7.3 Oxidation of Pyruvate and the Citric Acid Cycle

Pyruvate oxidation and the citric acid cycle are the stages of cellular respiration that finish breaking down glucose. After glycolysis splits glucose into two pyruvate molecules in the cytosol, these reactions take over inside the mitochondria to extract the remaining energy. The payoff isn't mainly ATP directly; instead, these steps load up electron carriers (NADH and FADH2) that will drive massive ATP production in the electron transport chain.

Pyruvate Oxidation

Pyruvate oxidation process

Before the citric acid cycle can begin, pyruvate from glycolysis has to be converted into acetyl-CoA. This happens in the mitochondrial matrix, catalyzed by the pyruvate dehydrogenase complex. Since pyruvate is a 3-carbon molecule and acetyl-CoA carries only a 2-carbon acetyl group, one carbon is lost as $CO_2$.

The conversion happens in three linked steps:

1. Decarboxylation: Pyruvate loses one carbon as $CO_2$. This requires the coenzyme thiamine pyrophosphate (TPP).
2. Oxidation: The remaining 2-carbon fragment is oxidized, and $NAD^+$ is reduced to $NADH$.
3. Acetyl group transfer: The oxidized 2-carbon acetyl group is attached to coenzyme A (CoA), forming acetyl-CoA.

Because glycolysis produces two pyruvate molecules per glucose, pyruvate oxidation runs twice per glucose. That means this step alone generates 2 $NADH$ and releases 2 $CO_2$ per glucose molecule.

Citric Acid Cycle

Citric acid cycle vs glycolysis

These two pathways differ in structure, location, and output:

• Glycolysis is a linear pathway in the cytosol. It partially oxidizes glucose to pyruvate.
• The citric acid cycle (also called the Krebs cycle) is a circular pathway in the mitochondrial matrix. It completes the oxidation of glucose by breaking down acetyl-CoA.

The circular nature matters: the last step regenerates the molecule needed for the first step, so the cycle can keep turning as long as acetyl-CoA is available.

Steps of the citric acid cycle

Each turn of the cycle processes one acetyl-CoA (2 carbons) and produces electron carriers plus one ATP/GTP. Here are all eight steps:

1. Condensation: Acetyl-CoA (2C) joins with oxaloacetate (4C) to form citrate (6C). Catalyzed by citrate synthase. CoA is released.
2. Isomerization: Aconitase rearranges citrate into isocitrate (6C). This is a reversible reaction.
3. Oxidative decarboxylation: Isocitrate dehydrogenase converts isocitrate into $\alpha$-ketoglutarate (5C). One $CO_2$ is released and one $NAD^+$ is reduced to $NADH$.
4. Oxidative decarboxylation: The $\alpha$-ketoglutarate dehydrogenase complex converts $\alpha$-ketoglutarate into succinyl-CoA (4C). Another $CO_2$ is released and another $NADH$ is produced.
5. Substrate-level phosphorylation: Succinyl-CoA synthetase converts succinyl-CoA into succinate (4C). The energy stored in the thioester bond is used to generate GTP (or ATP). CoA is released.
6. Oxidation: Succinate dehydrogenase converts succinate into fumarate (4C). $FAD$ is reduced to $FADH_2$ (not $NADH$, because there isn't enough energy in this reaction to reduce $NAD^+$).
7. Hydration: Fumarase adds water to fumarate, forming malate (4C).
8. Oxidation: Malate dehydrogenase oxidizes malate back into oxaloacetate (4C), reducing $NAD^+$ to $NADH$. Oxaloacetate is now ready to accept another acetyl-CoA in Step 1.

Carbon dioxide release points

Two carbons enter the cycle as acetyl-CoA, and two carbons leave as $CO_2$:

• Step 3: $CO_2$ released during oxidative decarboxylation of isocitrate
• Step 4: $CO_2$ released during oxidative decarboxylation of $\alpha$-ketoglutarate

This is where the carbon atoms from your food ultimately become the $CO_2$ you exhale.

NADH and FADH2 production

Per turn of the cycle:

• NADH is produced at three steps:
  • Step 3 (isocitrate → $\alpha$-ketoglutarate)
  • Step 4 ($\alpha$-ketoglutarate → succinyl-CoA)
  • Step 8 (malate → oxaloacetate)
• FADH2 is produced at one step:
  • Step 6 (succinate → fumarate)

So each turn yields 3 NADH + 1 FADH2. Since two acetyl-CoA enter per glucose, the cycle produces 6 NADH + 2 FADH2 per glucose.

