The main function of the citric acid cycle is to extract chemical energy from nutrients and store it in the form of adenosine triphosphate (ATP), the cell’s primary energy source. The citric acid cycle, also known as the tricarboxylic acid cycle or the Krebs cycle, is a vital biochemical process that takes place in the mitochondria of eukaryotic cells.
How does the citric acid cycle work?
Simply put, the citric acid cycle first takes in acetyl-CoA, which can come from the nutrients we eat. These nutrients (carbohydrates, fats, proteins) are digested and further broken down in various metabolic pathways (glycolysis, beta-oxidation, proteolysis) to ultimately form acetyl-CoA. This acetyl-CoA is then converted into a starting compound called citrate.
During the cycle, citrate passes through various stages, gradually transforming into other substances. At the same time, small energy molecules, such as NADH and FADH₂, are produced. These molecules act like tiny batteries that are later used to produce ATP, the energy source for our cells.
The Exact Process
- Pyruvate + CoA → Acetyl-CoA Pyruvate dehydrogenase is a so-called multienzyme complex consisting of three different enzymes and located in the mitochondrion. In order for its reactions to be fully catalyzed, PDH requires a total of five coenzymes: Thiamine pyrophosphate (TPP) = activated thiamine, lipoic acid, coenzyme A, FAD, NAD+
- Acetyl-CoA + oxaloacetate → citrate Enzyme: Citrate synthase Co-substrate: H₂O Byproduct: Coenzyme A (CoA-SH)
- Citrate → Isocitrate Enzyme: Aconitase
- Isocitrate → α-Ketoglutarate Enzyme: Isocitrate dehydrogenase Co-substrate: NAD+
- α-Ketoglutarate → Succinyl-CoA α-Ketoglutarate dehydrogenase is a large enzyme complex that closely resembles pyruvate dehydrogenase. It requires the following cofactors for the oxidative decarboxylation of α-ketoglutarate to succinyl-CoA: thiamine pyrophosphate, liponamide, coenzyme A, FAD, and NAD+. This reaction again produces CO₂ as well as 1 additional NADH+H+ for the respiratory chain.
- Succinyl-CoA → Succinate + CoA + GTP Enzyme: Succinyl-CoA synthetase Co-substrate: GDP, Pi
- Succinate → Fumarate + FADH2 FAD-dependent succinate dehydrogenase catalyzes the oxidation of succinate to fumarate. This occurs with the formation of a double bond and the release of 1 FADH2. An important characteristic is that succinate dehydrogenase is the only enzyme in the citric acid cycle that is not present freely in the matrix, but is anchored in the inner mitochondrial membrane. This allows it to transfer electrons from FADH₂ directly into the respiratory chain, and it is therefore referred to as Complex II. Fumarate + H₂O → Malate Enzyme: Fumarase
- Malate → Oxaloacetate Enzyme: Malate dehydrogenase Co-substrate: NAD⁺
What happens to NADH and FADH2?
Once the citric acid cycle is complete, NADH and FADH2 contribute the energy stored during the cycle to ATP synthesis. These molecules act as “energy carriers” and serve as the main sources of electrons and protons for the electron transport chain (ETC) in the mitochondria.
- Electron Transport Chain (ETC): NADH and FADH2 transfer their electrons to the protein complexes of the electron transport chain, which are located in the inner mitochondrial membrane. These electrons pass through a series of protein complexes, causing protons (H+) to be pumped from the interior of the mitochondria into the intermembrane space.
- Proton Pumps and Electron Transport: As the electrons travel through the protein complexes, the proton gradient energy generated by these proton pumps is used to actively pump protons into the intermembrane space. This creates a proton gradient across the inner mitochondrial membrane.
- ATP Synthesis: The proton gradient is utilized by an enzyme called ATP synthase to produce ATP. ATP synthase functions as a “turbine” that rotates due to the flow of protons, converting ADP to ATP in the process.
- Water formation: At the end of the electron transport chain, the electrons and protons combine with oxygen (O₂) to form water (H₂O). Regeneration of NAD+ and FAD:
The electrons that travel through the electron transport chain are transferred to oxygen, and NADH and FADH2 are regenerated to their original states (NAD+ and FAD). This is important because NAD+ and FAD are needed again in the citric acid cycle to continue the cycle.
In summary, NADH and FADH₂ contribute the energy stored during the citric acid cycle to the electron transport chain, thereby generating ATP, which serves as the cell’s primary energy currency. The process ends with the formation of water and the regeneration of the NAD⁺ and FAD molecules, which can then be reused at the beginning of the citric acid cycle.