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Bacteria and archaea use three main routes to
catabolize glucose:
- 1. Glycolysis, or the Embden-Meyerhof-Parnas
- (EMP) pathway
- 2. Entner-Doudoroff (ED) pathway
- 3. Pentose phosphate pathway (PPP), also
- known as the pentose phosphate shunt
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The EMP Pathway (glycolysis)
- Glucose molecule (6C) undergoes a stepwise
- breakdown to two pyruvate molecules (2x3C).
- The most common form of glycolysis.
- - It is central for animals and plants, as well as
- many bacteria.
- It occurs in the cytoplasm of the cell.
- It functions in the presence or absence of O2.
- It involves ten steps that are divided into two stages:
- 1 - glucose activation/phosphorylation (requires
- energy - 2ATP), glucose is split in half
- 2 - energy-yielding: 4ATP+2NADH
- Total: 2ATP+2NADH
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The ED Pathway (Entner-Doudoroff)
- Probably evolved earlier than EMP pathway.
- Glucose is activated by one phosphorylation reaction,
- and then dehydrogenated to 6-phosphogluconate.
- - Then dehydrated and cleaved to pyruvate and
- glyceraldedyde-3-P, which enters the EMP
- pathway to form pyruvate
- The ED pathway produces 1 ATP, 1 NADH, and
- 1 NADPH.
- Many gut flora use the ED pathway as their primary
- glycolytic pathway.
- - Escherichia coli feeds on gluconate from mucus
- secretion.
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The Pentose Phosphate Pathway
- Like the ED pathway, the pentose phosphate shunt
- forms 6-phosphogluconate.
- - It is then converted to the key intermediate
- ribulose-5-phosphate, which in turn produces a
- series of sugars, each containing three to seven
- carbons.
- This pathway produces 1 ATP and 2 NADPH for
- biosynthesis.
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Fermentation
- is the completion of catabolism
- without the electron transport system and a terminal
- electron acceptor.
- - The hydrogens from NADH + H+ are transferred back
- onto the products of pyruvate, forming partly oxidized
- fermentation products.
- Most fermentations do not generate ATP beyond that
- produced by substrate-level phosphorylation.
- - Microbes compensate for the low efficiency of
- fermentation by consuming large quantities of
- substrate and excreting large quantities of products.
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Fermentation Pathways
- Homolactic fermentation
- - Produces two molecules of lactic acid
- Ethanolic fermentation
- - Produces two molecules of ethanol and two CO2
- Heterolactic fermentation
- - Produces one molecule of lactic acid, one ethanol,
- and one CO2
- Mixed-acid fermentation
- - Produces acetate, formate, lactate, and succinate, as
- well as ethanol, H2, and CO2
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Products of the TCA Cycle
- For each pyruvate oxidized:
- - 3CO2 are produced by decarboxylation
- - 4 NADH and 1 FADH2 are produced by redox reactions.
- - 1 ATP (or GTP) is produced by substrate-level
- phosphorylation.
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Aromatic Pollutants
- Catabolism of aromatic molecules by bacteria and
- fungi recycles lignin and other important substances
- within ecosystems. (also: toxic pollutants)
- Benzoate and related compounds undergo aerobic
- catabolism to catechol.
- - Catechols are degraded through several
- alternative pathways to the TCA cycle.
- In anaerobic conditions, benzoate undergoes
- reductive degradation instead of oxidation.
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