The Citric Acid Cycle: Tricarboxylic Acid Cycle

1. The Citric Acid Cycle:Tricarboxylic Acid Cycle Roundabouts, or traffic circles, function as hubs to facilitate traffic flow. The citric acid cycle is the biochemical hub of the cell, oxidizing carbon fuels, usually in the form of acetyl CoA, as well as serving as a source of precursors for biosynthesis. [(Above) Chris Warren/International Stock.] Dr. M. ZeeshanHyder Chapter 17 Biochemistry, LubertStryer, 5th Edition

2. An Overview of the Citric Acid Cycle • The citric acid cycle is the central metabolic hub of the cell. • It is the gateway to the aerobic metabolism of any molecule that can be transformed into an • acetyl group or • dicarboxylic acid. • The cycle is also an important source of precursors for: • the storage forms of fuels • the building blocks of many other molecules • amino acids, • nucleotide bases • cholesterol, and porphyrin (the organic component of heme).

3. An Overview of the Citric Acid Cycle • The glycolysis produces pyruvate in the end • Pyruvate has many fates depending upon the cellular conditions i.e. • Ethanol Fermentation • Lactate production • gluconeogenesis • The pyruvate is shuttled into mitochondria by an antiporter which transport pyruvate in exchange of an OH- group • Inside the mitochondria pyruvate is oxidativelydecaroboxylated into Acetyl CoA by pyruvate dehydrogenase complex • Acetyl Co A enters into the TCA cycle and produces NADH, FADH and GTP

4. An Overview of the Citric Acid Cycle • The NADH and FADH2 produced during the TCA cycle are then utilized through electron transport chain to harvest energy. • The energy generated by TCA cycle in association with ETC provides most of the energy. • The TCA cycle is strictly aerobic process as NAD+ and FAD+ need to be regenerated through ETC to carry on this process The function of the citric acid cycle is the harvesting of high energy electron from carbon fuels

5. The Citric Acid Cycle Oxidizes Two-Carbon Units • The entry point into TCA is Acetyl CoA which is the fuel of this cycle. • AcetyleCoA comes from: • Glycolysis through pyruvate • Breakdown of glycogen • Fats • Many amio acids • Oxidative Decarboxylation of pyruvate for form Acetyl CoA by pyruvate dehydrogenase complex is the link between Glycolysis and TCA • Pyruvate Dehydrogenase Complex is a multienzyme complex each composed of several polypeptide chains, and five coenzymes: thiamine pyrophosphate (TPP), lipoic acid, and FAD serve as catalytic cofactors • pyruvate dehydrogenase component (E1), (decarboxylation activity) • Dihydrolipoyltransacetylase (E2) (transfer the acetyl group from acetyllipoamide to CoA to form acetyl CoA). • dihydrolipoyldehydrogenase (E3) (Regenerate the the oxidized form of lipoamide). • The activity of Pyruvate Dehydrogenase Complex is regulated to control the TCA cycle.

7. Stoichiometry of the Citric Acid Cycle • 1. Two carbon atoms enter the cycle in the condensation of an acetyl unit (from acetyl CoA) with oxaloacetate. Two carbon atoms leave the cycle in the form of CO2 in the successive decarboxylations catalyzed by isocitrate dehydrogenase and a-ketoglutarate dehydrogenase. • 2. Four pairs of hydrogen atoms leave the cycle in four oxidation reactions. Two molecules of NAD+ are reduced in the oxidative decarboxylations of isocitrate and a -ketoglutarate, one molecule of FAD is reduced in the oxidation of succinate, and one molecule of NAD+ is reduced in the oxidation of malate. • 3. One compound with high phosphoryl transfer potential, usually GTP, is generated from the cleavage of the thioester linkage in succinyl CoA. • 4. Two molecules of water are consumed: one in the synthesis of citrate by the hydrolysis of citryl CoA and the other in the hydration of fumarate. • Recall also that NADH is generated in the formation of acetyl CoA from pyruvate by the pyruvate dehydrogenase reaction.

8. Entry to the Citric Acid Cycle and Metabolism Through It Are Controlled • The TCA cycle is the final common pathway for the aerobic oxidation of fuels molecules • TCA is also a good source of building blocks and intermediate metabolites • Therefore TCA is rigorously regulated for its: • Entry point • For its pace

9. The pyruvate Dehydrogenase Complex is Regulated Allosterically and by Reversible Phosphorylation • The formation of Acetyl Co A from pyruvate is an irreversible step in animals and commits it to two principal fates: • Oxidation to CO2 by TCA cycle • Incorporation into lipids • The activity of this enzyme complex is stringently controlled • It is allosterically inhibited by its own products • NADH (inhibit dihydrolipoyl dehydrogenease E3) • Acetyl Co A (Inhibit transacetylase component E2) • The enzyme complex is regulated by reversible phosphorylation by specific kinase and phosphatase which regulate pyruvate dehydrogenase component E1 • Kinase (adds phosphate group and inhibit the activity) • Phosphatase (removes phosphate group and activate the enzyme)

10. Activity is inhibited by increase in: • NADH, • Acetyl Co A • ATP • While activity is stimulated by: • Pyruvate • ADP The pyruvate dehydrogenase is switched off when the energy charge is high and biosynthetic intermediates are abundant.

11. The Citric Acid Cycle is Controlled at Several Points • The rate of citric acid cycle is precisely adjusted to meet an anime cells need for ATP • The primary allostertic control points are : • Isocitrate Dehydrogenase • Activated by ADP • ATP and NADH inhibits it • α-ketoglutarate Dehydrogenase • Inhibited by succinyl Co A, NADH and ATP

12. The Citric Acid Cycle Is a Source of Biosynthetic Precursors • Although TCA is primarily provides ATP for energy but it also provides intermediates for biosynthesis • It provides intermediate for synthesis of: • Amino acids • Fatty acids ands sterols • Purines and pyrimidines • Porphyrins, heme and chlorphyll

13. The Citric Acid Cycle Must Be Capable of Being Rapidly Replenished • Being producer of intermediatory metabolitesf TCA cycle should be replenished if • The intermediates are used for biosynthesis • As TCA is a cyclic conversion it can be replenished by the production of any intermediate theoretically. • In TCA oxaloacetate functions cayalytically • To replenish the TCA oxaloacetate is regenrated from pyruvate by the action of pyruvate carboxylase; an enzyme involved in gluconeogenesis • The activity of pyruvate carboxylase is stimulated when actyle CoA is present meaning need for more oxaloacetate • If the energy charge is high, oxaloaccetate is converted into glucose • When energy charge is low, oxaloacetate replensihs the citric acid cycle

14. The Glyoxylate Cycle Enables Plants and Bacteria to Grow on Acetate • Many bacteria and plants are able subsist on acetate or other compounds to produce acetyl CoA • They utilize another pathway which is absent in most of the organisms that coverts two-carbon unit acetyl units into four carbon units (succinate) to produce: • Energy • And biosynteses • The glycolate cycle utilizes: • Two acetyl CoA molecules per cycle • Bypasses the two decarboxylation steps • In plants, these reactions take place in organelles called glyoxysomes • Bacteria and plants can synthesize acetyl CoA from acetate as well utilzing ATP