KREBS CYCLE (Tricarboxylic Acid Cycle) (Citric Acid Cycle)

www.web.virginia.edu/heidi/chapter20/chp20.htm Comprises of a series of reactions that oxidises pyruvate under aerobic conditions Pyruvate is oxidised to acetyl-CoA via an oxidative decarboxylation reaction which also produces NADH = LINK BETWEEN GLYCOLYSIS AND TCA CYCLE* *

OCCURS IN THE MITOCHONDRIAL MATRIX Acetyl CoA is impermeable to the mitochondrial matrix and is formed from: Pyruvate produced from glycolysis crosses the mitochondrial membrane into the matrix where it is metabolised to acetyl CoA. RXN is catalysed by pyruvate dehydrogenase -oxidation of fatty acid Some amino acids such as leucine OCCURS IN THE MITOCHONDRIAL MATRIX

Thus TCA cycle: Amphibolic malate carbohydrate Gluconeogenesis Asp Fatty acid General pathway for the oxidation of proteins, fatty acids and carbohydrates A source of reducing agents for the production of ATP Amino acid 3. Also responsible for the supply of metabolites for anabolic processes: Amino acid Thus TCA cycle: Amphibolic Amino acid Amino acid Fatty acid

Electron Transport Chain Electron Transport Chain www.sp.uconn.edu/.../fa02/terry/metabolism.html

Step 1. Formation of citrate  Acetyl-CoA + oxaloacetate + H2O —> CoA-SH + citrate enzyme = citrate synthase (allosteric enzyme) G = -7.7kcal/mol (- 32.17 kJ/mol) The activity of citrate synthase is controlled byTCA metabolites Acetyl CoA Oxaloacetate Succinyl CoA – inhibits ATP – Inhibits NADH – Inhibits Long chain fatty acyl CoA – Inhibits Citrate - Inhibits Binding of oxaloacetate induces conformational change that facilitates the binding of acetyl CoA Also catalyses the formation of monofluorocitrate from monofluoroacetyl CoA. Monofluorocitrate inhibits aconitase which catalyses the next rxn in the TCA cycle (fluoracetate fluoroacetyl CoA fluorocitrate courses.cm.utexas.edu/.../Lecture-Ch16.html

Step 2: Formation of isocitrate   Citrate cis-aconitate + H2O isocitrate G = + 2.0 kcal/mol (+8.36 kJ/mol) enzyme = aconitase Under physiological conditions, and at equilibrium 93% is in the form of citrate while the remaining 7% is isocitrate. But citrate does not accumulate in the cell because the oxidation of isocitrate to -ketoglutarate occurs at a fast rate.

Step 3. Oxidation of isocitrate to a-ketoglutarate + CO2 -ketoglutarate + CO2 + NAD(P)H + H+       isocitrate + NAD(P)+ G = -5.0kcal/mol (- 20.9 kJ/mol) enzyme = isocitrate dehydrogenase (allosteric enzyme) Loss of 1 hydrogen and 1 molecule fo CO2 1st redox reaction in the TCA cycle 1st NADH formed in TCA cycle Isocitrate dehydrogenase that uses NADP+ as the coenzyme is found in the cytoplasm. -ketoglutarate in cytoplasm is for amino acid synthesis and NADPH for anabolic processes Acitvity of isocitrate dehydrogenase is regulated by ATP/ADP and NADH/NAD+ levels ATP and NADH are negative modulators bio.winona.edu/berg/308s01/Lec-note/14-new.htm

Step 4. Oxidation of a-ketoglutarate to succinyl-CoA + CO2        -ketoglutarate + CoA-SH + NAD+ —> succinyl-CoA + CO2 + NADH + H+ G = -8.0kcal/mol (- 33.44 kJ/mol) enzyme = -ketoglutarate dehydrogenase complex Loss of 1 hydrogen and 1 molecule fo CO2 2nd redox rxn 2nd oxidative decarboxylation rxn in the TCA cycle 2nd NADH formed in TCA cycle A complex enzyme comprising of i. -ketoglutarate dehydrogenase ii. Transsuccinylase iii. Dihidrolypoil dehydrogenase Rxn similar to pyruvate to acetyl CoA rxn. 5 coenzyme involved Succinyl CoA is a high energy compound bio.winona.edu/berg/308s01/Lec-note/14-new.htm

Step 5. Deacylation of succinyl-CoA, making succinate     succinyl-CoA + GDP + Pi succinate + GTP + CoA-SH G = -0.7kcal/mol (- 2.93 kJ/mol) enzyme = succinyl-CoA synthetase GTP is formed. Substrate level phosphorylation GTO can be used to form ATP (nucleoside diphosphate kinase): ATP and GTP are energetically equivalent

Step 5. Deacylation of succinyl-CoA, making succinate     succinyl-CoA + GDP + Pi succinate + GTP + CoA-SH G = - 0.7kcal/mol (- 2.93 kJ/mol) enzyme = succinyl-CoA synthetase GTP is formed. Substrate level phosphorylation GTO can be used to form ATP (nucleoside diphosphate kinase): ATP and GTP are energetically equivalent

Fumarate Step 6. Oxidation of succinate to fumarate. succinate + FAD          succinate + FAD G = 0 kcal/mol (0 kJ/mol) fumarate + FADH2 enzyme = succinate dehydrogenase (allosteric enzyme) Succinate dehidrogenase is tightly bound to the michondrial membrane (Other TCA cycle enzymes are in the matrix) 3rd redox rxn in TCA cycle Hydrogen donated from succinate is accepted by FAD to form FADH2 FAD, ferum and sulphur are the cofactors of succinate dehydrogenase Succinate, phosphate, ATP and coenzyme QH2 activates the enzyme Fumarate Question: Why is FAD and not NAD+ the acceptor of hydrogen donated by succinate

Step 7. Hydration of fumarate to malate fumarate + H2O L-malate G = -0.9 kcal/mol (-3.77 kJ/mol) enzyme = fumarase Fumarase bertindak secara stereospesifik & memangkin pembentukan L-malat          Carbanion

Step 8. Oxidation of malate to oxaloacetate G = 7.09 kcal/mol (29.7 kJ/mol) L-malate + NAD+ oxaloacetate + NADH + H+ enzyme = malate dehydrogenase 4th redox reaction in the TCA cycle Production of the 3rd NADH Although the rxn has a positive G, the rate at shich oxaloacetate and NADH is used up, pulls (next rxn has a the reaction forward G of -7.7 kcal/mol

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courses.cm.utexas.edu/.../Lecture-Ch16.html

Overall reaction : Acetyl CoA + 3 NAD+ + FAD + GDP + Pi + 2H2O CoASH + 3 NADH + FADH2 + GTP + 2CO2 + 3H+ G’ = -8.7kcal/mol (2.08 kJ/mol

Production of ATP for every molecule of pyruvate oxidised Reaction Coenzyme ATP a. Pyruvate to acetyl CoA 1 NADH 3 b. Isocitrate to -ketoglutarate c. -ketoglutarate to succinyl CoA d. Succinate to fumarate 1 FADH2 2 e. L-malate to oxaloacetate f. Succinly –CoA to succinate GTP 1 Total 15