Microbial Metabolism

1. Catabolic and
Anabolic Reactions
2. Enzymes

3. Energy Production
Oxidation-reduction reactions
Generation of ATP

4. Carbohydrate Catabolism
Aerobic respiration
Anaerobic respiration
Fermentation

5. Lipid and Protein Catabolism

6. Photosynthesis

7. Metabolic Diversity
8. Metabolic Pathways
of Energy Use
Objectives:

15. List the three processes that are required for the complete catabolism of glucose to carbon dioxide.

16. Define and describe the following processes:

· Glycolysis
· Pentose phosphate pathway
· Entner-Doudoroff pathway

17. Define cell respiration and describe where it occurs in the eukaryotic cell and the prokaryotic cell.

18. List the types of final electron acceptors for:

· Aerobic respiration
· Anaerobic respiration

19. Define and describe the following processes:

· Oxidative decarboxylation of pyruvate (preparatory or transition step)
· Kreb’s cycle (citric acid cycle)
· Electron transport chain

20. List the components of the electron transport chain.

21. List the electron carrier coenzymes used in:

· Glycolysis
· Oxidative decarboxylation of pyruvate (preparatory or transition step)
· Kreb’s cycle (citric acid cycle)

22. Define and describe chemiosmosis.

23. Explain how chemiosmosis is used to produce ATP.

24. Give the amount of NADH + H+ produced in:

· Glycolysis
· Oxidative decarboxylation of pyruvate
· Kreb’s cycle (citric acid cycle)

25. Give the amount of FADH2, if any, (directly) produced in:

· Glycolysis
· Oxidative decarboxylation of pyruvate
· Kreb’s cycle (citric acid cycle)

26. Give the amount of ATP, if any, (directly) produced in:

· Glycolysis
· Oxidative decarboxylation of pyruvate
· Kreb’s cycle (citric acid cycle)

27. Give the amount of ATP, produced via the electron chain from coenzyme energy carriers which were reduced in:

· Glycolysis
· Oxidative decarboxylation of pyruvate
· Kreb’s cycle (citric acid cycle)

28. Give the amount of ATP, if any, used in:

· Glycolysis
· Oxidative decarboxylation of pyruvate
· Kreb’s cycle (citric acid cycle)

29. Differentiate between ATP and GTP.

30. Explain why 38 ATPs can be produced from the oxidation of one molecule of glucose in a prokaryotic cell whereas only 36 ATPs are produced from the same process in a eukaryotic cell.

31. Define fermentation.
32. Describe how a strictly fermentative bacterium produces energy.
33. Differentiate among the following:

· Lactic acid fermentation (homolactic)
· Alcohol fermentation
· Heterolactic fermentation

34. Describe the use of fermentation end products in laboratory identification of bacteria.

Carbohydrate Catabolism

Most of a cell’s energy is produced from the oxidation of carbohydrates.

 

Glucose is the most commonly used carbohydrate.

 

The two major types of glucose catabolism are respiration, in which glucose is completely broken down, and fermentation, in which it is partially broken down.

 

 

Glycolysis (Embden-Meyerhof pathway)

 

The most common pathway for the oxidation of glucose is glycolysis.  Pyruvic acid is the end-product.

 

Two ATP and two NADH molecules are produced from one glucose molecule.

 

 

 

Alternatives to Glycolysis

 

The pentose phosphate pathway (hexose monophosphate shunt) is used to metabolize five-carbon sugars; one ATP and 2 NADPH molecules are produced from oxidation of one glucose molecule.

 

Produces intermediates for nucleic acid synthesis, glucose synthesis from CO2 in photosynthesis, and some amino acids.

 

The Entner-Doudoroff pathway yields one ATP and two NADPH molecules from one glucose molecule.

 

Used by Gram negatives (e.g. Rhizobium, Psuedomonas, Agrobacterium), usually not by Gram positives.

