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OpenStax-CNX module: m56028 1 Bis2A 07.3 Oxidation of Pyruvate and ∗ the Citric Acid Cycle Mitch Singer Based on Oxidation of Pyruvate and the Citric Acid Cycle† by OpenStax College This work is produced by OpenStax-CNX and licensed under the ‡ Creative Commons Attribution License 4.0 Abstract By the end of this section, you will be able to: • Explain how a circular pathway, such as the citric acid cycle, fundamentally diers from a linear pathway, such as glycolysis • Describe how pyruvate, the product of glycolysis, is prepared for entry into the citric acid cycle 1 Introduction to Pyruvate oxidation and the TCA cycle A Note from the Instructor As with the module on glycolysis, there is a lot of material in this module. I do not expect you to memorize specic names of compounds or enzymes. However, I will give you those names for completeness. For exams I will always provide you with the pathways we discuss in class and in the BioStax Biology text modules. What you need to be able to do is understand what is going on in each reaction. We will go over in lecture, problems that will be similar to those I will ask of you on exams. Do not be overwhelmed with specic enzyme names and specic structures. What you should know are the general types of enzymes used and the types of structures found. For example you do not need to memorize the structures of malate or succinate. You will need to know that both are carboxylic acids if the structure is given to you and should be able to identify the important functional groups. In addition, you will not need to know which reactions specically generate GTP or NADH, but if given the reactions you should be able to tell if a red/ox reaction is occurring. Finally, you will not be expected to memorize enzyme names, but like in glycolysis you will be expected to know the various types of reactions a type of enzyme can catalyze, for example, a dehydrogenase catalyzes a red/ox reaction. That is the level of understanding I expect. If you have any questions please ask. ∗ Version 1.2: Jun 19, 2015 11:49 am -0500 † http://cnx.org/content/m44433/1.7/ ‡ http://creativecommons.org/licenses/by/4.0/ http://cnx.org/content/m56028/1.2/ OpenStax-CNX module: m56028 2 2 Pyruvate oxidation and the TCA cycle The end-product of glycolysis are 2 pyruvate molecules, 2 ATPs and 2 NADH molecules. The question becomes, what does the cell do with them. ATP can be used for a variety of cellular functions including biosynthesis, transport, replication etc. NADH, is a problem, it needs to be recycled to NAD+. This occurs either through fermentation, in the absence of an electron transport chain, or can be used to generate a proton motive force (PMF) or "energized membrane", which can then lead to either ATP formation or other forms of work (transport of nutrients, cellular locomotion, etc. and will be discussed in later modules). That leaves the cell to deal with pyruvate. The fate of cellular pyruvate • • • Pyruvate can be used as a terminal electron acceptor in fermentation reactions, as was discussed in Module 7.2. Pyruvate could be secreted from the cell as a waste product. Pyruvate could be further oxidized to extract even more usable cellular energy, which is what will be discussed below. The further oxidation of pyruvate The pruvate formed in glycolysis has a variety of fates depending upon the cell type, physiology and environment the cell is in. In many instances, cells can further oxidize pyruvate, generating additional energy in the form of GTP and reducing power, the formation of NADH (and FADH2) along with the production of a variety of additional precursors, which can be used for biosynthesis as required by the cell. In aerobically respiring eukaryotic cells, the pyruvate molecules produced at the end of glycolysis are transported into mitochondria, which are the sites of cellular respiration and house the oxygen consuming electron transport chain. In respiring bacteria and archaea, the pyruvate is further oxidized in the cytoplasm. All three use similar mechanisms to further oxidize the pyruvate to CO2. Regardless of the organism, if pyruvate is to be further oxidized, the reactions are basically universal: rst pyruvate will be transformed into an acetyl group that will be picked up and activated by a carrier compound called coenzyme A (CoA) and the resulting acetyl-CoA feeds directly into the Tricarboxylic Acid Cycle also referred to as the TCA cycle or the Krebs Cycle. This process is detailed below. 