxidation, so this step rearranges it into a molecule with the right geometry for the next step. It is not considered a major regulatory point.

Step 3: Isocitrate Dehydrogenase (Rate-Limiting)

Isocitrate dehydrogenase converts isocitrate into alpha-ketoglutarate — one of the most important steps in the cycle, and commonly treated as the rate-limiting step. Two things happen at once: isocitrate is oxidized (NAD⁺ → NADH), and the molecule undergoes decarboxylation, releasing the cycle's first CO₂. A six-carbon molecule becomes a five-carbon molecule.

Step 4: Alpha-Ketoglutarate Dehydrogenase

This enzyme converts alpha-ketoglutarate into succinyl-CoA through another oxidative decarboxylation, similar in theme to pyruvate dehydrogenase. A carbon is removed as CO₂ (the cycle's second), the remaining fragment is oxidized (NAD⁺ → NADH), and CoA attaches. At this point, both CO₂ molecules from one turn of the cycle have been released.

Step 5: Succinyl-CoA Synthetase

Succinyl-CoA synthetase converts succinyl-CoA into succinate. The high-energy thioester bond in succinyl-CoA drives the formation of GTP from GDP and inorganic phosphate — this is the cycle's substrate-level phosphorylation step. GTP is then readily converted to ATP by nucleoside diphosphate kinase.

Step 6: Succinate Dehydrogenase

Succinate dehydrogenase converts succinate into fumarate, an oxidation reaction in which electrons removed from succinate reduce FAD to FADH₂ — the only step in the citric acid cycle that produces FADH₂. This enzyme is unique because it's embedded directly in the inner mitochondrial membrane, where it also functions as Complex II of the electron transport chain.

Step 7: Fumarase

Fumarase converts fumarate into malate through a hydration reaction — water is added across fumarate's double bond. This step prepares the molecule for the cycle's final oxidation.

Step 8: Malate Dehydrogenase

Malate dehydrogenase converts malate back into oxaloacetate, an oxidation reaction that reduces NAD⁺ to NADH — the third and final NADH of the cycle. This step regenerates oxaloacetate, readying the cycle to accept another acetyl-CoA. The reaction isn't strongly favorable on its own, but it's pulled forward because oxaloacetate is continuously consumed by citrate synthase in step 1.

Net Yield Per Turn of the Cycle

For one acetyl-CoA entering the cycle, one full turn produces:

• 3 NADH

• 1 FADH₂

• 1 GTP

• 2 CO₂

Coenzyme A is also regenerated.

From Pyruvate to ATP: The Full Yield Calculation

Combining the pyruvate dehydrogenase complex (1 NADH + 1 CO₂ per pyruvate) with one turn of the citric acid cycle (3 NADH + 1 FADH₂ + 1 GTP per acetyl-CoA), the total output per pyruvate is:

4 NADH + 1 FADH₂ + 1 GTP

Using MCAT conventions — each NADH ≈ 2.5 ATP, each FADH₂ ≈ 1.5 ATP, and 1 GTP = 1 ATP — this works out to:

(4 × 2.5) + (1 × 1.5) + 1 = 10 + 1.5 + 1 = 12.5 ATP per pyruvate

Since one glucose produces two pyruvate molecules, this doubles to 25 ATP per glucose from PDC and the citric acid cycle combined. Glycolysis itself contributes roughly another 7 ATP (2 ATP from substrate-level phosphorylation, plus 2 NADH worth about 5 ATP depending on the shuttle system used) — bringing the total picture together for the full ATP yield discussion later in this chapter.

Keep in mind: aside from the one GTP per turn, the ATP associated with the citric acid cycle isn't produced during the cycle itself — it comes later, when NADH and FADH₂ deliver their electrons to the electron transport chain.

Regulating the Citric Acid Cycle

Upstream: Pyruvate Dehydrogenase

Regulation starts before the cycle even begins. Pyruvate dehydrogenase controls how much carbon is allowed to flow into the cycle as acetyl-CoA, and its activity is controlled by reversible phosphorylation — PDC kinase turns it off, PDC phosphatase turns it on (see Acetyl-CoA for the full mechanism).

