Aerobic Respiration: Glycolysis, Pyruvate Oxidation, and the Citric Acid Cycle

Once inside the mitochondrial matrix, pyruvate undergoes oxidative decarboxylation to become acetyl CoA. This reaction is catalyzed by a massive enzyme assembly known as the pyruvate dehydrogenase complex. This step is crucial because it links glycolysis to the citric acid cycle by converting a three-carbon pyruvate into a two-carbon acetyl group attached to Coenzyme A.

The stoichiometry for the conversion of two pyruvates (from one glucose) is:

$$\begin{aligned} 2\text{Pyruvate} + 2\text{HS-CoA} + 2\text{NAD}^{+} \rightarrow 2\text{Acetyl-CoA} + 2\text{NADH} + 2\text{H}^{+} + 2\text{CO}_2 \end{aligned}$$

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The Citric Acid Cycle (TCA/Krebs Cycle)

The citric acid cycle, also known as the TCA cycle or the Krebs cycle, takes place within the mitochondrial matrix. This cycle serves as the central metabolic hub of the cell.

The cycle begins when the two-carbon acetyl group from acetyl CoA is condensed with a four-carbon molecule, oxaloacetate, to form a six-carbon molecule called citrate. Through a series of seven enzymatic steps, the citrate is oxidized, releasing two molecules of $\text{CO}_2$ and regenerating the original oxaloacetate to allow the cycle to continue.

The primary purpose of the cycle is not the direct production of ATP, but rather the coenzyme reduction of $\text{NAD}^{+}$ and $\text{FAD}$. For every turn of the cycle (one acetyl CoA), the following are produced:

• 3 NADH
• 1 $\text{FADH}_2$
• 1 GTP (which is readily converted to ATP)
• 2 $\text{CO}_2$

Because one glucose molecule produces two pyruvates, the cycle turns twice per glucose molecule.

Exam Tip: Amphibolic Nature The citric acid cycle is described as amphibolic because it functions in both catabolic (degradative) and anabolic (synthetic) pathways. While it oxidizes acetyl groups for energy, its intermediates (like $\alpha$-ketoglutarate or oxaloacetate) can be drawn off to serve as precursors for synthesizing amino acids or other molecules. To prevent the cycle from running out of intermediates, the cell uses anaplerotic reactions to replenish them.

Oxidative Phosphorylation

The final and most productive stage of aerobic respiration is oxidative phosphorylation, which occurs in the inner mitochondrial membrane. This stage consists of two closely linked processes: the electron transport chain (ETC) and chemiosmosis.

The Electron Transport Chain (ETC)

The ETC is composed of four major mitochondrial electron-transport complexes (I, II, III, and IV) and two mobile carriers: ubiquinone (Coenzyme Q) and cytochrome c.

The process begins when reduced coenzymes, NADH and $\text{FADH}_2$, donate their high-energy electrons to the chain. NADH enters at Complex I, while $\text{FADH}_2$ enters at Complex II. As electrons move through the complexes toward the final electron acceptor, oxygen ($\text{O}_2$), they move from carriers with more negative redox potentials to those with more positive potentials.

As electrons pass through Complexes I, III, and IV, the energy released is used to pump protons ($\text{H}^{+}$) from the mitochondrial matrix into the intermembrane space. This creates an electrochemical gradient, also known as the proton-motive force.

ATP Synthesis via ATP Synthase

The potential energy stored in the proton gradient is harvested by ATP synthase (Complex V). This enzyme acts like a molecular turbine. Protons flow back into the matrix through a channel in the $\text{F}_0$ portion of the enzyme, causing the $\text{F}_0$ ring to rotate. This mechanical rotation is transmitted to the $\text{F}_1$ catalytic subunit, which drives the phosphorylation of ADP to produce ATP.

Summary of ATP Yield

The total amount of ATP generated from the complete aerobic oxidation of one glucose molecule is approximately 30 to 32 ATP. This yield is a combination of ATP produced directly via substrate-level phosphorylation in glycolysis and the citric acid cycle, and the much larger amount produced via oxidative phosphorylation.

Note on NADH Transport: NADH produced during glycolysis in the cytosol cannot cross the inner mitochondrial membrane directly. It must use specific shuttle mechanisms to transfer its electrons into the mitochondria, which is why the ATP yield from cytosolic NADH can vary (yielding either 3 or 5 ATP).