1. BCM208 Metabolic Biochemistry
Topic 3:
The Citric Acid Cycle

2. Learning objectives
Define cellular respiration and identify three stages of respiration
Describe the role of coenzyme A as an activator of acetate and other carboxylic acids
Describe in general terms the pyruvate dehydrogenase complex and know the names of the five co-factors and three enzymes involved
List the steps of the citric acid cycle including names of intermediates, cofactors and enzymes (not structures)
Describe the major controls operating on the pyruvate dehydrogenase reaction and the citric acid cycle
Define the term anaplerosis and identify:
Anabolic pathways drawing carbon from citric acid cycle carbon atoms
Anaplerotic pathways which can replenish citric acid cycle intermediates
Define the term malolactic fermentation and identify the enzymatic steps in malolactic fermentation

3. Cellular respiration
Fig. 16-1

4. Steps in respiration
Conversion of pyruvate (formed in glycolysis pathway) to acetyl-coenzyme A (Pyruvate dehydrogenase complex)
Combination of acetyl-CoA with oxaloacetate to form citrate then conversion back to oxaloacetate to produce a pool of electrons
Release of energy by the electron transport chain

5. Production of acetyl CoA
3 steps:
Decarboxylation: form a 2 carbon molecule + CO2
Oxidation: aldehyde/ketone group converted to carboxylic acid
Acylation reaction: carboxylic group attached to thiol group of HSCoA to form acetyl Coenzyme A

6. Production of Acetyl-CoA

7. Pyruvate dehydrogenase complex enzymes
E1: pyruvate decarboxylase (with thiamine pyrophosphate attached)
E2: dihydrolipoyl transacetylase (with two lipoic acid residues attached)
E3: dihydrolipoyl dehydrogenase (with FAD attached)

8. Pyruvate dehydrogenase complex cofactors
Thiamine pyrophosphate (TPP): Vit B1, carboxyl carrying coenzyme
Flavin adenine dinucleotide (FAD)
Coenzyme A: contains reactive thiol group
NAD+
Lipoate: hydrogen carrier

9. Coenzyme A (CoA)

10. Citric acid cycle

11. 1. Formation of citrate

12. 2. Formation of isocitrate

13. 3. Oxidation of Isocitrate to a-Ketoglutarate and CO2

14. 4. Oxidation of a-Ketoglutarate to Succinyl-CoA and CO2

15. 5. Conversion of Succinyl-CoA to Succinate

16. 6. Oxidation of succinate to fumarate

17. 7. Hydration of fumarate to malate

18. 8. Oxidation of Malate to oxaloacetate

19. Conservation of oxidation energy

20. Regulation of citric acid cycle

21. Role of citric acid cycle in anabolism

22. Anaplerosis
Replenishment of intermediates from other pathways

23. Malolactic Fermentation
Malate is transported into the bacterium
Malate is decarboxlyated by a single enzyme: malolactic enzyme
Malic acid -----> Lactic acid + CO2
CO2 reduces intracellular pH:
CO2 + H20 ---> HCO3- + H+
Protons generated creates a proton gradient which is used to create ATP