 

 

Cellular Respiration

 

During respiration, organic molecules are oxidized.  Energy is generated from the electron transport chain and the final electron acceptor is an inorganic molecule.

 

In aerobic respiration, O2 functions as the final electron acceptor.

 

In anaerobic respiration, the final electron acceptor is an inorganic molecule other than O2.

 

Aerobic Respiration

The Krebs Cycle

 

Decarboxylation of pyruvic acid produces one CO2 molecule and one acetyl group linked to CoA (acetyl-CoA).

 

NAD is reduced to NADH (2 electrons removed from pyruvate).

 

Two acetyl-CoA groups are oxidized in the Krebs cycle for each glucose molecule (one six carbon glucose is oxidized to two 3 carbon pyruvic acid molecules, each of which is decarboxylated to produce an acetyl-CoA molecule). 

 

Electrons are picked up by NAD+ and FAD for the electron transport chain.

 

From one molecule of glucose, oxidation in the Krebs cycle produces six molecules of NADH, two molecules of FADH2, and two molecules of ATP.

 

Decarboxylation produces six molecules of CO2.

 

 

The Electron Transport Chain (System)

 

Electrons are brought to the electron transport chain by NADH.

 

The electron transport chain consists of carriers, including flavoproteins, cytochromes, and ubiquinones.

 

Electrons are passed from one carrier to the next, the energy is used to drive proton pumps.

 

The final electron acceptor is irreversibly reduced; it may be oxygen (aerobic) or another inorganic molecule (anaerobic).

 

 

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The Chemiosmotic Mechanism of ATP Generation

 

Protons being pumped across the membrane generate proton motive force as electrons move through a series of acceptors or carriers.

 

Energy produced from movement of the protons back across the membrane is used by ATP synthase to make ATP form ADP and phosphate.

 

In eukaryotes, electron carriers are located in the inner mitochondrial membrane; in prokaryotes, electron carriers are in the plasma membrane.

 

 

 

A Summary of Aerobic Respiration

 

In aerobic prokaryotes, 38 ATP molecules can be produced form complete oxidation of a glucose molecule in glycolysis, the Krebs cycle, and the electron transport chain.

 

In eukaryotes, 36 ATP molecules are produced from complete oxidation of a glucose molecule (2 ATP are required to shuttle the 2 electrons from the NADH produced in glycolysis across the mitochondrial membrane to the electron transport chain).

 

 

Anaerobic Respiration

 

The final electron acceptors in anaerobic respiration include NO3-, SO4 2-, and CO3 2-.

NO3- (nitrate) is reduced to NO2- (nitrite)

 

SO42- (sulfate) is reduced to H2S (hydrogen sulfide)

 

CO32- (carbonate) is reduced to CH4 (methane)

The total ATP yield is less than in aerobic respiration because only part of the Krebs cycle operates under anaerobic conditions.

 

Fermentation

 

Fermentation is any process that releases energy from sugars or other organic molecules by oxidation, does not require O2, the Krebs cycle, or an electron transport chain, and uses an organic molecule as the final electron acceptor.

 

Fermentation can sometimes occur in the presence of O2.

 

Fermentation produces two ATP molecules by substrate-level phosphorylation.

 

Electrons removed from the substrate reduce NAD+ to NADH.

 

 

    In lactic acid fermentation, pyruvic acid is reduced by NADH to lactic acid (lactic acid fermenters include Streptococcus and Lactobacillus).

     

    Lactic acid can be fermented to propionic acid and CO2 by Propionibacterium freudenreichii (Swiss cheese).

     

    In alcohol fermentation, acetaldehyde is reduced by NADH to produce ethanol (alcohol fermenters include yeasts and bacteria).

     

    Ethanol can be fermented to acetic acid (vinegar) by Acetobacter.

     

    Acetic acid can be fermented to methane by Methanosarcina.

     

    Heterolactic fermenters can use the pentose phosphate pathway to produce lactic acid and ethanol.

     

    Homolactic fermenters produce only lactic acid.

     

     

     

     

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