Substrate-level phosphorylation role

Step 5 is the only point in the citric acid cycle where ATP (or GTP) is made directly. The energy comes from breaking the thioester bond in succinyl-CoA. This is substrate-level phosphorylation, the same type of ATP production that occurs in glycolysis.

It's a small contribution compared to the ATP that NADH and FADH2 will generate later through oxidative phosphorylation, but it's immediate and doesn't require oxygen.

Regulation of the citric acid cycle

The cycle speeds up when the cell needs energy and slows down when energy is plentiful. The logic is straightforward: high ATP and NADH signal that the cell has enough energy, so the cycle slows. High ADP signals energy demand, so the cycle speeds up.

Three key regulatory enzymes control the rate:

• Citrate synthase (Step 1): Inhibited by ATP and NADH; activated by ADP
• Isocitrate dehydrogenase (Step 3): Inhibited by ATP and NADH; activated by ADP
• $\alpha$-Ketoglutarate dehydrogenase complex (Step 4): Inhibited by ATP, NADH, and succinyl-CoA; activated by ADP

Notice that all three regulatory points are at irreversible steps, which makes sense because those are the committed steps that control flow through the pathway.

Energy yield comparisons

Per acetyl-CoA (one turn of the citric acid cycle):

• 1 ATP (or GTP) via substrate-level phosphorylation
• 3 NADH → approximately 7.5 ATP via oxidative phosphorylation
• 1 FADH2 → approximately 1.5 ATP via oxidative phosphorylation
• Total: ~10 ATP per acetyl-CoA

Per glucose through glycolysis:

• 2 ATP (net) via substrate-level phosphorylation
• 2 NADH → approximately 3–5 ATP via oxidative phosphorylation (depending on the shuttle used to transport electrons into the mitochondria)
• Total: ~5–7 ATP per glucose

Since each glucose yields 2 acetyl-CoA, the citric acid cycle alone accounts for ~20 ATP per glucose, making it a far larger contributor to total ATP yield than glycolysis.

Oxaloacetate regeneration importance

Oxaloacetate is the 4-carbon molecule that accepts acetyl-CoA in Step 1. If it isn't regenerated, the cycle stops. Step 8 handles this: malate dehydrogenase oxidizes malate back to oxaloacetate, producing one more NADH in the process.

This regeneration is what makes the pathway a cycle rather than a linear pathway. Oxaloacetate is not consumed overall; it acts as a carrier that is recycled with each turn.

Connections to metabolic pathways

The citric acid cycle isn't just for energy. Several of its intermediates get pulled out to serve as building blocks for other molecules:

• Citrate → fatty acid and sterol (cholesterol) synthesis
• $\alpha$-Ketoglutarate → amino acids like glutamate, glutamine, proline, and arginine
• Succinyl-CoA → heme synthesis (the iron-containing group in hemoglobin)
• Oxaloacetate → amino acids like aspartate and asparagine; also used in gluconeogenesis

When intermediates are pulled out, the cycle needs to be topped off. Anaplerotic reactions do this. The most common example is pyruvate carboxylase, which converts pyruvate directly into oxaloacetate, replenishing the cycle so it can keep running.

Mitochondrial enzyme localization

Seven of the eight citric acid cycle enzymes are soluble in the mitochondrial matrix: citrate synthase, aconitase, isocitrate dehydrogenase, $\alpha$-ketoglutarate dehydrogenase complex, succinyl-CoA synthetase, fumarase, and malate dehydrogenase.

The exception is succinate dehydrogenase (Step 6), which is embedded in the inner mitochondrial membrane. It doubles as Complex II of the electron transport chain. This is why FADH2 from Step 6 feeds electrons directly into the chain rather than being released as a free molecule.

Significance in energy production

The citric acid cycle sits at the center of aerobic metabolism. It completes the oxidation of carbon fuels (not just glucose, but also fatty acids and amino acids), generates the bulk of the electron carriers that drive ATP synthesis, and supplies precursors for biosynthesis. Without it, aerobic organisms would be limited to the small ATP yield of glycolysis alone.

Oxidative phosphorylation and the electron transport chain

The NADH and FADH2 generated by the citric acid cycle carry high-energy electrons to the electron transport chain (ETC), located in the inner mitochondrial membrane. Electrons pass through a series of protein complexes via redox reactions, and the energy released is used to pump $H^+$ ions across the membrane. This proton gradient then drives ATP synthase to produce ATP. Oxidative phosphorylation is the final stage of aerobic respiration and generates the vast majority of ATP, roughly 26–28 ATP per glucose.