The Citric Acid Cycle

The citric acid cycle is a closed loop of reactions that occur in the mitochondrial matrix, including redox, dehydration, hydration, and decarboxylation reactions.

It's name is derived from the intermediate compound citric acid as the steps were first described by Hans Krebs, this aerobic pathway is also known as the Krebs cycle, which over a series of eight enzymatic steps, is critical in glucose catabolism.

To begin, acetyl CoA, the resulting compound from pyruvate oxidation donates its acetal group to a four carbon molecule oxaloacetate forming a six carbon intermediate citrate. While it's CoA group is bound to a self hydro group and diffused away to eventually combine with another acetal group. A water molecule is then removed and replaced, transforming citrate into its isomer isocitrate. The molecule is then oxidized, reducing NAD+ to NADH and H+, and a carbon dioxide molecule, forming a five carbon alpha-ketoglutarate.

This product releases another carbon dioxide molecule and two electrons, reducing another NAD+ to NADH and a proton. Leaving the molecule with an unstable bond, where a coenzyme A attaches forming succinyl CoA. In the next step, the coenzyme is replaced by a phosphate group. Then the phosphate is transferred to GDP forming succinate and GTP, which can be used to generate ATP.

During step six, succinate is oxidized with two electrons from hydrogen atoms, transformed to the electron carrier flavin adenine dinucleotide, FAD to produce FADH2 and fumarate. Water is then added to the resulting molecule and after bond rearrangement forms into malate.

Finally this molecule is oxidized, reducing NAD+ to NADH and H+, regenerating the original compound, oxaloacetate. In the end, each cycle produces three NADH and one FADH2. High energy electron carriers that are used in the electron transport chain.