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

3.
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* *

4. 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

5. 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

6. Electron
Transport
Chain
Electron
Transport
Chain

7. Step 1. Formation of citrate
 Acetyl-CoA + oxaloacetate + H2O —> CoA-SH + citrate
enzyme = citrate synthase (allosteric enzyme)
G = -7.7kcal/mol
( 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

8. Step 2: Formation of isocitrate
Citrate
cis-aconitate + H2O
isocitrate
G = 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.

9. Step 3. Oxidation of isocitrate to a-ketoglutarate + CO2
-ketoglutarate + CO2 + NAD(P)H + H+
      isocitrate + NAD(P)+
G = -5.0kcal/mol
( 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

10. Step 4. Oxidation of a-ketoglutarate to succinyl-CoA + CO2
       -ketoglutarate + CoA-SH + NAD+ —> succinyl-CoA + CO2 + NADH + H+
G = -8.0kcal/mol
( 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

11. Step 5. Deacylation of succinyl-CoA, making succinate
  succinyl-CoA + GDP + Pi
succinate + GTP + CoA-SH
G = -0.7kcal/mol
( 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

12. Step 5. Deacylation of succinyl-CoA, making succinate
  succinyl-CoA + GDP + Pi
succinate + GTP + CoA-SH
G = - 0.7kcal/mol
( 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

13. 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

14. 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

15. 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

16. employees.csbsju.edu/.../04syl112.html

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

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

19. 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