1. Bacterial Physiology (Micr430)
Lecture 5
Central Pathways of
Carbohydrate Metabolism
(Text Chapter: 8)

2. Central Metabolic Pathways
The central metabolic pathways are the pathways that provide the precursor metabolites to all the other pathways
They are the pathways for the metabolism of carbohydrates and carboxylic acids.
The major carbohydrate pathways:
Embden-Meyerhof-Parnas (glycolysis or EMP)
Pentose phosphate pathway (PPP)
Entner-Doudoroff pathway (ED)

3. Embden-Meyerhof-Parnas Pathway
This is also called Glycolysis or Fructose Bisphophate Aldolase Pathway
Important reactions:
Phosphorylation of glucose and fructose-6-phosphate by ATP
Cleavage of fructose-1,6-bisphosphate to trioses by a specific aldolase
Structural rearrangements
Oxidation-reduction and Pi assimilation

4. EMP Pathway or Glycolysis
Fig 8.2

5. EMP pathway
The enzyme fructose bisphosphate aldolase is one of the most critical steps in the pathway
In general, glycolysis in muscle tissue, yeast and many bacterial species are identical in terms of intermediates involved
Pyruvate is the last common intermediate

6. Pentose Phosphate Pathway
Significance of this pathway:
It produces pentose phosphates – precursors to the ribose and deoxyribose in nucleic acids
It produces erythrose phosphate – precursor to the aromatic amino acids
NADPH is produced – major source of electrons for biosynthesis

7. Pentose Phosphate Pathway
In this cycle, G-6-P is converted to ribulose-5-phosphate and CO2.
Ribulose-5-phosphate is maintained in equilibrium with ribose-5-phosphate and xylulose-5-phosphate by isomerase and epimerase
Through a series of transketolase and transaldolase reactions, three turns of cycle are required to produce one triose phosphate (plus 3 CO2) from one hexose

8. Pentose Phosphate Pathway
Fig 8.11

9. Entner-Doudoroff Pathway
This pathway is absence in eukaryotes
This pathway is active in E. coli and many other Gram negative bacteria
Two key enzymes: 6-phosphogluconate dehydratase (edd; reaction 2) and 2-keto-3-deoxy-6-phosphogluconate aldolase (eda; reaction 4)

10. Entner-Doudoroff Pathway
Fig 8.12

11. Oxidation of Pyruvate to Acetyl-CoA
Pyruvate is the common product of sugar catabolism in all major carbohydrate catabolic pathways
One fate of pyruvate is to be oxidized to acetyl-CoA.
During aerobic growth, this reaction is catalyzed by the enzyme complex pyruvate dehydrogenase

12. Oxidation of Pyruvate to Acetyl-CoA
Pyruvate dehydrogenase from E. coli consists of
24 molecules of enzyme E1 (pyruvate dehydrogenase)
24 molecules of enzyme E2 (dihydrolipoate transacetylase)
12 molecules of enzyme E3 (dihydrolipoate dehydrogenase)

13. Pyruvate Dehydrogenase Complex
Fig 8.14

14. TCA Cycle
Tricarboxylic acid cycle also known as Krebs or citric acid cycle
This is a cycle to generate remaining energy and reducing equivalents from end product of glycolysis, pyruvate
Its intermediates serve as precursors for biosynthesis of many other important cellular compounds (amino acids)

15. TCA Cycle
Fig 8.16

16. TCA Cycle under anaerobic conditions
Under anaerobic conditions, TCA cycle cannot go full circle due to absence of terminal electron acceptor (O2)
It now functions as a branched biosynthetic pathway:
one branch operating as a reductive pathway reversing the sequence from succinate to oxaloacetate
The other branch continuing to operate oxidatively to convert oxaloacetate to a-ketoglutarate

17. TCA variant under anaerobic condition

18. Glyoxylate Cycle
Activation of acetate with CoA to form acetyl-CoA and the combined activities of the enzymes isocitritase (isocitrate lyase) and malate synthase provide for the operation of a C4 cycle called glyoxylate cycle
It results in the net formation of malate from 2 mol of acetate
Activity of this bypass cycle explains the ability of organisms to utilize acetate as a sole source of carbon for growth.

19. The Glyoxylate Cycle
Fig 8.24