In eukaryotic cells, the pyruvate molecules produced at the end of glycolysis are transported into the mitochondria, which are the sites of cellular respiration. There, pyruvate is transformed into an acetyl group that will be picked up and activated by a carrier compound called coenzyme A CoA.
The resulting compound is called acetyl CoA. CoA is derived from vitamin B5, pantothenic acid. Acetyl CoA can be used in a variety of ways by the cell, but its major function is to deliver the acetyl group derived from pyruvate to the next stage of the pathway in glucose catabolism.
In order for pyruvate, the product of glycolysis, to enter the next pathway, it must undergo several changes. The conversion is a three-step process Figure. Step 1. A carboxyl group is removed from pyruvate, releasing a molecule of carbon dioxide into the surrounding medium. This reaction creates a two-carbon hydroxyethyl group bound to the enzyme pyruvate dehydrogenase.
We should note that this is the first of the six carbons from the original glucose molecule to be removed. This step proceeds twice because there are two pyruvate molecules produced at the end of glycolsis for every molecule of glucose metabolized anaerobically; thus, two of the six carbons will have been removed at the end of both steps. Step 2. Step 3. The enzyme-bound acetyl group is transferred to CoA, producing a molecule of acetyl CoA.
Note that during the second stage of glucose metabolism, whenever a carbon atom is removed, it is bound to two oxygen atoms, producing carbon dioxide, one of the major end products of cellular respiration.
In the presence of oxygen, acetyl CoA delivers its acetyl 2C group to a four-carbon molecule, oxaloacetate, to form citrate, a six-carbon molecule with three carboxyl groups; this pathway will harvest the remainder of the extractable energy from what began as a glucose molecule and release the remaining four CO 2 molecules.
This single pathway is called by different names: the citric acid cycle for the first intermediate formed—citric acid, or citrate—when acetate joins to the oxaloacetate , the TCA cycle because citric acid or citrate and isocitrate are tricarboxylic acids , and the Krebs cycle , after Hans Krebs, who first identified the steps in the pathway in the s in pigeon flight muscles.
Like the conversion of pyruvate to acetyl CoA, the citric acid cycle takes place in the matrix of mitochondria. Almost all of the enzymes of the citric acid cycle are soluble, with the single exception of the enzyme succinate dehydrogenase, which is embedded in the inner membrane of the mitochondrion.
Unlike glycolysis, the citric acid cycle is a closed loop: the last part of the pathway regenerates the compound used in the first step.
This is considered an aerobic pathway because the NADH and FADH 2 produced must transfer their electrons to the next pathway in the system, which will use oxygen. If this transfer does not occur, the oxidation steps of the citric acid cycle also do not occur.
Note that the citric acid cycle produces very little ATP directly and does not directly consume oxygen. Prior to the first step, a transitional phase occurs during which pyruvic acid is converted to acetyl CoA. Then, the first step of the cycle begins: This condensation step combines the two-carbon acetyl group with a four-carbon oxaloacetate molecule to form a six-carbon molecule of citrate. CoA is bound to a sulfhydryl group -SH and diffuses away to eventually combine with another acetyl group.
This step is irreversible because it is highly exergonic. The rate of this reaction is controlled by negative feedback and the amount of ATP available. If ATP levels increase, the rate of this reaction decreases. 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. Privacy Policy. Skip to main content. Cellular Respiration. Search for:. Oxidation of Pyruvate and the Citric Acid Cycle.
Breakdown of Pyruvate After glycolysis, pyruvate is converted into acetyl CoA in order to enter the citric acid cycle. Learning Objectives Explain why cells break down pyruvate. Key Takeaways Key Points In the conversion of pyruvate to acetyl CoA, each pyruvate molecule loses one carbon atom with the release of carbon dioxide. In the final step of the breakdown of pyruvate, an acetyl group is transferred to Coenzyme A to produce acetyl CoA. Key Terms acetyl CoA : a molecule that conveys the carbon atoms from glycolysis pyruvate to the citric acid cycle to be oxidized for energy production.
This molecule of acetyl CoA is then further converted to be used in the next pathway of metabolism, the citric acid cycle. Learning Objectives Explain why cells break down pyruvate. Key Points In the conversion of pyruvate to acetyl CoA, each pyruvate molecule loses one carbon atom with the release of carbon dioxide. In the final step of the breakdown of pyruvate, an acetyl group is transferred to Coenzyme A to produce acetyl CoA. Key Terms acetyl CoA : a molecule that conveys the carbon atoms from glycolysis pyruvate to the citric acid cycle to be oxidized for energy production.
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