TRICARBOXYLIC ACID CYCLE (TCA CYCLE) Biochem II Unit 1.pptx

**Tricarboxylic Acid Cycle (TCA Cycle) – Biochemistry II Unit 1**

This unit describes the sequence of reactions in the TCA cycle, also known as the Krebs cycle or citric acid cycle. It explains how acetyl-CoA is oxidized to produce energy in the form of ATP, NADH, and FADH₂. The unit outlines each step of the cycle, the enzymes involved, and the regulation of the process. It also shows how the TCA cycle connects carbohydrate, fat, and protein metabolism and its central role in cellular energy production.

1. TRICARBOXYLIC ACID CYCLE (TCA CYCLE) UNIT ONE BIOCHEMISTRY II LECTURE 3

2. TRICARBOXYLIC ACID CYCLE (TCA CYCLE) This cycle is the aerobic phase of carbohydrate metabolism and follows the anaerobic pathway from the stage of pyruvate and is called as citric acid cycle or TCA cycle. The name citric acid cycle stems from citric acid which is formed in the first step of this cycle. This cycle is also named "Krebs cycle" after Hans Adolf Krebs, an English biochemist who worked on it.

3. The citric acid cycle is the most important metabolic pathway for energy supply to the body. About 65 - 70%of the ATP is synthesized in Krebs cycle. Krebs cycle is the most important central pathway connecting almost all the individual metabolic pathways ( either directly or indirectly). Krebs cycle is both catabolic and anabolic in nature, hence regarded as Amphibolic. The enzymes of TCA cycle are located in mitochondrial matrix.

4. Oxidative decarboxylation of pyruvate Pyruvate, the end product of aerobic glycolysis, must be transported into the mitochondrion before it can enter the TCA cycle. This is accomplished by a specific pyruvate transporter that helps pyruvate cross the inner mitochondrial membrane. Once in the matrix, pyruvate is converted to acetyl CoA by the pyruvate dehydrogenase complex, which is a multienzyme complex. The pyruvate dehydrogenase complex is not part of the TCA cycle proper, but is a major source of acetyl CoA-the two-carbon substrate for the cycle.

5. COMPONENT OF PDH COMPLEX ► The pyruvate dehydrogenase complex {PDH complex) consists of three enzymes, pyruvate decarboxylase {E1), dihydrolipoyl transacetylase {E2), and dihydrolipoyl dehydrogenase (E3). ► The PDH complex contains five coenzymes that act as carriers or oxidants for the intermediates of the reactions. E1 requires thiamine pyrophosphate (TPP). E2 requires lipoic acid and CoA, and E3 requires FAD and NAD+.

6. Regulation of PDH complex ► Covalent modification by the two regulatory enzymes that are part of the complex alternately activate and inactivate E1 (PDH). ► The PDH kinase phosphorylates and, thereby, inhibits E1, whereas PDH phosphatase dephosphorylates and activates E1. ► The kinase itself is allosterically activated by ATP, acetyl CoA, and NADH.

7. Therefore, in the presence of these high-energy signals, the PDH complex is turned off. ► Calcium is a strong activator of PDH phosphatase, stimulating E1 activity. ► This is particularly important in skeletal muscle.

8. Pyruvate dehydrogenase deficiency ► A deficiency in the E1 component of the PDH complex, although rare, is the most common biochemical cause of congenital lactic acidosis. ► This enzyme deficiency results in an inability to convert pyruvate to acetyl CoA, causing pyruvate to be shunted to lactic acid via lactate dehydrogenase. ► This causes particular problems for the brain, which relies on the TCA cycle for most of its energy, and is particularly sensitive to acidosis.

9. ► The El defect is X-linked, but because of the importance of the enzyme in the brain, it affects both males and females. Therefore, the defect is classified as X-linked dominant. ► There is no proven treatment for pyruvate dehydrogenase deficiency: however, dietary restriction of carbohydrate and supplementation with TPP may reduce symptoms in select patients.

