Which of the following sequences of the reaction systems A → D is correct?
(1) NAD⁺ → NADH+H⁺, → NAD⁺,CO₂ → NADH+H⁺, GDP, CO₂ → GTP, FAD, iP → FADH₂
(2) NAD⁺ → NADH+H⁺, NAD⁺, CO₂ → NADH+H⁺, ADP, CO₂ → ATP, FAD, iP → FADH₂
(3) NAD⁺ → NADH+H⁺, FAD⁺, CO₂ → FADH₂, ADP → ATP, NAD⁺, iP → NADH+H⁺ ,CO₂
(4) NAD⁺ → NADH+H⁺, FAD⁺, CO₂ → FADH₂, GDP → GTP, NAD⁺,iP → NADH+H⁺ ,CO₂

The citric acid cycle (TCA cycle) is a series of enzymatic reactions that oxidize acetyl-CoA to CO₂ while generating energy-rich molecules such as NADH, FADH₂, and GTP. Each step involves specific changes in electron carriers and substrate molecules. Correctly sequencing these changes is essential for understanding the cycle’s function.

This article identifies the correct sequence of conversions involving NAD+, FAD, GDP, and CO₂ during the TCA cycle.

Key Points About the Citric Acid Cycle Reactions

• NAD+ → NADH + H⁺: Occurs in three oxidative decarboxylation steps:

  • Isocitrate to α-ketoglutarate (releases CO₂)

  • α-Ketoglutarate to succinyl-CoA (releases CO₂)

  • Malate to oxaloacetate

• GDP → GTP: Occurs during the conversion of succinyl-CoA to succinate (substrate-level phosphorylation).

• FAD → FADH₂: Occurs during the oxidation of succinate to fumarate.

• CO₂ release: Happens during the oxidative decarboxylation steps (isocitrate to α-ketoglutarate and α-ketoglutarate to succinyl-CoA).

Evaluating the Options

Explanation

• The cycle begins with NAD+ reduction coupled with CO₂ release (isocitrate and α-ketoglutarate steps).

• Succinyl-CoA synthetase converts succinyl-CoA to succinate, producing GTP (from GDP).

• Succinate dehydrogenase oxidizes succinate to fumarate, reducing FAD to FADH₂.

• Finally, malate dehydrogenase converts malate to oxaloacetate, reducing NAD+ to NADH.

Option (1) best reflects this sequence.

Conclusion

The correct sequence of electron carrier and energy molecule changes in the citric acid cycle is:

(1) NAD+ → NADH + H⁺ → NAD+, CO₂ → NADH + H⁺, GDP, CO₂ → GTP, FAD, iP → FADH₂