Pathways

The citric acid cycle, which is also known as the tricarboxylic acid (TCA) cycle or the Krebs cycle, is a collection of 9 enzyme-catalyzed chemical reactions that occurs in all living cells undergoing aerobic respiration. The citric acid cycle itself was officially identified in 1937 by Hans Adolf Krebs, who received the Nobel Prize for this discovery in 1953. In eukaryotes, the citric acid cycle occurs in the mitochondria. In prokaryotes, the TCA cycle occurs in the cytoplasm. The TCA cycle starts with acetyl-CoA, which is the “fuel” for the entire cycle. This important molecule is formed from the breakdown of glycogen (a stored form of glucose), fats, and many amino acids. At the start of the cycle, acetyl-CoA first transfers its 2-carbon acetyl group to the 4-carbon acceptor compound called oxaloacetate to form the 6-carbon compound (citrate) for which the cycle is named. The resulting citrate molecule then goes through a series of chemical transformations, whereby it loses one carboxyl group (leading to the 5-carbon compound called alpha-ketoglutarate) and then a second carboxyl group (leading to the 4-carbon compound called succinate). Succinate molecule is further oxidized to fumarate, then malate and finally oxaloacetate. The regeneration of the 4-carbon oxaloacetate, allows the TCA cycle to continue. Most of the energy generated by the oxidation steps in the TCA cycle is transferred as energy-rich electrons to NAD+, forming NADH. For each acetyl group that enters the citric acid cycle, three molecules of NADH are produced.

The citric acid cycle (also named tricarboxylic acid (TCA) cycle or the Krebs cycle), is a collection of 9 enzyme-catalyzed chemical reactions that occur in all living cells undergoing aerobic respiration. The citric acid cycle itself was officially identified in 1937 by Hans Adolf Krebs, who received the Nobel Prize for this discovery in 1953. In eukaryotes, the citric acid cycle occurs in the mitochondria. In prokaryotes, the TCA cycle occurs in the cytoplasm. The TCA cycle starts with acetyl-CoA, which is the “fuel” for the entire cycle. This important molecule comes from the breakdown of glycogen (a stored form of glucose), fats, and many amino acids. At beginning, acetyl-CoA first transfers its 2-carbon acetyl group to the 4-carbon acceptor compound called oxaloacetate to form the 6-carbon compound (citrate) for which the cycle is named. The resulting citrate will have numbers of chemical transformations, whereby it loses one carboxyl group (leading to the 5-carbon compound called alpha-ketoglutarate) and then a second carboxyl group (leading to the 4-carbon compound called succinate). Succinate molecule is further oxidized to fumarate, then malate and finally oxaloacetate. The regeneration of the 4-carbon oxaloacetate, allows the TCA cycle to continue. Oxidation step generates energy that is transferring energy-rich electrons for NAD+ to form NADH in TCA cycle. Each acetyl group will generate 3 NADH in TCA cycle.

The citric acid cycle (also named tricarboxylic acid (TCA) cycle or the Krebs cycle), is a collection of 9 enzyme-catalyzed chemical reactions that occur in all living cells undergoing aerobic respiration. The citric acid cycle itself was officially identified in 1937 by Hans Adolf Krebs, who received the Nobel Prize for this discovery in 1953. In eukaryotes, the citric acid cycle occurs in the mitochondria. In prokaryotes, the TCA cycle occurs in the cytoplasm. The TCA cycle starts with acetyl-CoA, which is the “fuel” for the entire cycle. This important molecule comes from the breakdown of glycogen (a stored form of glucose), fats, and many amino acids. At beginning, acetyl-CoA first transfers its 2-carbon acetyl group to the 4-carbon acceptor compound called oxaloacetate to form the 6-carbon compound (citrate) for which the cycle is named. The resulting citrate will have numbers of chemical transformations, whereby it loses one carboxyl group (leading to the 5-carbon compound called alpha-ketoglutarate) and then a second carboxyl group (leading to the 4-carbon compound called succinate). Succinate molecule is further oxidized to fumarate, then malate and finally oxaloacetate. The regeneration of the 4-carbon oxaloacetate, allows the TCA cycle to continue. Oxidation step generates energy that is transferring energy-rich electrons for NAD+ to form NADH in TCA cycle. Each acetyl group will generate 3 NADH in TCA cycle.

