Citric acid cycle
The citric acid cycle—also known as the Krebs cycle, Szent–Györgyi–Krebs cycle, or TCA cycle —is a series of biochemical reactions that release the energy stored in nutrients through acetyl-CoA oxidation. The energy released is available in the form of ATP. The Krebs cycle is used by organisms that generate energy via respiration, either anaerobically or aerobically. In addition, the cycle provides precursors of certain amino acids, as well as the reducing agent NADH, which are used in other reactions. Its central importance to many biochemical pathways suggests that it was one of the earliest metabolic components. Even though it is branded as a "cycle", it is not necessary for metabolites to follow a specific route; at least three alternative pathways of the citric acid cycle are recognized.
Its name is derived from the citric acid that is consumed and then regenerated by this sequence of reactions. The cycle consumes acetate and water and reduces NAD+ to NADH, releasing carbon dioxide. The NADH generated by the citric acid cycle is fed into the oxidative phosphorylation pathway. The net result of these two closely linked pathways is the oxidation of nutrients to produce usable chemical energy in the form of ATP.
In eukaryotic cells, the citric acid cycle occurs in the mitochondrial matrix. In prokaryotic cells, such as bacteria, which lack mitochondria, the citric acid cycle reaction sequence is performed in the cytosol with the proton gradient for ATP production being across the cell's surface rather than the inner membrane of the mitochondrion.
For each pyruvate molecule, the overall yield of energy-containing compounds from the citric acid cycle is three NADH, one FADH2, and one GTP or ATP.
Discovery
Several of the components and reactions of the citric acid cycle were established in the 1930s by the research of Albert Szent-Györgyi, who received the Nobel Prize in Physiology or Medicine in 1937 specifically for his discoveries pertaining to fumaric acid, a component of the cycle. He made this discovery by studying pigeon breast muscle. Because this tissue maintains its oxidative capacity well after breaking down in the Latapie mincer and releasing in aqueous solutions, breast muscle of the pigeon was very well qualified for the study of oxidative reactions. The citric acid cycle itself was finally identified in 1937 by Hans Adolf Krebs and William Arthur Johnson while at the University of Sheffield, for which the former received the Nobel Prize for Physiology or Medicine in 1953, and for whom the cycle is sometimes named the "Krebs cycle". Independently citric acid cycle was identified in 1937 by German biochemists Carl Martius and Franz Knoop.Overview
The citric acid cycle is a metabolic pathway that connects carbohydrate, fat, and protein metabolism. The reactions of the cycle are carried out by eight enzymes that completely oxidize acetate, in the form of acetyl-CoA, into two molecules each of carbon dioxide. Through catabolism of sugars, fats, and proteins, the two-carbon organic product acetyl-CoA is produced which enters the citric acid cycle. The reactions of the cycle also convert three equivalents of nicotinamide adenine dinucleotide into three equivalents of reduced NAD, one equivalent of flavin adenine dinucleotide into one equivalent of FADH2, and one equivalent each of guanosine diphosphate and inorganic phosphate into one equivalent of guanosine triphosphate. The NADH and FADH2 generated by the citric acid cycle are, in turn, used by the oxidative phosphorylation pathway to generate energy-rich ATP.One of the primary sources of acetyl-CoA is from the breakdown of sugars by glycolysis which yield pyruvate that in turn is decarboxylated by the pyruvate dehydrogenase complex generating acetyl-CoA according to the following reaction scheme:
The product of this reaction, acetyl-CoA, is the starting point for the citric acid cycle. Acetyl-CoA may also be obtained from the oxidation of fatty acids. Below is a schematic outline of the cycle:
- The citric acid cycle begins with the transfer of a two-carbon acetyl group from acetyl-CoA to the four-carbon acceptor compound to form a six-carbon compound.
- The citrate then goes through a series of chemical transformations, losing two carboxyl groups as CO2. The carbons lost as CO2 originate from what was oxaloacetate, not directly from acetyl-CoA. The carbons donated by acetyl-CoA become part of the oxaloacetate carbon backbone after the first turn of the citric acid cycle. Loss of the acetyl-CoA-donated carbons as CO2 requires several turns of the citric acid cycle. However, because of the role of the citric acid cycle in anabolism, they might not be lost, since many citric acid cycle intermediates are also used as precursors for the biosynthesis of other molecules.
- Most of the electrons made available by the oxidative steps of the cycle are transferred to NAD+, forming NADH. For each acetyl group that enters the citric acid cycle, three molecules of NADH are produced. The citric acid cycle includes a series of redox reactions in mitochondria.
