Anaerobic Respiration and Fermentation

Study Notes

Anaerobic respiration is a type of cellular respiration that occurs in the absence of oxygen.
The final electron acceptor is an inorganic compound, unlike oxygen in aerobic respiration.
The electron transport chain is similar in structure to aerobic respiration, differing only in the final electron acceptor.
Nitrate (NO₃⁻) and sulfate (SO₄²⁻) are common inorganic electron acceptors in anaerobic respiration.
E. coli bacteria can use nitrate as the final electron acceptor when glucose is oxidized, producing nitrate derivatives and nitrogen gas.
Desulfovibrio sulfuricans bacteria reduce sulfate to sulfide (H₂S or atomic sulfur). This process can lead to the blackening of mud, like in the Black Sea.
The process involves a series of electron carriers in the cytoplasmic membrane similar to aerobic respiration but with different final electron acceptors.

Fermentation is an anaerobic pathway for breaking down glucose in many microorganisms.
Microorganisms may lack or repress electron transport chains under anaerobic conditions, favoring fermentation.
In fermentation, pyruvate from glycolysis does not proceed through the citric acid cycle or electron transport chain.
Pyruvate or its derivatives act as electron acceptors, reoxidizing NADH.
Common fermentations include lactic acid fermentation and alcohol fermentation.

This process occurs in some Bacillus and Lactobacillus species, playing a role in cheese production.
NADH transfers electrons directly to pyruvate, creating lactate as a byproduct.
The equation for the process is: Glucose → 2 Lactic Acid.

Certain yeasts and bacteria perform alcohol fermentation.
Pyruvate is converted into ethanol and carbon dioxide in a two-step process.
Step 1: The carboxyl group is removed from pyruvate, releasing carbon dioxide and forming acetaldehyde.
Step 2: Acetaldehyde is reduced to ethanol using NADH.

Heterolactic fermentation (in addition to lactic acid, produce ethanol, acetic acid, carbon dioxide, and glycerol).
Butyric fermentation (involving Clostridium bacteria) produces carbon dioxide, butyrate, and gas.
Propionic fermentation yields propionic acid, carbon dioxide, and acetic acid.
Butanediol fermentation produces butanediol, ethanol, lactic acid, and formic acid. Many bacteria in Klebsiella or Enterobacter genera use this fermentation as a pathway.

Monosaccharides (glucose, fructose, mannose, and galactose) are converted to glucose or glucose derivatives for catabolism.
Glucose, fructose, and mannose are phosphorylated using ATP to enter the Embden-Meyerhof pathway.
Galactose requires conversion to UDP-galactose and then to glucose-6-phosphate.

Maltose, sucrose, and lactose are hydrolyzed to their constituent monosaccharides.
Maltose, cellobiose, and sucrose are also split by phosphorolysis (phosphate attack on the bond joining two sugars).

Polysaccharides (like starch and glycogen) are cleaved by enzymes such as amylases into smaller units.
Many fungi and some other microorganisms utilize extracellular enzymes (cellulases) to hydrolyze cellulose into cellobiose and glucose.
Soil bacteria, like Azotobacter, hydrolyze poly-β-hydroxybutyrate (PHB) to 3-hydroxybutyrate.
Pectin and lignin are degraded by different groups of bacteria and fungi, utilizing specific enzymes involved in the process.
Agar is degraded by agarase enzymes produced by some actinomycetes and Cytophaga members.