Citric Acid Cycle. Carbohydrate Metabolism Overview. Elmhurst College. Form Acetyl CoA. Citric Acid Cycle Summary. Cori Cycle. Chemistry Department. Citric Acid Cycle Reactions. Virtual ChemBook. Click for larger image. Citric Acid Cycle Summary Introduction to Citric Acid Cycle: The diagram at the left puts the citric acid cycle into perspective as the most important part of metabolism. Quiz: Starting with glycogen producing one glucosephosphate, what is the end product of aerobic glycolysis?
Starting with glycogen to make glucosephosphate, how many ATP are made using aerobic glycolysis and the citric acid cycle? ATP Summary for Glucose - Complete Metabolism: The phrase "complete metabolism" means do reactions until you end up with carbon dioxide and water. Net complete glucose. This cycle is catalyzed by several enzymes and is named in honor of the British scientist Hans Krebs who identified the series of steps involved in the citric acid cycle.
The usable energy found in the carbohydrates , proteins , and fats we eat is released mainly through the citric acid cycle. Although the citric acid cycle does not use oxygen directly, it works only when oxygen is present. The citric acid cycle, however, occurs in the matrix of cell mitochondria. Prior to the beginning of the citric acid cycle, pyruvic acid generated in glycolysis crosses the mitochondrial membrane and is used to form acetyl coenzyme A acetyl CoA. Acetyl CoA is then used in the first step of the citric acid cycle.
Each step in the cycle is catalyzed by a specific enzyme. The two-carbon acetyl group of acetyl CoA is added to the four-carbon oxaloacetate to form the six-carbon citrate. The conjugate acid of citrate is citric acid, hence the name citric acid cycle. Oxaloacetate is regenerated at the end of the cycle so that the cycle may continue. Citrate loses a molecule of water and another is added. In the process, citric acid is converted to its isomer isocitrate.
Isocitrate loses a molecule of carbon dioxide CO2 and is oxidized forming the five-carbon alpha ketoglutarate. Alpha ketoglutarate is converted to the 4-carbon succinyl CoA. CoA is removed from the succinyl CoA molecule and is replaced by a phosphate group. The phosphate group is then removed and attached to guanosine diphosphate GDP thereby forming guanosine triphosphate GTP. The final product from the removal of CoA from succinyl CoA is succinate. Succinate is oxidized and fumarate is formed.
Another molecule of NADH is produced. Two carbon atoms come into the citric acid cycle from each acetyl group, representing four out of the six carbons of one glucose molecule. Two carbon dioxide molecules are released on each turn of the cycle; however, these do not necessarily contain the most recently-added carbon atoms.
The two acetyl carbon atoms will eventually be released on later turns of the cycle; thus, all six carbon atoms from the original glucose molecule are eventually incorporated into carbon dioxide. These carriers will connect with the last portion of aerobic respiration to produce ATP molecules.
Several of the intermediate compounds in the citric acid cycle can be used in synthesizing non-essential amino acids; therefore, the cycle is amphibolic both catabolic and anabolic. In order for pyruvate, the product of glycolysis, to enter the next pathway, it must undergo several changes to become acetyl Coenzyme A acetyl CoA.
Acetyl CoA is a molecule that is further converted to oxaloacetate, which enters the citric acid cycle Krebs cycle. The conversion of pyruvate to acetyl CoA is a three-step process. Breakdown of Pyruvate : Each pyruvate molecule loses a carboxylic group in the form of carbon dioxide. A carboxyl group is removed from pyruvate, releasing a molecule of carbon dioxide into the surrounding medium. Note: carbon dioxide is one carbon attached to two oxygen atoms and is one of the major end products of cellular respiration.
The result of this step is a two-carbon hydroxyethyl group bound to the enzyme pyruvate dehydrogenase; the lost carbon dioxide is the first of the six carbons from the original glucose molecule to be removed. This step proceeds twice for every molecule of glucose metabolized remember: there are two pyruvate molecules produced at the end of glycolysis ; thus, two of the six carbons will have been removed at the end of both of these steps. Step 3. The enzyme-bound acetyl group is transferred to CoA, producing a molecule of acetyl CoA.
This molecule of acetyl CoA is then further converted to be used in the next pathway of metabolism, the citric acid cycle. The citric acid cycle is a key component of the metabolic pathway by which all aerobic organisms generate energy.
The citric acid cycle, shown in —also known as the tricarboxylic acid cycle TCA cycle or the Krebs cycle—is a series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetate—derived from carbohydrates, fats, and proteins—into carbon dioxide. The cycle provides precursors including certain amino acids as well as the reducing agent NADH that is used in numerous biochemical reactions. Its central importance to many biochemical pathways suggests that it was one of the earliest established components of cellular metabolism; it may have originated abiogenically.
The Citric Acid Cycle : The citric acid cycle, or Krebs cycle, is a series of chemical reactions used by all aerobic organisms to generate energy through the oxidization of acetate—derived from carbohydrates, fats, and proteins—into carbon dioxide. In addition, the cycle provides precursors including certain amino acids as well as the reducing agent NADH that is used in numerous biochemical reactions.
The name of this metabolic pathway is derived from citric acid, a type of tricarboxylic acid that is first consumed and then regenerated by this sequence of reactions to complete the cycle. The net result of these two closely linked pathways is the oxidation of nutrients to produce usable energy in the form of ATP. Components of the TCA cycle were derived from anaerobic bacteria, and the TCA cycle itself may have evolved more than once.
Theoretically there are several alternatives to the TCA cycle, however the TCA cycle appears to be the most efficient. If several alternatives independently evolved, they all rapidly converged to the TCA cycle.
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