Step 5. A phosphate group is substituted for coenzyme A, and a high- energy bond is formed. This energy is used in substrate-level phosphorylation during the conversion of the succinyl group to succinate to form either guanine triphosphate GTP or ATP.
There are two forms of the enzyme, called isoenzymes, for this step, depending upon the type of animal tissue in which they are found. One form is found in tissues that use large amounts of ATP, such as heart and skeletal muscle. This form produces ATP. The second form of the enzyme is found in tissues that have a high number of anabolic pathways, such as liver.
This form produces GTP. In particular, protein synthesis primarily uses GTP. Step 6. Step six is a dehydration process that converts succinate into fumarate. Unlike NADH, this carrier remains attached to the enzyme and transfers the electrons to the electron transport chain directly.
This process is made possible by the localization of the enzyme catalyzing this step inside the inner membrane of the mitochondrion. Step 7. Water is added to fumarate during step seven, and malate is produced.
The last step in the citric acid cycle regenerates oxaloacetate by oxidizing malate. 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 two acetyl-carbon atoms will eventually be released on later turns of the cycle; in this way, all six carbon atoms from the original glucose molecule will be eventually released as carbon dioxide. It takes two turns of the cycle to process the equivalent of one glucose molecule.
These high-energy carriers will connect with the last portion of aerobic respiration to produce ATP molecules. One ATP or an equivalent is also made in each cycle. Several of the intermediate compounds in the citric acid cycle can be used in synthesizing non-essential amino acids; therefore, the cycle is both anabolic and catabolic. Unless otherwise noted, images on this page are licensed under CC-BY 4. Text adapted from: OpenStax , Concepts of Biology.
OpenStax CNX. Skip to content 47 Aerobic Respiration: The Citric Acid Cycle In eukaryotic cells, if there is oxygen available, the pyruvate molecules produced at the end of glycolysis are transported into the mitochondria. Figure 1 Pyruvate is converted into acetyl-CoA before entering the citric acid cycle.
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