CHAPTER 23: Unit 1. The Citric Acid Cycle

Introduction

The citric acid cycle is a series of reactions that connect the intermediate acetyl CoA from the catabolic pathways in stage 2 with electron transport and the synthesis of ATP in stage 3. As a central pathway in metabolism, the Citric Acid Cycle uses the two-carbon acetyl group of acetyl CoA to produce CO₂ and reduced coenzymes NADH and FADH₂.

Catabolic Stages

Reference: https://byjus.com/chemistry/catabolism/

i) Catabolism in Different Organisms

The exact nature of catabolic reactions differs from organism to organism and can be classified based on their sources of energy and carbon which are as given below-Organotrophsuse organic sources as a source of energy.Lithotrophs use inorganic substrates.Phototrophs uses sunlight as chemical energy.

ii.) Stages of Catabolism

Catabolism can be broken down into 3 main stages. The three stages are as explained as follows-Stage 1 – Stage of DigestionThe large organic molecules of organic chemistry like proteins, lipids, and polysaccharides are digested into their smaller components outside cells. This stage acts on starch, cellulose or proteins that cannot be directly absorbed by the cells.Stage 2 – Release of energyOnce the molecules are broken down, these molecules are taken up by cells and converted to yet smaller molecules, usually acetyl coenzyme A, which releases some energy.Stage 3 – Energy StoredThe released energy is stored by reducing the coenzyme nicotinamide adenine dinucleotide into NADH.This process provides the chemical energy necessary for the maintenance and growth of cells. Some of the examples of the catabolic processes include the glycolysis, the citric acid cycle, the breakdown of muscle protein in order to use the amino acids as substrates for gluconeogenesis, the breakdown of fat in adipose tissue to fatty acids, and oxidative deamination of neurotransmitters by monoamine oxidase.

Key PointsThe four-carbon molecule, oxaloacetate, that began the cycle is regenerated after the eight steps of the citric acid cycle.The eight steps of the citric acid cycle are a series of redox, dehydration, hydration, and decarboxylation reactions.Each turn of the cycle forms one GTP or ATP as well as three NADH molecules and one FADH2 molecule, which will be used in further steps of cellular respiration to produce ATP for the cell.Key Termscitric acid cycle: 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 dioxideKrebs cycle: a series of enzymatic reactions that occurs in all aerobic organisms; it involves the oxidative metabolism of acetyl units and serves as the main source of cellular energymitochondria: in cell biology, a mitochondrion (plural mitochondria) is a membrane-enclosed organelle, often described as “cellular power plants” because they generate most of the ATP.

iii.) Citric Acid Cycle (Krebs Cycle)

Like the conversion of pyruvate to acetyl CoA, the citric acid cycle takes place in the matrix of the 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. 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. 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.

Figure Above: The citric acid cycle: 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. In the process, three NAD+ molecules are reduced to NADH, one FAD molecule is reduced to FADH2, and one ATP or GTP (depending on the cell type) is produced (by substrate-level phosphorylation). Because the final product of the citric acid cycle is also the first reactant, the cycle runs continuously in the presence of sufficient reactants.

Reference: https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/7%3A_Cellular_Respiration/7.3%3A_Oxidation_of_Pyruvate_and_the_Citric_Acid_Cycle/7.3C%3A_Citric_Acid_Cycle

iv) Steps in the Citric Acid Cycle

Step 1. The first step is a condensation step, combining the two-carbon acetyl group (from acetyl CoA) 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. If ATP is in short supply, the rate increases.Step 2. Citrate loses one water molecule and gains another as citrate is converted into its isomer, isocitrate.Steps 3 and 4. In step three, isocitrate is oxidized, producing a five-carbon molecule, α-ketoglutarate, together with a molecule of CO₂and two electrons, which reduce NAD+ to NADH. This step is also regulated by negative feedback from ATP and NADH and by a positive effect of ADP. 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. 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 insufficient 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.

