Micro Biology Vh 13

Front 1

ENERGY ACQUISITION

Back 1

• Living organisms obtain energy from chemical reactions, based on electron transfer between molecules
- organotrophs use organic compounds as a source of electrons- lithotrophs use inorganic compounds as a source of electrons

Front 2

GLUCOSE CATABOLISM

Back 2

• Most organisms use glucose as a source of energy (for growth) and carbon (for biosynthesis)
• Glucose (and related sugars) are catabolized through a series of phosphorylated sugar derivatives - These typically yield two molecules of pyruvate
C6H12O6 → 2 C3H4O3 + 4H (on NADH or NADPH)
• Bacteria can catabolize glucose via three main routes:

Front 3

Embden-Meyerhoff-Parnas Pathway

Back 3

• The EMP pathway is the most common form of glycolysis

• It occurs in the cytoplasm of the cell • It functions in the presence or absence of o2
- The EMP pathway has both catabolic and anabolic functions- Therefore, it is said to be amphipathic

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Embden-Meyerhoff-Parnas Pathway
1) Glucose activation stage

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- Glucose is “activated” by phosphorylations that ultimately convert it into
fructose-1,6-bisphosphate
- Two ATPs are expended
- Fructose-1,6-bisphosphate is cleaved into two 3-carbon-phosphate isomers
- Dihydroxyacetone phosphate
- Gyceraldehyde-3-phosphate- There are no reduction-oxidation (REDOX) reactions, and so no coenzymes are produced

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Embden-Meyerhoff-Parnas Pathway 2) Energy-yielding stage

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Each glyceraldehyde-3-phosphate molecule is ultimately converted to pyruvate
- REDOX reactions produce two molecules of nicotinamide adenine dinucleotide (NADH)
- Four ATP molecules are produced by substrate-level phosphorylation

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• Pasteur Effect I

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- Microbes grown aerobically use glucose less rapidly than those grown anaerobically
- The rate-limiting step of the EMP pathway is the reaction catalyzed by phosphofructokinase
- PFK is an allosteric enzyme
- An allosteric enzyme is one that has two binding sites
- The active site binds the substratrate- The allosteric site binds the effector
- Allosteric activators make the substrate bind better- Allosteric inhibitors inhibit the binding of the substrate

Front 7

Pasteur Effect II

Back 7

- ATP is an allosteric inhibitor
- AMP and ADP are allosteric activator
- In the presence of oxygen, the cell is undergoing aerobic respiration
==> Number of ATPs per glucose molecule is high
==> PFK is inhibited
- In the absence of oxygen, the cell is undergoing fermentation
==> Number of ATPs per glucose molecule is low
==> PFK is not inhibited

Front 8

Entner-Doudoroff Pathway

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- NADPH is the coenzyme most commonly associated with biosynthesis
• The ED pathway is most commonly found in GRAM -bacteria • The ED pathway is rare in eukaryotes

Front 9

FERMENTATION

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• Fermentation is an energy-yielding process in which organic molecules serve as both electron donors and acceptors
• Why the need?
- For every glucose molecule converted into pyruvate, two NAD+ molecules are consumed
- The specific reaction is:
Glyceraldehyde 3-phosphate 1,3-bisphosphoglycerate
- There is a limited amount of NAD+ in the cell
- So if it is not regenerated the oxidation of glyceraldehyde-3-phosphate will cease
and glycolysis will STOP
• In the absence of O2 and other inorganic electron acceptors, heterotrophic cells must transfer the hydrogens from NADH to organic electron acceptors

Front 10

• Many kinds of fermentation pathways exist (Fig. 13.21)

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- These pathways have two unifying themes1. NADH is oxidized into NAD+2. The electron acceptor is a pryuvate derivative

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Lactic Acid Fermentation 1. Homolactic Fermentation -

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Involves the EMP pathway - Overall reaction
Glucose + 2 ADP + 2 Pi à 2 lactic acid + 2 ATP
- It is utilized by Streptococcus, some Lactobacillus - It is also utilized by algae, protozoa and even human skeletal muscles- It is an important pathway for the production of yogurt and cheese - It is also a cause of dental caries

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Lactic Acid Fermentation 2. Heterolactic Fermentation

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- Involves a modified Pentose Phosphate Shunt- Overall reaction
Glucose + ADP + Pi à lactic acid + ethanol + CO2 + ATP
- It is utilized by Leuconostoc, some Lactobacillus
- It is an important pathway for sauerkraut production

Front 13

• Ethanolic Fermentation

Back 13

- Involves the EMP pathway - Overall reaction
Glucose + 2 ADP + 2 Pi à 2 ethanol + 2 CO2 + 2 ATP- It is utilized by yeast, some bacteria
- It is an important pathway in the food and brewing industrie - Brewer’s yeast- Baker’s yeast

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- Mixed acid fermentation

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Products = Acetic, formic, lactic and succinic acids
Ethanol, H2 and CO2- Bacterium = E. coli

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Propionic acid fermentation

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- Propionic acid fermentation- Products = Propionic and acetic acids, and CO2 Bacterium = propioni bacterism sp.
- This is an important pathway in the production of swiss cheese

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1. Phenol red broth test (Fig. 13.23A)

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- Contains a pH indicator and an inverted durham tube

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2. Sorbitol-MacConkey agar (Fig. 13.23B)

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- Differentiates between E. coli o157:h1, which does not ferment sorbitol, and
non-pathogenic E. coli, which do