Question

Connecting the link between glycolysis and Krebs cycle is OR Before entering the Krebs cycle, pyruvate is changed to -
(a) Oxaloacetate
(b) PEP
(c) Pyruvate
(d) Acetyl CoA

Answer 1

Hint: The end product of glycolysis is two molecules of pyruvate and the first substrate of the Krebs cycle is citrate. Hence, pyruvate is converted to a link which needs to be stably carried into the Krebs cycle to form citric acid.

Complete answer:
- The link between glycolysis and Krebs cycle is Acetyl-CoA or Acetyl-Coenzyme A. The molecule is an acetyl group carried by the coenzyme A and is also known as CoASH or CoA.
- Glycolysis is the first step in respiration that uses the breakdown product of carbohydrates or fats, simpler molecules like glucose, to give pyruvate as the end product. Glycolysis occurs in both aerobic and anaerobic respiration, while the Krebs cycle occurs in aerobic respiration.
- Pyruvate undergoes oxidative decarboxylation to form an acetyl group, which is acetylated to the CoA. After entering the Krebs cycle, it is oxidized to carbon dioxide and water and energy are released.
- The reaction of pyruvate to Acetyl CoA is ${ CH }_{ 3 }C(=O)C(=O){ O }^{ - } + HSCoA +{ NAD }^{ + } + \longrightarrow { CH }_{ 3 }C(=O)SCoA + NADH + { CO }_{ 2 }$. The reaction is catalyzed by pyruvate dehydrogenase complex, that decarboxylates pyruvate to form acetyl CoA.

Additional Information:
- Pyruvate is a 3C molecule that is decarboxylated to Acetyl-CoA, a 2C molecule. Acetyl-CoA is converted to citrate, which is a 3C molecule.
- NAD molecules are reduced to form NADH when acetyl-CoA is converted to carbon dioxide.
- NADH generated is fed into oxidative phosphorylation. These two pathways produce energy in the form of ATP.

So, the correct answer is ‘Acetyl CoA’.

Note:
- Krebs cycle is also known as the tricarboxylic acid cycle or the citric acid cycle.
- When glucose levels are low in the cell, CoA is acetylated using acetate by acetyl-CoA synthetase.
- In eukaryotic cells, Krebs cycle occurs in the mitochondria.