v)Products of the Citric Acid Cycle

The cells of most organisms—including plants and animals—obtain usable energy through aerobic respiration, the oxygen-requiring version of cellular respiration. Aerobic respiration consists of four major stages: glycolysis, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation. The third major stage, the citric acid cycle, is also known as the Krebs cycle or tricarboxylic acid (TCA) cycle.For every glucose molecule that undergoes cellular respiration, the citric acid cycle is carried out twice; this is because glycolysis (the first stage of aerobic respiration) produces two pyruvate molecules per glucose molecule. During pyruvate oxidation (the second stage of aerobic respiration), each pyruvate molecule is converted into one molecule of acetyl-CoA—the input into the citric acid cycle. Therefore, for every glucose molecule, two acetyl-CoA molecules are produced. Each of the two acetyl-CoA molecules goes once through the citric acid cycle.The citric acid cycle begins with the fusion of acetyl-CoA and oxaloacetate to form citric acid. For each acetyl-CoA molecule, the products of the citric acid cycle are two carbon dioxide molecules, three NADH molecules, one FADH₂molecule, and one GTP/ATP molecule. Therefore, for every glucose molecule (which generates two acetyl-CoA molecules), the citric acid cycle yields four carbon dioxide molecules, six NADH molecules, two FADH2 molecules, and two GTP/ATP molecules. The citric acid cycle also regenerates oxaloacetate, the molecule that starts the cycle.While the ATP yield of the citric acid cycle is modest, the generation of coenzymes NADH and FADH₂ is critical for ATP production in the final stage of cellular respiration, oxidative phosphorylation. These coenzymes act as electron carriers and donate their electrons to the electron transport chain, ultimately driving the production of most of the ATP produced by cellular respiration.

vi) Breakdown of Pyruvate

After glycolysis, pyruvate is converted into acetyl CoA in order to enter the citric acid cycle.

vii)Formation of Citrate

Formation of Citrate: The first reaction of the cycle is the condensation of acetyl-CoA with oxaloacetate to form citrate, catalyzed by citrate synthase: In this reaction the methyl carbon of the acetyl group is joined to the carbonyl group (C-2) of oxaloacetate. Citroyl-CoA is a transient intermediate.

viii) Isomerization

The citrate formed in the reaction 1 is tertiary alcohol, which cannot be oxidized. In reaction 2, citrate is rearranged to its isomer isocitrate, a secondary alcohol that can be oxidized. Initially, aconitase catalyzes the dehydration of citrate (tertiary alcohol) to yield cis-aconitate, which is followed by a hydration, that forms isocitrate (secondary alcohol).

In step 3 reaction oxidation and decarboxylation takes place for the first time in citric acid cycle. The Isocitrate is converted into alpha-ketogluterate by isocitrate dehydrogenase. The byproducts of which are NADH and CO₂. In reaction 4, Decarboxylation and Oxidation take place where Apha-ketogluterate is then converted into succynl-CoA by alpha-ketogluterate dehydrogenase. NADH and CO₂are once again produced. In the reaction 5, hydrolysis takes place where Succynl-CoA is then converted into succinate by succynl-CoA synthetase which yields one ATP per succynl-CoA. In the reaction 6, the oxidation of Succinate converts into fumerate by way of the enzyme succinate dehydrogenase and [FAD] is reduced to [FADH₂] which is a prosthetic group of succinate dehydrogenase. Succinate dehydrogenase is a direct part of the ETC. It is also known as electron carrier II. In the reaction 7, a hydration catalyzed by Fumerase adds water to the double bond of fumerate to yield malate which is a secondary alcohol.In reaction 8, the oxidation where the last step of the citric acid cycle, malate is converted into oxaloacetate by malate dehydrogenase the byproducts of which are NADH.

Figure above: In the citric acid cycle, oxidation reactions produce two CO₂and reduced coenzyme NADH and FADH₂, GTP and regenerate oxaloacetate.

CHEM 1152

CHAPTER 23: Unit 1. The Citric Acid Cycle