1. Krebs Cycle Tricarboxylic Acid Cycle
Citric Acid Cycle
Krebs Cycle
Tricarboxylic Acid Cycle

2. Citric Acid Cycle
Reduces Acetyl CoA to CO2, with production of NADH,FADH2 and GTP
Acetyl CoA comes from:
Lipids
Pyruvate from Glycolysis
Amino Acid breakdown
Acetoacetate or Acetyl-CoA
pyruvate
Produces oxaloacetate from various compounds for glucose synthesis

3. Oxidative Decarboxylation of Pyruvate
The first step of Citric Acid Cycle oxidation of carbohydrates is decarboxylation of pyruvate.
The Acetyl group is attached to CoA.
The process oxidation results in reduction of NAD+ to NADH
The process happens on a large complex in 3 stages.

4. Pyruvate Dehydrogenase Complex
Enzyme
Abbreviation
Number of chains
Prosthetic group
Reaction catalyzed
Pyruvate dehydrogenase component E1 24
TPP
Oxidative decarboxylation of pyruvate
Dihydrolipoyl transacetylase E2
Lipoamide
Transfer of the acetyl group to CoA
Dihydrolipoyl dehydrogenase E3 12
FAD
Regeneration of the oxidized form of lipoamide

5. Pyruvate Dehydrogenase Complex

6. Mechanism of Pyruvate Decarboxylation
TPP is first deprotonated.
It can then attack pyruvate to displace CO2.

7. Transfer of Acetyl to CoA
Transfer to Lipoamide
Transfer to CoA
Oxidation of Lipoamide

8. Flipping of Lipamide between subunits

9. Acetyl CoA breakdown Acetyl CoA is broken down to produce 1 GTP or ATP
3 NADH
1 FADH2

10. Citrate Synthase
Citrate Synthase adds acetyl group to oxaloacetate.

11. Citrate Synthase conformation change ensure correct reaction

12. Citryl CoA Synthesis Mechanism
Citrate synthase goes through a second conformation change after this reaction to allow citryl-CoA hydrolysis.

13. Aconitase converts Citrate to Isocitrate
The reaction is stereospecific to one side of citrate.
The enzyme uses an iron-sulfur complex for binding citrate.

14. Isocitrate is then oxidized & decarboxylated
Oxalosuccinate is an unstable intermediate, which is decarboxylated on the enzyme.
This is an important step in regulating the cycle.

15. a-Ketoglutarate dehydrogenase catalyzes oxidative decarboxylation
The reaction is by the same mechanism as pyruvate dehydrogenase.
The same kind of large complex with E1, E2 and E3 is used.
E3 (dihyrolipoyl dehydrogenase is identical in the two complexes.

16. Succinyl CoA synthase runs in reverse to generate GTP
Some organisms use ATP instead of GTP.
These NTPs are converted by dinucleotide kinase

17. Oxidation of Succinate to Oxaloacetate
Oxaloacetate is regenerated by oxidation of succinate to the keto acid in 3 steps.
Succinate Dehydrogenase oxidizes succinate to fumarate with reduction of an enzyme-bound FAD to FADH2.
Succinate dehydrogenase in membrane bound & directly transfers e- to e- transport chain.
Fumarate is hydrated to malate.
Hydration is stereospecific.
Malate is oxidized to oxaloacetate with production of NADH.

18. Citric Acid Cycle Summary

19. Regulation of Citric Acid Cycle
Decarboxylation of pyruvate is committed step, since in animals acetyl-CoA cannot be converted back to pyruvate & glucose.
This step is highly regulated.
Other regulated steps include the two decarboxylation steps.

20. Pyruvate Dehydrogenase Regulation

21. Citric Acid Cycle Generation of Biosynthetic Precursors
Anaplerotic reactions can replenish the reaction intermediates.
i.e. generation of oxaloacetate by pyruvate carboxylation.
No net conversion of Acetyl CoA to intermediates or pyruvate.

22. The Glyoxylate Cycle
Plants and many bacteria can convert acetyl CoA to TCA cycle intermediates by the Glyoxylate cycle.
Allows these organisms to grow on acetate.

23. Conclusions
The Citric Acid Cycle allows organisms to extract electrons from pyruvate and other Acetyl-CoA precursors for transport to the mitochondria electron transport chain.
One NADH is made converting pyruvate to Acetyl-CoA.
Three NADH, one FADH2 & 1 GTP/ATP is made in the citric acid cycle.
The citric acid cycle can be used to make precursors for other molecules.
Citric acid cycle intermediates can be made from pyruvate and amino acids to increase flow through or replace those used in biosynthesis.
Also from acetyl CoA in plants & some bacteria, but not animals.
The citric acid cycle is regulated by NADH, and ultimately by oxygen availability.