explain that reactions in the Krebs cycle involve decarboxylation and dehydrogenation and the reduction of the coenzymes NAD and FAD

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Biology 9700 – Respiration: Krebs Cycle Reactions

Respiration – The Krebs (Citric Acid) Cycle

The Krebs cycle is the central pathway of aerobic respiration. It occurs in the mitochondrial matrix and

processes the acetyl‑CoA derived from glycolysis and pyruvate oxidation. The key chemical changes in each

turn of the cycle are:

  • Decarboxylation – removal of carbon atoms as CO₂.

  • Dehydrogenation – removal of hydrogen atoms, which reduces the co‑enzymes NAD⁺ and FAD.

  • Substrate‑level phosphorylation – formation of a small amount of ATP (or GTP).

Overall Reaction

For one molecule of acetyl‑CoA entering the cycle, the net reaction can be written as:

\$\$\text{Acetyl‑CoA} + 3\,\text{NAD}^+ + \text{FAD} + \text{GDP} + \text{P}i + \text{H}2\text{O}

\;\longrightarrow\; 2\,\text{CO}2 + 3\,\text{NADH} + \text{FADH}2 + \text{GTP} + \text{CoA‑SH}\$\$

Step‑by‑Step Reactions

StepSubstrateProduct(s)EnzymeCo‑enzyme (Reduced)
1. Citrate formationAcetyl‑CoA + OxaloacetateCitrate + CoA‑SHCitrate synthase–
2. IsomerisationCitrateIsocitrateAconitase–
3. First decarboxylation & dehydrogenationIsocitrateα‑Ketoglutarate + CO₂ + NADHIsocitrate dehydrogenaseNADH
4. Second decarboxylation & dehydrogenationα‑KetoglutarateSuccinyl‑CoA + CO₂ + NADHα‑Ketoglutarate dehydrogenase complexNADH
5. Substrate‑level phosphorylationSuccinyl‑CoASuccinate + GTP + CoA‑SHSuccinyl‑CoA synthetase–
6. DehydrogenationSuccinateFumarate + FADH₂Succinate dehydrogenaseFADH₂
7. HydrationFumarateMalateFumarase–
8. DehydrogenationMalateOxaloacetate + NADHMalate dehydrogenaseNADH

Key Concepts

Decarboxylation

Steps 3 and 4 each remove a carbon atom from the substrate as carbon dioxide. This shortens the carbon

chain and releases energy that is captured by the reduction of NAD⁺ to NADH.

Dehydrogenation

During dehydrogenation, hydrogen atoms (as H⁺ and electrons) are transferred from the substrate to the

co‑enzymes:

  • NAD⁺ + 2H → NADH + H⁺

  • FAD + 2H → FADH₂

These reduced co‑enzymes then donate their electrons to the electron transport chain, driving ATP synthesis.

Reduction of NAD⁺ and FAD

Four molecules of NAD⁺ and one molecule of FAD are reduced per turn of the cycle, giving a total of

three NADH, one FADH₂, and one GTP (≈ ATP) directly from the cycle. The high‑energy electrons carried by

NADH and FADH₂ are the main source of ATP in oxidative phosphorylation.

Suggested diagram: A circular schematic of the Krebs cycle showing each substrate,

the point of CO₂ release, and where NAD⁺/FAD are reduced.

Why Decarboxylation and Dehydrogenation Matter

Both processes are essential for extracting the chemical energy stored in carbon bonds:

  1. Decarboxylation removes carbon atoms that would otherwise remain in a low‑energy form as CO₂,

    allowing the remaining carbon skeleton to be fully oxidised.

  2. Dehydrogenation transfers high‑energy electrons to NAD⁺ and FAD, creating powerful electron

    carriers that feed the electron transport chain.

Summary

The Krebs cycle is a series of eight enzyme‑catalysed reactions that achieve three fundamental

transformations:

  • Two decarboxylation steps that release CO₂.

  • Four dehydrogenation steps that reduce NAD⁺ (three times) and FAD (once).

  • One substrate‑level phosphorylation that yields GTP/ATP.

These transformations provide the reduced co‑enzymes (NADH, FADH₂) that drive the majority of ATP

production in aerobic respiration.