🔄 Krebs Cycle

Overview

In the Krebs cycle, acetyl coenzyme A breaks down and releases its acyl group, which enters the cycle.

• The acyl group (two carbons) combines with oxaloacetate (four carbons) to form citrate (six carbons). Citrate is the same as citric acid.

• Citrate undergoes multiple steps to regenerate oxaloacetate, allowing the cycle to continue.

• The primary goal is to generate products that will be used in the final stage, oxidative phosphorylation.

Process Breakdown

1. Citrate (six carbons) is converted into a five-carbon molecule, releasing carbon dioxide (CO2CO2​).

   • NAD is reduced to NADH.

   • This is another oxidative decarboxylation reaction.

2. The five-carbon molecule is converted into a four-carbon molecule, again releasing carbon dioxide (CO2CO2​).

   • NAD is reduced to NADH.

   • This is another oxidative decarboxylation reaction.

3. ATP is generated when an ADP molecule is phosphorylated.

   • This is an example of substrate-level phosphorylation.

4. FAD is reduced to FADH2FADH2​

5. NAD is reduced to NADH, and oxaloacetate is regenerated, restarting the cycle.

Products

From one pyruvate molecule, the Krebs cycle yields:

• Four reduced NADH molecules

• One reduced FADH2FADH2​ molecule

• Three carbon dioxide molecules

• One ATP molecule

Key Terminology

Fate of Products

• Carbon dioxide (CO2CO2​) is a byproduct.

• NADH and FADH2FADH2​ feed into oxidative phosphorylation.

• ATP is the final goal.