The Connecting Link between Glycolysis and Krebs Cycle: Formation of Acetyl CoA

The connecting link between glycolysis and the Krebs cycle is the conversion of pyruvate into acetyl CoA. This conversion occurs in the mitochondria and is an essential step for the further oxidation of glucose.

1. Glycolysis Overview
Glycolysis is the first step in the breakdown of glucose, which occurs in the cytoplasm of cells. It involves the conversion of one molecule of glucose into two molecules of pyruvate. This process generates a small amount of ATP and NADH.

2. Pyruvate Transport into the Mitochondria
Pyruvate, the end product of glycolysis, needs to be transported into the mitochondria for further oxidation. This transport occurs through specific transport proteins present in the mitochondrial membrane. Once inside the mitochondria, pyruvate undergoes several enzymatic reactions to form acetyl CoA.

3. Conversion of Pyruvate to Acetyl CoA
The conversion of pyruvate to acetyl CoA involves three key steps:

a) Decarboxylation: Pyruvate is decarboxylated, leading to the release of one molecule of carbon dioxide (CO2). This step is catalyzed by the enzyme pyruvate dehydrogenase complex, which requires thiamine pyrophosphate (TPP), coenzyme A (CoA), and lipoic acid as cofactors.

b) Oxidation: The remaining two-carbon fragment, known as an acetyl group, is oxidized. This oxidation is accompanied by the reduction of NAD+ to NADH. The enzyme responsible for this step is dihydrolipoamide dehydrogenase.

c) Formation of Acetyl CoA: The acetyl group is then transferred to CoA, forming acetyl CoA. This reaction is catalyzed by the enzyme acetyl CoA synthetase. The resulting acetyl CoA is then ready to enter the Krebs cycle.

4. Significance of Acetyl CoA Formation
Acetyl CoA serves as the entry point for the Krebs cycle, also known as the citric acid cycle or the tricarboxylic acid (TCA) cycle. In the Krebs cycle, the acetyl group is further oxidized, releasing energy in the form of ATP, NADH, and FADH2. These energy-rich molecules are then used in the electron transport chain to generate additional ATP through oxidative phosphorylation.

Conclusion
In summary, the conversion of pyruvate to acetyl CoA is the connecting link between glycolysis and the Krebs cycle. This conversion allows for the further oxidation of glucose and the generation of more ATP. Acetyl CoA serves as a crucial molecule in cellular respiration, providing the necessary fuel for the production of energy in the form of ATP.