10. REACTIONS OF THE CITRIC ACID CYCLE ► There are 8 steps in the cycle and the reactions are as follows: - Formation of citrate Formation of isocitrate via cis aconitate Oxidation of isocitrate to alpha AcetytCoA Citrate ketoglutarate and CO2 Oxidation of a-ketoglutarate to succinyl CoA and CO2 Conversion of succinyl CoA to succinate Oxidation of succinate to fumarate Hydration of fumarate to malate Oxidation of malate to oxaloacetate

11. 1. Formation of citrate The citric acid cycle begins with the irreversible condensation of acetyl CoA and oxaloacetate to form citrate, catalyzed by citrate synthase. Citrate synthase is inhibited by ATP, NADH, succinyl CoA, and citrate, and activated by ADP and calcium.

12. 2. Isomerization of citrate Aconitase catalyzes the reversible conversion of citrate to isocitrate through cis-aconitate. Fluoroacetate is a toxic chemical m ade in laboratories for use as rat poison or pesticide (commonly called compound 1080). Fluoroacetate (a poison) forms fluorocitrate , which blocks aconitase and causes citrate buildup and stops energy production.

13. 3. Oxidation and decarboxylation of isocitrate Isocitrate dehydrogenase catalyzes the irreversible conversion of isocitrate to α-ketoglutarate, producing NADH and releasing CO₂. It is a rate-limiting enzyme, activated by ADP and Ca²⁺, and inhibited by ATP and NADH. This means high ATP or NADH levels signal abundant energy, so isocitrate dehydrogenase slows down, reducing citric acid cycle activity.

14. 4. Oxidative decarboxylation of a-ketoglutarate ► The next step is another oxidative decarboxylation, in which a - ketoglutarate is converted to succinyl CoA and CO2 by the action of the a - ketoglutarate dehydrogenase complex. The reaction is irreversible. The reaction releases the second CO2 and produces the second NADH of the cycle. ► The coenzymes required are thiamine pyrophosphate, lipoic acid, FAD, NAD+, and CoA. Each functions as part of the catalytic mechanism in a way analogous to that described for the PDH Complex. ► a - Ketoglutarate dehydrogenase complex is inhibited by its products, NADH and succinyl CoA, and activated by ca2+.

15. 5. Cleavage of succinyl CoA ► Succinate thiokinase cleaves the high-energy thioester bond of succinyl CoA. This reaction is coupled to phosphorylation GDP to GTP. ► GTP and ATP are energetically interconvertible by the nucleoside diphosphate kinase reaction: ► NDP (Nucleoside Diphosphate) is a general term for molecules with two phosphate groups . Examples: GDP (Guanosine diphosphate)

16. 6. Oxidation of succinate to fumarate ► Succinate is oxidized to fumarate by succinate dehydrogenase, as FAD (its coenzyme) is reduced to FADH2. ► Succinate dehydrogenase is the only enzyme of the TCA cycle that is embedded in the inner mitochondrial membrane.

17. 7. Hydration of fumarate to malate ► Fumarate is hydrated to malate in a freely reversible reaction catalyzed by fumarase (fumarate hydratase). ► [Note: Fumarate is also produced by the urea cycle]

18. 8. Oxidation of malate to oxaloacetate ► Malate is oxidized to oxaloacetate by malate dehydrogenase. This reaction produces the third and final NADH of the cycle. ► [Note: Oxaloacetate is also produced by the transamination of the amino acid, aspartic acid]

19. ENERGY PRODUCED BY THE TCA CYCLE CO₂ release: Two carbons leave as CO₂; no ATP is made directly here. NADH production (3 per acetyl CoA): From isocitrate → α- ketoglutarate From α- ketoglutarate → succinyl CoA From malate → oxaloacetate Each NADH can produce ≈3 ATP via the electron transport chain. FADH₂ production (1 per acetyl CoA): From succinate → fumarate Each FADH₂ can produce ≈2 ATP via the electron transport chain. GTP/ATP production (substrate-level phosphorylation, 1 per acetyl CoA): From succinyl CoA → succinate by succinate thiokinase GTP is easily converted to ATP . Total ATP from one acetyl CoA: NADH: 3 × 3 = 9 ATP FADH₂: 1 × 2 = 2 ATP GTP/ATP: 1 × 1 = 1 ATP Total ≈ 12 ATP per acetyl CoA

20. To be Continued…. THANK YOU