The tricarboxylic acid (TCA) cycle, also known as the Krebs cycle, involves a series of biochemical reactions that convert acetyl-CoA into energy-rich molecules. Each step features specific intermediates critical for the cycle’s progression. Correctly identifying these intermediates is essential for understanding the metabolic flow.

This article clarifies the names of molecules A, B, C, and D in the TCA cycle based on their sequence and structure.

Overview of Key TCA Cycle Intermediates

The TCA cycle begins with the condensation of acetyl-CoA and oxaloacetate to form citrate, followed by its conversion into isocitrate, then α-ketoglutarate, and later oxaloacetate is regenerated at the end of the cycle.

The typical sequence of these intermediates is:

• Citrate (6 carbons): The first product formed by citrate synthase.

• Isocitrate (6 carbons): An isomer of citrate formed via aconitase.

• α-Ketoglutarate (5 carbons): Produced by oxidative decarboxylation of isocitrate.

• Oxaloacetate (4 carbons): Regenerated at the end of the cycle to combine with acetyl-CoA.

Assigning Molecules A, B, C, and D

Based on the sequence:

• A = Citrate (first intermediate after acetyl-CoA entry)

• B = Isocitrate (isomer of citrate)

• C = α-Ketoglutarate (produced after isocitrate oxidation)

• D = Oxaloacetate (regenerated at cycle end)

This matches option:

(2) A = citrate, B = isocitrate, C = α-ketoglutarate, D = oxaloacetate

Summary Table

Conclusion

The correct identification of the TCA cycle intermediates A, B, C, and D is:

A = citrate, B = isocitrate, C = α-ketoglutarate, D = oxaloacetate

This sequence reflects the order of metabolites in the citric acid cycle and their roles in energy metabolism.

Correct answer: (2) A= citrate, B = isocitrate, C = α-ketoglutarate, D = oxaloacetate