7.9: Oxidation of Pyruvate and the Citric Acid Cycle - Acetyl CoA to CO₂
- Describe the fate of the acetyl CoA carbons in the citric acid cycle
Acetyl CoA to CO 2
Acetyl CoA links glycolysis and pyruvate oxidation with the citric acid cycle. In the presence of oxygen, acetyl CoA delivers its acetyl group to a four-carbon molecule, oxaloacetate, to form citrate, a six-carbon molecule with three carboxyl groups. During this first step of the citric acid cycle, the CoA enzyme, which contains a sulfhydryl group (-SH), is recycled and becomes available to attach another acetyl group. The citrate will then harvest the remainder of the extractable energy from what began as a glucose molecule and continue through the citric acid cycle.
In the citric acid cycle, the two carbons that were originally the acetyl group of acetyl CoA are released as carbon dioxide, one of the major products of cellular respiration, through a series of enzymatic reactions. For each acetyl CoA that enters the citric acid cycle, two carbon dioxide molecules are released in reactions that are coupled with the production of NADH molecules from the reduction of NAD + molecules.
In addition to the citric acid cycle, named for the first intermediate formed, citric acid, or citrate, when acetate joins to the oxaloacetate, the cycle is also known by two other names. The TCA cycle is named for tricarboxylic acids (TCA) because citric acid (or citrate) and isocitrate, the first two intermediates that are formed, are tricarboxylic acids. Additionally, the cycle is known as the Krebs cycle, named after Hans Krebs, who first identified the steps in the pathway in the 1930s in pigeon flight muscle.
Key Points
- The citric acid cycle is also known as the Krebs cycle or the TCA (tricarboxylic acid) cycle.
- Acetyl CoA transfers its acetyl group to oxaloacetate to form citrate and begin the citric acid cycle.
- The release of carbon dioxide is coupled with the reduction of NAD + to NADH in the citric acid cycle.
Key Terms
- TCA cycle : an alternative name for the Krebs cycle or citric acid cycle
- Krebs cycle : a series of enzymatic reactions that occurs in all aerobic organisms; it involves the oxidative metabolism of acetyl units and serves as the main source of cellular energy
- oxaloacetate : a four carbon molecule that receives an acetyl group from acetyl CoA to form citrate, which enters the citric acid cycle