3 Breakdown of Pyruvate 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 1). Step 1. A carboxyl group is removed from pyruvate, releasing a molecule of carbon dioxide into the surrounding medium. The result of this step is a two-carbon hydroxyethyl group bound to the enzyme (pyruvate dehydrogenase). This is the rst of the six carbons from the original glucose molecule to be removed. This step proceeds twice (remember: there are two pyruvate molecules produced at the end of glycolsis) for every molecule of glucose metabolized; thus, two of the six carbons will have been removed at the end of both steps. Step 2. The hydroxyethyl group is oxidized to an acetyl group, and the electrons are picked up by NAD+, forming NADH. The high-energy electrons from NADH will be used later to generate ATP. Step 3. The enzyme-bound acetyl group is transferred to CoA, producing a molecule of acetyl CoA. http://cnx.org/content/m56028/1.2/ OpenStax-CNX module: m56028 Figure 1: 3 Upon entering the mitochondrial matrix, a multi-enzyme complex converts pyruvate into acetyl CoA. In the process, carbon dioxide is released and one molecule of NADH is formed. 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. 4 Acetyl CoA to CO2 In the presence of oxygen, acetyl CoA delivers its acetyl 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. This single pathway is called by dierent names: the citric acid cycle (for the rst intermediate formedcitric acid, or citratewhen acetate joins to the oxaloacetate), the TCA cycle (since citric acid or citrate and isocitrate are tricarboxylic acids), and the Krebs cycle, after Hans Krebs, who rst identied the steps in the pathway in the 1930s in pigeon ight muscles. 5 Citric Acid Cycle 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 rst step. http://cnx.org/content/m56028/1.2/ OpenStax-CNX module: m56028 4 The eight steps of the cycle are a series of redox, dehydration, hydration, and decarboxylation reactions that produce two carbon dioxide molecules, one GTP/ATP, and reduced forms of NADH and FADH2 (Figure 2). This is considered an aerobic pathway because the NADH and FADH2 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. http://cnx.org/content/m56028/1.2/ OpenStax-CNX module: m56028 Figure 2: 5 In the citric acid cycle, the acetyl group from acetyl CoA is attached to a four-carbon oxaloacetate molecule to form a six-carbon citrate molecule. Through a series of steps, citrate is oxidized, releasing two carbon dioxide molecules for each acetyl group fed into the cycle. + NAD 2 In the process, three molecules are reduced to NADH, one FAD molecule is reduced to FADH , and one ATP or GTP (depending on the cell type) is produced (by substrate-level phosphorylation). Because the nal product of the citric acid cycle is also the rst reactant, the cycle runs continuously in the presence of sucient reactants. (credit: modication of work by Yikrazuul/Wikimedia Commons) http://cnx.org/content/m56028/1.2/ OpenStax-CNX module: m56028 6 5.1 Steps in the Citric Acid Cycle Step 1. Prior to the start of the rst step, a transitional phase occurs during which pyruvic acid is converted to acetyl CoA. Then, the rst step of the cycle begins: This is a condensation step, combining 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 diuses 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. If ATP is in short supply, the rate increases. Step 2. In step two, citrate loses one water molecule and gains another as citrate is converted into its isomer, isocitrate. Step 3. In step three, isocitrate is oxidized, producing a ve-carbon molecule, α-ketoglutarate, together with a molecule of CO2 and two electrons, which reduce NAD+ to NADH. This step is also regulated by negative feedback from ATP and NADH, and a positive eect of ADP. Steps 3 and 4. Steps three and four are both oxidation and decarboxylation steps, which release electrons that reduce NAD+ to NADH and release carboxyl groups that form CO2 molecules. α-Ketoglutarate is the product of step three, and a succinyl group is the product of step four. CoA binds the succinyl group to form succinyl CoA. The enzyme that catalyzes step four is regulated by feedback inhibition of ATP, succinyl CoA, and NADH. Step 5. In step ve, 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. GTP is energetically equivalent to ATP; however, its use is more restricted. In particular, protein synthesis primarily uses GTP. Step 6. Step six is a dehydration process that converts succinate into fumarate. Two hydrogen atoms are transferred to FAD, producing FADH2. The energy contained in the electrons of these atoms is insucient to reduce NAD+ but adequate to reduce FAD. 