1. Metabolism: TCA Cycle

2. Looking back at glycolysis
Glucose + 2Pi + 2 ADP + 2 NAD+ ->
2 pyruvate + 2 ATP + 2 NADH + 2H+ + 2H2O

3. The Tricarboxylic Acid Cycle
Rotondas/Traffic circles facilitate traffic flow for many converging paths
Central metabolic hub of cell
TCA/Krebs Cycle is the final common pathway for the oxidation of fuel molecules (proteins, fatty acids, carbs)
Important source of precursors

4. THE TCA CYCLE 8 steps, most common entry point is CoA (C2)
Key: Oxidation of one acetyl group to two CO2
Function: harvesting high-energy e- (to be used later in oxidative phosphorylation or the e- transport chain

5. TCA in the Mitochondrion

6. Entry into the TCA cycle
Use of Pyruvate dehydrogenase complex
Pyruvate + CoA + NAD+ -> Acetyl CoA + CO2 + NADH
NET REACTION: Pyruvate is oxidatively decarboxylated to form acetyl-CoA
Pyruvate dehydrogenase uses TPP, CoASH, lipoic acid, FAD and NAD+

7. Entry into the TCA cycle
Three basic steps

8. Entry into the TCA cycle
Step 1: Decarboxylation
(pyruvate dehydrogenase E1)

9. Entry into the TCA cycle
Step 2: Oxidation of hydroxymethyl group on TPP (pyruvate dehydrogenase E1)

10. Entry into the TCA cycle
Step 3: Acetyl transfer to CoA (dihydrolipoyl transacetylase E2)

11. (Step 4: Regenerate lipoamide from dihydrolipoamide) (dihydrolipoyl dehydrogenase)

12. Amazing pyruvate dehydrogenase complex
Flexible linkages allow lipoamide to move between different active sites
Eight catalytic trimers

13. Begin the TCA cycle!

14. “RXN 1”: Oxaloacetate to Citrate
No true “first step” since it is a cycle. But assume here acetyl CoA is the entry point
Aldol condensation followed by hydrolysis
Citrate synthase

15. Acetyl CoA must not be wasted/hydrolyzed!
Exploring the citrate synthase
Oxaloacetate binds first
Structural rearragement creating acetyl CoA binding site
Efficiency
Acetyl CoA binds only after oxaloacetate
Catalytic residues are not positioned until citryl CoA is formed

16. RXN 2: Citrate to Isocitrate
Isomerization for proper oxidative decarboxylation later
Aconitase used

17. RNX “3”: Isocitrate to Ketoglutarate
Oxidation AND Decarboxylation
Rate of ketoglutarate formation important in over-all rate of cycle
By isocitrate dehydrogenase

18. RXN “4”: Ketoglutarate to Succinyl CoA
Another oxidative decarboxylation
Resembles pyruvate decarboxylation!
Ketoglutarate dehydrogenase

19. RXN “5”: Succinyl CoA to Succinate
Succinyl-CoA is a high-energy compound. Energy is transformed to phosphoryl transfer potential
Succinyl CoA synthetase

20. FINAL 3 STEPS Key: reactions of 4C species
Regeneration of oxaloacetate from succinate

21. RXN “6”: Succinate to Fumarate
Oxidation
Succinate dehydrogenase

22. RXN “7”: Fumarate to malate
By Fumarase

23. RXN “8”: Malate to Oxaloacetate
Oxidation
By malate dehydrogenase

26. Net of TCA Cycle SUMMARY
Acetyl CoA + 3 NAD+ + FAD + GDP + Pi + 2H2O -> 2CO2 + 3 NADH + FADH2 + GTP + 2H+ + CoA
SUMMARY
C2 enters and joins oxaloacetate (C4). Two C atoms leave as CO2
Four pairs of H leave in four redox rxns
One compound with high phosphorylation transfer potential (GTP) is generated
Two molecules of water are consumed

27. TCA Cycle

28. TCA is regulated

29. TCA is a source of precursors