34. Pyruvate generated by glycolysis, is converted to acetyl- coenzyme A, which is metabolized by the citric acid cycle generating energy-rich molecules. From the choices given below, select the right combination of these molecules produced from one molecule of acetyl-CoA.
    (1) 2 NADH + 2 FADH₂ + 1 GTP

(2) 3 NADH + 1 FADH₂ + 1 GTP
(3) 3 NADH + 1 GTP
(4) 4 NADH + 1 FADH₂ + 1 GTP

 The citric acid cycle (also known as the Krebs cycle or TCA cycle) is a central metabolic pathway that oxidizes acetyl-CoA to produce energy-rich molecules used in cellular respiration. Understanding the precise yield of these molecules per acetyl-CoA is crucial for appreciating how cells generate ATP efficiently.

This article explains the correct combination of NADH, FADH2, and GTP produced from one molecule of acetyl-CoA metabolized in the citric acid cycle.

Overview of the Citric Acid Cycle and Energy Production

• The citric acid cycle takes place in the mitochondrial matrix.

• One molecule of acetyl-CoA (2 carbons) enters the cycle by combining with oxaloacetate (4 carbons) to form citrate (6 carbons).

• Through a series of enzymatic reactions, citrate is oxidized, releasing two molecules of CO₂ and regenerating oxaloacetate.

• During these reactions, high-energy electron carriers and a nucleoside triphosphate are produced.

Energy-Rich Molecules Produced per Acetyl-CoA

For every turn of the citric acid cycle (i.e., per acetyl-CoA), the following molecules are generated:

• 3 NADH molecules: Produced in three separate oxidative steps, these carry electrons to the electron transport chain.

• 1 FADH₂ molecule: Generated during the oxidation of succinate to fumarate.

• 1 GTP molecule: Produced via substrate-level phosphorylation in the conversion of succinyl-CoA to succinate (GTP can be readily converted to ATP).

Additionally, two molecules of CO₂ are released as waste.

Evaluating the Given Options

Summary Table

Conclusion

The citric acid cycle produces 3 NADH, 1 FADH2, and 1 GTP per molecule of acetyl-CoA metabolized. These molecules are essential for driving oxidative phosphorylation and generating the bulk of cellular ATP.

Correct answer: (2) 3 NADH + 1 FADH2 + 1 GTP