This should be a comprehensive description of the pathway.

The citric acid cycle (also named tricarboxylic acid (TCA) cycle or the Krebs cycle), is a collection of 9 enzyme-catalyzed chemical reactions that occur in all living cells undergoing aerobic respiration. The citric acid cycle itself was officially identified in 1937 by Hans Adolf Krebs, who received the Nobel Prize for this discovery in 1953. In eukaryotes, the citric acid cycle occurs in the mitochondria. In prokaryotes, the TCA cycle occurs in the cytoplasm. The TCA cycle starts with acetyl-CoA, which is the “fuel” for the entire cycle. This important molecule comes from the breakdown of glycogen (a stored form of glucose), fats, and many amino acids. At beginning, acetyl-CoA first transfers its 2-carbon acetyl group to the 4-carbon acceptor compound called oxaloacetate to form the 6-carbon compound (citrate) for which the cycle is named. The resulting citrate will have numbers of chemical transformations, whereby it loses one carboxyl group (leading to the 5-carbon compound called alpha-ketoglutarate) and then a second carboxyl group (leading to the 4-carbon compound called succinate). Succinate molecule is further oxidized to fumarate, then malate and finally oxaloacetate. The regeneration of the 4-carbon oxaloacetate, allows the TCA cycle to continue. Oxidation step generates energy that is transferring energy-rich electrons for NAD+ to form NADH in TCA cycle. Each acetyl group will generate 3 NADH in TCA cycle.

The citric acid cycle (also named tricarboxylic acid (TCA) cycle or the Krebs cycle), is a collection of 9 enzyme-catalyzed chemical reactions that occur in all living cells undergoing aerobic respiration. The citric acid cycle itself was officially identified in 1937 by Hans Adolf Krebs, who received the Nobel Prize for this discovery in 1953. In eukaryotes, the citric acid cycle occurs in the mitochondria. In prokaryotes, the TCA cycle occurs in the cytoplasm. The TCA cycle starts with acetyl-CoA, which is the “fuel” for the entire cycle. This important molecule comes from the breakdown of glycogen (a stored form of glucose), fats, and many amino acids. At beginning, acetyl-CoA first transfers its 2-carbon acetyl group to the 4-carbon acceptor compound called oxaloacetate to form the 6-carbon compound (citrate) for which the cycle is named. The resulting citrate will have numbers of chemical transformations, whereby it loses one carboxyl group (leading to the 5-carbon compound called alpha-ketoglutarate) and then a second carboxyl group (leading to the 4-carbon compound called succinate). Succinate molecule is further oxidized to fumarate, then malate and finally oxaloacetate. The regeneration of the 4-carbon oxaloacetate, allows the TCA cycle to continue. Oxidation step generates energy that is transferring energy-rich electrons for NAD+ to form NADH in TCA cycle. Each acetyl group will generate 3 NADH in TCA cycle.

