Acetyl-CoA is the reactant needed in the citric acid cycle. It is produced by the oxidation of pyruvate which is the end product of glycolysis.

During the first step of cellular respiration, glycolysis, a 6-carbon glucose molecule is split into two 3-carbon molecules called pyruvate. These pyruvate molecules must by oxidized and converted to acetyl-CoA, which will subsequently move into the citric acid cycle, for the energy stored in them to be extracted. Acetyl-CoA acts as the link between glycolysis and the citric acid cycle. In eukaryotes, this step takes place in the mitochondrial matrix, the innermost compartment of mitochondria, so pyruvate is first transported across the mitochondrial membrane. In prokaryotes, the reaction takes place in the cytoplasm.

First, a carboxyl group is removed from pyruvate and a carbon dioxide molecule is released, as shown below. This leaves behind a two carbon molecule (shown in pink). Next, the two carbon molecule is oxidized and the electrons are accepted by NAD+, reducing it to NADH. Finally, the oxidized two carbon molecule which is now an acetyl group is attached to Coenzyme A, an organic molecule derived from vitamin B5, to form acetyl-CoA.

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Key Points

• The acetyl CoA is formed from the end product of glycolysis i.e. pyruvate.

• The formation of acetyl CoA takes place with the help of the enzyme pyruvate dehydrogenase and coenzyme A.

• During the breakdown of pyruvate, electrons are transferred to NAD+ to produce NADH, which will be used by the cell to produce ATP.

• The reaction in which pyruvate is broken down to form acetyl CoA acts as a link between glycolysis and the citric acid cycle.

• Acetyl CoA acts as the starting molecule and end product of the citric acid cycle.

Key Terms

acetyl CoA: it is a molecule that helps in the transfer of the carbon dioxide molecule from pyruvate to the citric acid cycle for the production of energy

citric acid cycle: it is a part of cellular respiration in which oxidation of acetate (derived from carbohydrates, fats, and proteins) to carbon dioxide takes place that helps in the production of energy

coenzyme A: this coenzyme helps in the oxidation of the pyruvate into acetyl CoA

glycolysis: the first step in the breakdown of glucose to extract energy for cellular metabolism

NADH: nicotinamide adenine dinucleotide (NAD) carrying two electrons and bonded with a hydrogen (H) ion; the reduced form of NAD

pyruvate: any salt or ester of pyruvic acid; the end product of glycolysis

mitochondria: membrane-bound cell organelles that generate energy