29.7 The Citric Acid Cycle

The Citric Acid Cycle

The citric acid cycle (also called the Krebs cycle or TCA cycle) is the final common pathway for oxidizing carbohydrates, fats, and proteins. It completes the breakdown of acetyl groups into $CO_2$ while capturing most of the released energy as the reduced electron carriers $NADH$ and $FADH_2$. Those carriers then feed into the electron transport chain, where the bulk of ATP is actually made through oxidative phosphorylation.

Acetyl-CoA: Entry into the Cycle

Acetyl-CoA is the molecule that feeds carbon into the citric acid cycle. It's produced primarily by the oxidative decarboxylation of pyruvate (the end product of glycolysis), a reaction catalyzed by the pyruvate dehydrogenase complex. This enzyme complex requires several cofactors: thiamine pyrophosphate (TPP), lipoic acid, $FAD$, $NAD^+$, and CoA itself.

Structurally, acetyl-CoA consists of a two-carbon acetyl group ($-COCH_3$) linked to coenzyme A through a high-energy thioester bond. That bond is what makes the next reaction thermodynamically favorable.

The cycle begins when citrate synthase transfers the acetyl group from acetyl-CoA onto oxaloacetate (a four-carbon molecule), forming citrate (six carbons). This condensation reaction is essentially irreversible under cellular conditions, which is why it commits the acetyl group to the cycle. The freed CoA is recycled back to the pyruvate dehydrogenase complex to pick up another acetyl group.

Oxidation and Decarboxylation Reactions

The middle portion of the cycle is where carbon is lost as $CO_2$ and most of the electron carriers are generated. Here are the key steps in order:

1. Citrate → Isocitrate. The enzyme aconitase isomerizes citrate to isocitrate through a cis-aconitate intermediate. No carbon is lost or gained here; the hydroxyl group simply shifts position to set up the next oxidation.

2. Isocitrate → α-Ketoglutarate. Isocitrate dehydrogenase performs an oxidative decarboxylation: one $CO_2$ is released, and $NAD^+$ is reduced to $NADH$. This is a major regulatory step in the cycle.

3. α-Ketoglutarate → Succinyl-CoA. The α-ketoglutarate dehydrogenase complex catalyzes a second oxidative decarboxylation. Another $CO_2$ is released and another $NADH$ is produced. This enzyme complex is structurally and mechanistically similar to the pyruvate dehydrogenase complex, requiring the same set of cofactors (TPP, lipoic acid, $FAD$). The product, succinyl-CoA, contains a high-energy thioester bond.

4. Succinyl-CoA → Succinate. Succinyl-CoA synthetase cleaves the thioester bond and couples that energy to substrate-level phosphorylation, producing one $GTP$ (or $ATP$, depending on the tissue). This is the only step in the cycle that directly generates a high-energy phosphate.

5. Succinate → Fumarate. Succinate dehydrogenase oxidizes succinate, reducing $FAD$ to $FADH_2$. This enzyme is unique because it's embedded in the inner mitochondrial membrane and is also Complex II of the electron transport chain.

6. Fumarate → Malate. Fumarase adds water across the double bond of fumarate, producing malate. A straightforward hydration reaction.

7. Malate → Oxaloacetate. Malate dehydrogenase oxidizes malate back to oxaloacetate, reducing $NAD^+$ to $NADH$. Although this reaction has an unfavorable equilibrium on its own, it's pulled forward because citrate synthase rapidly consumes oxaloacetate in the next turn.

Oxaloacetate Regeneration and Energy Yield

The regeneration of oxaloacetate at the end of the cycle is what makes it a cycle rather than a linear pathway. Each newly formed oxaloacetate accepts another acetyl group from acetyl-CoA, and the whole sequence repeats.

One complete turn of the cycle produces:

1. 3 $NADH$ (from isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, and malate dehydrogenase)
2. 1 $FADH_2$ (from succinate dehydrogenase)
3. 1 $GTP$ (or $ATP$) (from succinyl-CoA synthetase)

When the electron carriers are oxidized by the electron transport chain, each $NADH$ yields approximately 2.5 ATP and each $FADH_2$ yields approximately 1.5 ATP. So the total ATP yield per turn is roughly:

$3 \times 2.5 + 1 \times 1.5 + 1 = 10 \text{ ATP}$

The electron transport chain works by using the energy from $NADH$ and $FADH_2$ to pump protons across the inner mitochondrial membrane, creating an electrochemical gradient. Protons flow back through ATP synthase, which harnesses that gradient to phosphorylate ADP to ATP.

Cellular Location and $CO_2$ Production

The citric acid cycle takes place in the mitochondrial matrix, the innermost compartment of the mitochondrion. This location matters because it places the cycle's products (especially $NADH$ and $FADH_2$) right next to the inner membrane where the electron transport chain operates.

Each turn of the cycle releases two molecules of $CO_2$ (at the isocitrate dehydrogenase and α-ketoglutarate dehydrogenase steps). These two carbons balance the two carbons that entered as the acetyl group, so there is no net gain or loss of carbon in the cycle intermediates. The $CO_2$ diffuses out of the mitochondria, into the blood, and is eventually exhaled by the lungs.

The cycle was elucidated by Hans Krebs in 1937, work for which he received the Nobel Prize in Physiology or Medicine in 1953.