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

Sourav PanOctober 31, 2024

Answered

The Krebs cycle, also known as the citric acid cycle or tricarboxylic acid (TCA) cycle, is a series of enzyme-catalyzed reactions in the mitochondrial matrix that play a central role in cellular respiration. The main purpose of the Krebs cycle is to fully oxidize acetyl-CoA, producing energy-rich molecules that can be used to generate ATP. Two types of reactions—decarboxylation and dehydrogenation—occur repeatedly in the cycle, with the involvement of coenzymes NAD and FAD, which get reduced and carry electrons to the electron transport chain. Here’s how these processes work:

1. Decarboxylation Reactions

• Decarboxylation is the removal of a carbon dioxide (CO₂) molecule from an organic compound. In the Krebs cycle, decarboxylation reactions help to gradually break down the carbon backbone of acetyl-CoA.
• Two decarboxylation steps occur in the Krebs cycle:
  • Isocitrate to α-ketoglutarate: Isocitrate is oxidized, and one molecule of CO₂ is released, resulting in the formation of α-ketoglutarate.
  • α-Ketoglutarate to Succinyl-CoA: α-Ketoglutarate undergoes further oxidation and loses another CO₂, forming succinyl-CoA.
• These decarboxylation reactions reduce the carbon atoms in the cycle, which eventually results in the release of all carbons that entered the cycle from acetyl-CoA as CO₂.

2. Dehydrogenation Reactions

• Dehydrogenation involves the removal of hydrogen atoms (protons and electrons) from a molecule, effectively oxidizing it. In the Krebs cycle, dehydrogenation reactions occur multiple times, where hydrogen atoms are transferred to the coenzymes NAD⁺ and FAD, reducing them to NADH and FADH₂, respectively.
• Key dehydrogenation steps:
  • Isocitrate to α-ketoglutarate: Isocitrate is dehydrogenated, and NAD⁺ is reduced to NADH.
  • α-Ketoglutarate to Succinyl-CoA: This reaction also involves dehydrogenation, reducing NAD⁺ to NADH.
  • Succinate to Fumarate: Succinate is dehydrogenated, and FAD is reduced to FADH₂.
  • Malate to Oxaloacetate: Malate undergoes dehydrogenation, reducing NAD⁺ to NADH.
• These dehydrogenation reactions transfer high-energy electrons and protons to NAD⁺ and FAD, creating NADH and FADH₂, which are electron carriers crucial for ATP synthesis in the electron transport chain.

3. Reduction of NAD and FAD

• NAD⁺ and FAD are coenzymes that act as electron carriers in the Krebs cycle. When they accept electrons during dehydrogenation reactions, they become reduced:
  • NAD⁺ + 2H⁺ + 2e⁻ → NADH + H⁺
  • FAD + 2H⁺ + 2e⁻ → FADH₂
• NADH and FADH₂ then transport these high-energy electrons to the electron transport chain, where they are ultimately used to produce ATP through oxidative phosphorylation.

Summary of Krebs Cycle Reactions

• Decarboxylation releases CO₂, reducing the carbon backbone.
• Dehydrogenation transfers electrons and hydrogen ions to NAD⁺ and FAD, forming NADH and FADH₂.
• NADH and FADH₂ carry high-energy electrons to the electron transport chain, essential for ATP generation.