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 in the process. : Click through each step of the citric acid cycle here1 . 1 http://openstaxcollege.org/l/krebs_cycle http://cnx.org/content/m56028/1.2/ OpenStax-CNX module: m56028 7 5.2 Products of the Citric Acid Cycle 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. Each turn of the cycle forms three NADH molecules and one FADH2 molecule. These carriers will connect with the last portion of aerobic respiration to produce ATP molecules. One GTP or ATP 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 amphibolic (both catabolic and anabolic). 6 Additional Links Here are some additional links to videos and pages that you may nd useful. Chemwiki Links • Chemwiki TCA cycle2 Khan Academy Links • Khan Academy TCA cycle3 7 Section Summary In the presence of oxygen, pyruvate is transformed into an acetyl group attached to a carrier molecule of coenzyme A. The resulting acetyl CoA can enter several pathways, but most often, the acetyl group is delivered to the citric acid cycle for further catabolism. During the conversion of pyruvate into the acetyl group, a molecule of carbon dioxide and two high-energy electrons are removed. The carbon dioxide accounts for two (conversion of two pyruvate molecules) of the six carbons of the original glucose molecule. The electrons are picked up by NAD+, and the NADH carries the electrons to a later pathway for ATP production. At this point, the glucose molecule that originally entered cellular respiration has been completely oxidized. Chemical potential energy stored within the glucose molecule has been transferred to electron carriers or has been used to synthesize a few ATPs. The citric acid cycle is a series of redox and decarboxylation reactions that remove high-energy electrons and carbon dioxide. The electrons temporarily stored in molecules of NADH and FADH2 are used to generate ATP in a subsequent pathway. One molecule of either GTP or ATP is produced by substrate-level phosphorylation on each turn of the cycle. There is no comparison of the cyclic pathway with a linear one. 8 Review Questions Exercise 1 What is removed from pyruvate during its conversion into an acetyl group? a. oxygen b. ATP c. B vitamin d. carbon dioxide (Solution on p. 9.) 2 http://chemwiki.ucdavis.edu/Biological_Chemistry/Metabolism/Kreb%27s_Cycle 3 https://www.khanacademy.org/science/biology/cellular-molecular-biology/cellular-respiration/v/krebs-citric-acid-cycle#! http://cnx.org/content/m56028/1.2/ OpenStax-CNX module: m56028 8 Exercise 2 (Solution on p. 9.) Exercise 3 (Solution on p. 9.) Exercise 4 (Solution on p. 9.) What do the electrons added to NAD+ do? a. They become part of a fermentation pathway. b. They go to another pathway for ATP production. c. They energize the entry of the acetyl group into the citric acid cycle. d. They are converted to NADP. GTP or ATP is produced during the conversion of ________. a. isocitrate into α-ketoglutarate b. succinyl CoA into succinate c. fumarate into malate d. malate into oxaloacetate How many NADH molecules are produced on each turn of the citric acid cycle? a. one b. two c. three d. four 9 Free Response Exercise 5 (Solution on p. 9.) What is the primary dierence between a circular pathway and a linear pathway? http://cnx.org/content/m56028/1.2/ OpenStax-CNX module: m56028 9 Solutions to Exercises in this Module to Exercise (p. 7) D to Exercise (p. 8) B to Exercise (p. 8) B to Exercise (p. 8) C to Exercise (p. 8) In a circular pathway, the nal product of the reaction is also the initial reactant. The pathway is selfperpetuating, as long as any of the intermediates of the pathway are supplied. Circular pathways are able to accommodate multiple entry and exit points, thus being particularly well suited for amphibolic pathways. In a linear pathway, one trip through the pathway completes the pathway, and a second trip would be an independent event. Glossary Denition 1: acetyl CoA combination of an acetyl group derived from pyruvic acid and coenzyme A, which is made from pantothenic acid (a B-group vitamin) Denition 2: citric acid cycle (also, Krebs cycle) series of enzyme-catalyzed chemical reactions of central importance in all living cells Denition 3: Krebs cycle (also, citric acid cycle) alternate name for the citric acid cycle, named after Hans Krebs who rst identied the steps in the pathway in the 1930s in pigeon ight muscles; see citric acid cycle Denition 4: TCA cycle (also, citric acid cycle) alternate name for the citric acid cycle, named after the group name for citric acid, tricarboxylic acid (TCA); see citric acid cycle http://cnx.org/content/m56028/1.2/