Three Control Points Within the Cycle

Three reactions in the cycle are strongly exergonic and effectively irreversible, making them the cycle's major regulatory points:

The logic is consistent throughout: high ATP and high NADH signal that the cell already has enough energy, so the cycle slows down. Low-energy signals like ADP and NAD⁺ speed the cycle up. Calcium (especially important in muscle, where it rises during contraction) couples the cycle to muscle activity, and insulin couples upstream PDC activity to the fed state.

Common MCAT Mistakes

• Thinking the citric acid cycle's main job is making ATP directly. It isn't — only 1 GTP (→ATP) is made per turn via substrate-level phosphorylation. The cycle's real payoff is NADH and FADH₂, which power ATP synthesis later at the electron transport chain.

• Losing track of which step makes which electron carrier. NADH is made at three steps (isocitrate dehydrogenase, alpha-ketoglutarate dehydrogenase, malate dehydrogenase); FADH₂ is made at exactly one step (succinate dehydrogenase) — mixing these up is a common source of ATP-calculation errors.

• Forgetting succinate dehydrogenase's dual identity. It's both a citric acid cycle enzyme and Complex II of the electron transport chain — the only citric acid cycle enzyme embedded in the inner mitochondrial membrane rather than free in the matrix.

• Misidentifying the rate-limiting step. Isocitrate dehydrogenase, not citrate synthase, is the step most often treated as rate-limiting — though citrate synthase, isocitrate dehydrogenase, and alpha-ketoglutarate dehydrogenase are all major regulatory points.

MCAT-Style Concept Check

Question: A researcher measures a sharp drop in ADP and a sharp rise in ATP and NADH within a cell's mitochondria. Based on the regulation of the citric acid cycle, which of the following is the most likely effect on cycle flux?

• A) Flux increases, because ATP and NADH directly activate isocitrate dehydrogenase

• B) Flux decreases, because citrate synthase and isocitrate dehydrogenase are inhibited by high ATP and NADH

• C) Flux is unaffected, because the citric acid cycle is regulated only by calcium and insulin

• D) Flux increases, because low ADP relieves inhibition at succinate dehydrogenase

Answer: B

Explanation: High ATP and NADH are the cycle's key "enough energy" signals, inhibiting citrate synthase and isocitrate dehydrogenase (and alpha-ketoglutarate dehydrogenase), which slows the cycle; low ADP removes one of the main activating signals at isocitrate dehydrogenase, reinforcing the slowdown rather than opposing it. Option A is wrong because ATP and NADH inhibit these enzymes, not activate them. Option C is wrong because calcium and insulin are only part of the regulatory picture — substrate/product levels (ATP, ADP, NADH, NAD⁺, succinyl-CoA, citrate) are the primary controls. Option D is wrong because succinate dehydrogenase is not one of the cycle's three regulated control points.

FAQ

Does the citric acid cycle require oxygen directly?

No — no step in the citric acid cycle uses oxygen directly. It's still classified as aerobic metabolism because it depends on NAD⁺ and FAD being regenerated from NADH and FADH₂ by the electron transport chain, which does require oxygen as the final electron acceptor.

How many ATP, NADH, FADH₂, and CO₂ come from one turn of the citric acid cycle?

One turn (one acetyl-CoA) yields 3 NADH, 1 FADH₂, 1 GTP (→ 1 ATP), and 2 CO₂.

Why is isocitrate dehydrogenase considered the rate-limiting step?

It catalyzes one of the cycle's three strongly exergonic, effectively irreversible reactions, and it's especially sensitive to the cell's energy status — strongly inhibited by ATP/NADH and activated by ADP/NAD⁺ — making it the primary control point for how fast the cycle runs.

How does the citric acid cycle connect to total ATP yield per glucose?

Each pyruvate entering PDC and the citric acid cycle yields 4 NADH + 1 FADH₂ + 1 GTP, or about 12.5 ATP. Since one glucose produces two pyruvate, this stage contributes about 25 ATP per glucose — on top of what glycolysis and the electron transport chain contribute separately.