Formation of Acetyl-CoA through the Transition Reaction

II. BACTERIAL GROWTH AND MICROBIAL METABOLISM

D. Cellular Respiration

1. Aerobic Respiration

b. Transition Reaction

Fundamental statements for this learning object:

1. Aerobic respiration involves four stages: glycolysis, a transition reaction that forms acetyl coenzyme A, the citric acid (Krebs) cycle, and an electron transport chain and chemiosmosis.
2. The transition reaction connects glycolysis to the citric acid (Krebs) cycle.
3. The transition reaction converts the two molecules of the 3-carbon pyruvate from glycolysis (and other pathways) into two molecules of the 2-carbon molecule acetyl Coenzyme A (acetyl-CoA) and 2 molecules of carbon dioxide.
4. As the two pyruvates undergo oxidative decarboxylation, two molecules of NAD⁺ become reduced to 2NADH + 2H⁺ which carry protons and electrons to the electron transport chain to generate additional ATP by oxidative phosphorylation.
5. The overall reaction for the transition reaction is: 2 pyruvate + 2 NAD⁺ + 2 coenzyme A yields 2 acetyl-CoA + 2 NADH + 2 H⁺ + 2 CO₂.
6. The two molecules of acetyl-CoA can now enter the citric acid cycle.

Learning Objectives for this Section

Aerobic respiration (def) is the aerobic catabolism of nutrients to carbon dioxide, water, and energy, and involves an electron transport system (def) in which molecular oxygen is the final electron acceptor. Most eukaryotes and prokaryotes use aerobic respiration to obtain energy from glucose. The overall reaction is:

C₆H₁₂O₆ + 6O₂ yields 6CO₂ + 6H₂O + energy (as ATP)

Note that glucose (C₆H₁₂O₆ ) is oxidized to produce carbon dioxide (CO₂) and oxygen (O₂) is reduced to produce water (H₂O).

Aerobic respiration involves four stages: glycolysis, a transition reaction that forms acetyl coenzyme A, the citric acid (Krebs) cycle, and an electron transport chain and chemiosmosis. We will now look at the transition reaction.

Formation of Acetyl-CoA through the Transition Reaction

The transition reaction connects glycolysis to the citric acid (Krebs) cycle. Through a process called oxidative decarboxylation, the transition reaction converts the two molecules of the 3-carbon pyruvate from glycolysis (and other pathways) into two molecules of the 2-carbon molecule acetyl Coenzyme A (acetyl-CoA) and 2 molecules of carbon dioxide. First, a carboxyl group of each pyruvate is removed as carbon dioxide and then the remaining acetyl group combines with coenzyme A (CoA) to form acetyl-CoA. As the two pyruvates undergo oxidative decarboxylation, two molecules of NAD⁺ become reduced to 2NADH + 2H⁺ (see Fig. 1 and Fig. 2). The 2NADH + 2H⁺ carry protons and electrons to the electron transport chain to generate additional ATP by oxidative phosphorylation (def).

The overall reaction for the transition reaction is:

2 pyruvate + 2 NAD⁺ + 2 coenzyme A

yields 2 acetyl-CoA + 2 NADH + 2 H⁺ + 2 CO₂

In prokaryotic cells (see Fig. 5), the transition step occurs in the cytoplasm; in eukaryotic cells the pyruvates must first enter the mitochondria (see Fig. 4) because the transition reaction and the citric acid cycle take place in the matrix of the mitochondria.

The two molecules of acetyl-CoA can now enter the citric acid cycle. Acetyl-CoA is also a precursor metabolite (def) for fatty acid synthesis, as shown in Fig. 3.