- In addition, electrons from the succinate oxidation step are transferred first to the FAD cofactor of succinate dehydrogenase, reducing it to FADH2, and eventually to ubiquinone in the mitochondrial membrane, reducing it to ubiquinol which is a substrate of the electron transfer chain at the level of Complex III.
- For every NADH and FADH2 that are produced in the citric acid cycle, 2.5 and 1.5 ATP molecules are generated in oxidative phosphorylation, respectively.
- At the end of each cycle, the four-carbon oxaloacetate has been regenerated, and the cycle continues.
Steps
| Reaction type | Substrates | Enzyme | Products | Comment | |
| Aldol condensation | Oxaloacetate + Acetyl CoA + H2O | Citrate synthase | Citrate + CoA-SH | irreversible, extends the 4C oxaloacetate to a 6C molecule | |
| 1 | Dehydration | Citrate | Aconitase | cis-Aconitate + H2O | reversible isomerisation |
| 2 | Hydration | cis-Aconitate + H2O | Aconitase | Isocitrate | reversible isomerisation |
| 3 | Oxidation | Isocitrate + NAD+ | Isocitrate dehydrogenase | Oxalosuccinate + NADH + H + | generates NADH |
| 4 | Decarboxylation | Oxalosuccinate | Isocitrate dehydrogenase | α-Ketoglutarate + CO2 | rate-limiting, irreversible stage, generates a 5C molecule |
| 5 | Oxidative decarboxylation | α-Ketoglutarate + NAD+ + CoA-SH | α-Ketoglutarate dehydrogenase, Thiamine pyrophosphate, Lipoic acid, Mg++,transsuccinytase | Succinyl-CoA + NADH + H + + CO2 | irreversible stage, generates NADH, regenerates the 4C chain |
| 6 | Substrate-level phosphorylation | Succinyl-CoA + GDP + Pi | Succinyl-CoA synthetase | Succinate + CoA-SH + GTP | or ADP→ATP instead of GDP→GTP, generates 1 ATP or equivalent. Condensation reaction of GDP + Pi and hydrolysis of succinyl-CoA involve the H2O needed for balanced equation. |
| 7 | Oxidation | Succinate + ubiquinone | Succinate dehydrogenase | Fumarate + ubiquinol | uses FAD as a prosthetic group in the enzyme. These two electrons are later transferred to QH2 during Complex II of the ETC, where they generate the equivalent of 1.5 ATP |
| 8 | Hydration | Fumarate + H2O | Fumarase | L-Malate | Hydration of C-C double bond |
| 9 | Oxidation | L-Malate + NAD+ | Malate dehydrogenase | Oxaloacetate + NADH + H+ | reversible, generates NADH |
| 10 / 0 | Aldol condensation | Oxaloacetate + Acetyl CoA + H2O | Citrate synthase | Citrate + CoA-SH | This is the same as step 0 and restarts the cycle. The reaction is irreversible and extends the 4C oxaloacetate to a 6C molecule |
Two carbon atoms are oxidized to CO2, the energy from these reactions is transferred to other metabolic processes through GTP, and as electrons in NADH and QH2. The NADH generated in the citric acid cycle may later be oxidized to drive ATP synthesis in a type of process called oxidative phosphorylation. FADH2 is covalently attached to succinate dehydrogenase, an enzyme which functions both in the citric acid cycle and the mitochondrial electron transport chain in oxidative phosphorylation. FADH2, therefore, facilitates transfer of electrons to coenzyme Q, which is the final electron acceptor of the reaction catalyzed by the succinate:ubiquinone oxidoreductase complex, also acting as an intermediate in the electron transport chain.
Mitochondria in animal cells, possess two succinyl-CoA synthetases: one that produces GTP from GDP, and another that produces ATP from ADP. Plant cells have the type that produces ATP. Several of the enzymes in the cycle may be loosely associated in a multienzyme protein complex within the mitochondrial matrix.
The GTP that is formed by GDP-forming succinyl-CoA synthetase may be utilized by nucleoside-diphosphate kinase to form ATP.
Products
Products of the first turn of the cycle are one GTP, three NADH, one FADH2 and two CO2.Because two acetyl-CoA molecules are produced from each glucose molecule, two cycles are required per glucose molecule. Therefore, at the end of two cycles, the products are: two GTP, six NADH, two FADH2, and four CO2.
The above reactions are balanced if Pi represents the H2PO4− ion, ADP and GDP the ADP2− and GDP2− ions, respectively, and ATP and GTP the ATP3− and GTP3− ions, respectively.
The total number of ATP molecules obtained after complete oxidation of one glucose in glycolysis, citric acid cycle, and oxidative phosphorylation is estimated to be between 30 and 38.