5.2.5 The Krebs Cycle

What is the Krebs Cycle?

• The Krebs cycle is a series of enzyme-controlled reactions that occur in the matrix of the mitochondria.
• It is the second stage of aerobic respiration and produces molecules necessary for oxidative phosphorylation.

Steps of the Krebs Cycle

1. Formation of a 6-Carbon Compound:

• Acetyl CoA (2C) donates its 2-carbon acetate group to a 4-carbon molecule (oxaloacetate), forming a 6-carbon molecule (citrate).
• Coenzyme A is released and reused in the link reaction.

2. Decarboxylation and Dehydrogenation:

• The 6-carbon molecule undergoes a series of reactions:
• Decarboxylation: Two molecules of $CO₂$ are removed.
• Dehydrogenation: Hydrogen atoms are removed and used to reduce NAD to NADH and $FAD\ to\ FADH₂$.

3. Regeneration of Oxaloacetate:

• The original 4-carbon molecule (oxaloacetate) is regenerated to accept another acetate group from acetyl CoA, continuing the cycle.

Products of the Krebs Cycle (Per Glucose Molecule)

Since the Krebs cycle turns twice for each molecule of glucose (because two acetyl CoA molecules are formed in the link reaction):

• 2 ATP: Produced by substrate-level phosphorylation.
• 6 NADH: Used in oxidative phosphorylation to generate ATP.
• 2 FADH₂: Used in oxidative phosphorylation to generate ATP.
• 4 CO₂: Released as a waste product.

Reaction Summary (Per Turn)

Acetyl CoA + 3 NAD + FAD + ADP + Pi → 2 CO₂ + CoA + 3 NADH + FADH₂ + ATP