The TCA cycle (tricarboxylic acid cycle) is also known as the citric acid cycle and the Krebs cycle. This pathway is the catabolism of aerobic respiration which produces energy and reducing power. It also initiates the production of precursors necessary for biosynthesis. If the carbon source for the cycle is acetate then citrate synthase becomes rate-limiting. Respiration produces ATP through a process of compounds acting as electron donors transferring electrons to electron acceptors. During this electron transport chain, a proton motive force is generated by the transport of protons outside the cytoplasmic membrane. As protons return to the cytoplasm, a multisubunit ATPase catalyzes the production of ATP from the proton motive force energy. During aerobic respiration, the final electron acceptor is oxygen. During anaerobic respiration, several organic compounds act as acceptors such as hydrogen, fumarate and nitrate. The cycle can start from Acetyl-CoA interacting with Oxalacetic acid and water through a citrate synthase monomer resulting in a hydrogen ion, CoA and a Citric Acid. The latter compound is dehydrated by a Citrate hydro-lyase resulting in the release of water and a cis-Aconitic acid. This compound is then hydrated through a Citrate hydro-lyase resulting in a D-threo-Isocitric acid. This compound is decarboxylated by an NADP dependent Citrate dehydrogenase, resulting in a release of carbon dioxide and NADPH and Oxoglutaric acid. The oxoglutaric acid interacts with a Coenzyme A through a NAD driven 2-oxoglutarate dehydrogenase resulting in a release of carbon dioxide, an NADH and succinyl-CoA. The succinyl-CoA interacts with a phosphate and an ADP through a 2-oxoglutarate dehydrogenase resulting in a CoA, an ATP and Succinic Acid. Succinic acid interacts with a ubiquinone, in this case a ubiquinone 1 through a succinate:quinone oxidoreductase resulting in an ubiquinol, in this case a ubiquinol-1 and a fumaric acid. The fumaric acid interacts with water through a fumarase hydratase resulting in a L-Malic acid. Malic acid can either react with a NAD dependent dehydrogenase resulting in the release of pyruvate. Malic Acid acid can also react with a malate dehydrogenase resulting in the release of oxalacetic acid

The citric acid cycle (also named tricarboxylic acid (TCA) cycle or the Krebs cycle), is a collection of 9 enzyme-catalyzed chemical reactions that occur in all living cells undergoing aerobic respiration. The citric acid cycle itself was officially identified in 1937 by Hans Adolf Krebs, who received the Nobel Prize for this discovery in 1953. In eukaryotes, the citric acid cycle occurs in the mitochondria. In prokaryotes, the TCA cycle occurs in the cytoplasm. The TCA cycle starts with acetyl-CoA, which is the “fuel” for the entire cycle. This important molecule comes from the breakdown of glycogen (a stored form of glucose), fats, and many amino acids. At beginning, acetyl-CoA first transfers its 2-carbon acetyl group to the 4-carbon acceptor compound called oxaloacetate to form the 6-carbon compound (citrate) for which the cycle is named. The resulting citrate will have numbers of chemical transformations, whereby it loses one carboxyl group (leading to the 5-carbon compound called alpha-ketoglutarate) and then a second carboxyl group (leading to the 4-carbon compound called succinate). Succinate molecule is further oxidized to fumarate, then malate and finally oxaloacetate. The regeneration of the 4-carbon oxaloacetate, allows the TCA cycle to continue. Oxidation step generates energy that is transferring energy-rich electrons for NAD+ to form NADH in TCA cycle. Each acetyl group will generate 3 NADH in TCA cycle.

The citric acid cycle (also named tricarboxylic acid (TCA) cycle or the Krebs cycle), is a collection of 9 enzyme-catalyzed chemical reactions that occur in all living cells undergoing aerobic respiration. The citric acid cycle itself was officially identified in 1937 by Hans Adolf Krebs, who received the Nobel Prize for this discovery in 1953. In eukaryotes, the citric acid cycle occurs in the mitochondria. In prokaryotes, the TCA cycle occurs in the cytoplasm. The TCA cycle starts with acetyl-CoA, which is the “fuel” for the entire cycle. This important molecule comes from the breakdown of glycogen (a stored form of glucose), fats, and many amino acids. At beginning, acetyl-CoA first transfers its 2-carbon acetyl group to the 4-carbon acceptor compound called oxaloacetate to form the 6-carbon compound (citrate) for which the cycle is named. The resulting citrate will have numbers of chemical transformations, whereby it loses one carboxyl group (leading to the 5-carbon compound called alpha-ketoglutarate) and then a second carboxyl group (leading to the 4-carbon compound called succinate). Succinate molecule is further oxidized to fumarate, then malate and finally oxaloacetate. The regeneration of the 4-carbon oxaloacetate, allows the TCA cycle to continue. Oxidation step generates energy that is transferring energy-rich electrons for NAD+ to form NADH in TCA cycle. Each acetyl group will generate 3 NADH in TCA cycle.