Pyruvate, produced during glycolysis, must be processed in order to drive other metabolic pathways.
After pyruvate is produced by glycolysis, it needs to be moved from the cytoplasm (the jellylike fluid around cell structures) into the mitochondria. This happens only if enough oxygen is available to continue this aerobic form of respiration. While in the mitochondria, pyruvate is transformed into an acetyl group that is activated by a molecule called coenzyme A (CoA). The result of this activation leads to the formation of acetyl CoA. Acetyl CoA is the final product made in pyruvate processing. This molecule is used in the next pathway of cellular respiration, the citric acid cycle. There are three steps involved in transforming pyruvate into acetyl CoA: decarboxylation, oxidation, and the attachment of CoA.
Step 1. During decarboxylation, a carboxyl group (C-O) must be removed from pyruvate. This process results in the first of six carbon atoms being removed from the original glucose molecule. Because there are two pyruvate molecules produced in glycolysis, this step occurs twice. That means a total of two carbons have been removed from the original glucose molecule.
Step 2. A transfer of electrons occurs during an oxidation reaction involving NAD+. This results in the reduction of NAD+ to NADH and the acetyl group is converted to acetate.
Step 3. CoA is added to the acetate in order to make acetyl CoA.
The process of making acetyl CoA from pyruvate is an irreversible reaction. This reaction is responsible for linking glycolysis to the citric acid cycle. Adding CoA makes the molecule unstable, which means it is more prone to releasing energy because it easily reacts with other molecules. This explains why it is necessary to create a glucose derivative that can fuel the citric acid cycle and release more energy.