TCA Cycle

Citric Acid Cycle

1. Citric acid cycle Introduction Site Pathway Energetics Regulation Clinical importance

2. Glycolysis Link reaction Kreb’s cycle Oxidative Phosphorylation (ETC)

3. introduction Citric acid cycle Krebs cycle ( 1937; NP-1953) Tricarboxylic acid cycle Reduced co-enzymes are produced – act as substrate for for ETC Most important ATP producing pathway About 65-70% ATP synthesized Sir Hans Krebs [1900-1981] – on his bike!

4. Takes place in the matrix of the mitochondria . It happens once for every pyruvate molecule in glycolysis…. Purpose Conversion of Acetyl- CoA to CO 2 Generates reducing equivalents ( NADH + H + , FADH2 ) & GTP to be oxidized in the respiratory chain to generate ATP

5. Overview

6. Steps involved in tca cycle

7. Succinate thiokinase

8. Let’s have a short break

9. Tips to learn biochemistry Do not rely on passive reading and highlighting/underlining of the textbook. Do not sit and stare at the handouts Do not try to read 50 review books. (Make your own review book instead!) Do focus on identifying key concepts Do actively draw and redraw pathways and connections Do learn to identify relevant information

10. Do prioritize: What is the purpose of a pathway? What are the starting and ending molecules? Where is the pathway (in the cell, in a tissue, in an organ system)? How does the pathway connect to other pathways? What metabolic conditions turn the pathway on and off? What are the control points for regulating the pathway? reactants, products and enzyme name of each regulatory step additional regulatory molecules involved (vitamins, cofactors) make sure you know every step that makes or uses ATP What specific drugs or diseases associated with the pathway?

11. TCA cycle is an open cycle Operates only under aerobic conditions This is the Final common pathway of oxidative metabolism Two carbon dioxide molecules are released as a waste product of respiration GTP + H +

12. Energetics : 2 Acetyl CoA from 2 Pyruvate 1NADH+H + = 3/2.5 ATP 1FADH 2 = 2/1.5 ATP 1GTP = 1 ATP Acetyl- CoA + 3 NAD + + [FAD] + GDP + Pi + 2 H 2 O CoASH + 3 NADH+3 H + +[FADH 2 ] + GTP + 2 CO 2 ×2=24

13. ATP generation during oxidation of Glucose Net ATP production depends on shuttle used for the transfer of reducing equivalents from cytosol to mitochondria. 8 / 7 6 / 5 24 / 20 38 / 32

14. Significance of tca cycle: Complete oxidation of Acetyl CoA As provider of energy Final common oxidative pathway Integration of major metabolic pathways Fat is burned on the wick of carbohydrates Excess carbohydrates are converted to Neutral fat No net synthesis of carbohydrates from fat Amino acids enters TCA cycle Amphibolic pathway Anaplerotic role

15. 1.Complete oxidation of Acetyl CoA 2. ATP generation 12/10 ATP are generated via substrate level phosphorylation and ETC Oxaloacetate Citrate Isocitrate α - ketogluterate

16. 3. Final common oxidative pathway

17. 4. Integration of major metabolic pathways

18. 5.Fat is burned on the wick of carbohydrates For complete oxidation of acetyl CoA , it needs the help of oxaloacetate The major source of oxaloacetate is pyruvate, derived from carbohydrate 6. Excess carbohydrates are converted to Neutral fat Glucose Pyruvate Acetyl CoA Fatty Acid PDH is irreversible – fat can not be converted to glucose.

19. Participation of TCA cycle in fatty acid synthesis from glucose

20. 7.No net synthesis of carbohydrates from fat Acetyl CoA entering in the cycle is completely oxidized to CO 2 by the time the cycle reaches Succinyl CoA. So acetyl CoA can not be used for the synthesis of carbohydrates.

21. 8. Amino acids finally enters the TCA cycle

22. 9. Amphibolic Pathway Both anabolic and catabolic Metabolic traffic cycle

23. 10. Anaplerotic role of TCA cycle “Filling up” reactions or “influx” reactions or “replenishing” reactions Pyruvate Oxaloacetate Glutamate α -KG Aspartate Oxaloacetate Pyruvate Malate

24. Regulation of TCA cycle Indicator molecules of higher energy state i.e. ATP, NADH, citrate, Acetyl CoA – inhibit TCA cycle Indicator molecules of low energy state i.e. ADP, AMP, NAD + – stimulate TCA cycle * * * *

25. INHIBITORS OF TCA CYCLE Fluoroacetate Condensation FluoroacetylCoA with Oxaloacetate Fluorocitrate inhibit Aconitase enzyme accumulation of citrate Fluoroacetate pesticide Malonate Succinate dehydrogenase enzyme Arsenite α - ketoglutarate dehydrogenase enzyme

26. Metabolic defects Extremely rare 1. Defect in PDH - Lactic acidosis - Neurologycal dosorders 2. Defect In Pyruvate carboxylase - Oxaloacetate - Hyperammonemia - Lactic acidosis - Hyperalaninemia .

27. Summary Pyruvate is converted to acetyl-CoA by the action of p yruvate dehydrogenase complex, a huge enzyme complex. Acetyl- CoA is converted to 2 CO 2 via the eight-step citric acid cycle, generating three NADH, one FADH 2 , and one ATP (by substrate-level phophorylation ).

28. Intermediates of citric acid cycle are also used as biosynthetic precursors for many other biomolecules, including fatty acids, steroids, amino acids, heme, pyrimidines , and glucose. Oxaloacetate can get replenished from pyruvate, via a carboxylation reaction catalyzed by the biotin-containing pyruvate carboxylase .

29. questions? TCA cycle is the final common metabolic pathway for the oxidation of food stuffs. – Justify Conversion of Pyruvate to Acetyl CoA Energetics of citric acid cycle

30. THANK YOU