Anaerobic Respiration and Fermentation

Anaerobic Respiration

• Cellular respiration is usually linked to the cytoplasmic membrane
• In anaerobic respiration, inorganic compounds act as the final electron acceptors, instead of oxygen.
• The electron transport chain is similar to aerobic respiration, but with a different final electron acceptor.
• Nitrate (NO₃⁻) and sulfate (SO₄²⁻) are common electron acceptors in anaerobic respiration.
• E. coli bacteria can use nitrate as the final electron acceptor during anaerobic glucose oxidation, producing nitrate derivatives (NO₂⁻) and nitrogen gas (N₂).
• Desulfovibrio sulfuricans bacteria reduce sulfate (SO₄²⁻) to sulfide ions (S²⁻) in the form of hydrogen sulfide (H₂S) or atomic sulfur (S), which is responsible for the blackening of mud, including the Black Sea.

Fermentation

• Fermentation is an anaerobic process for glucose breakdown in various microorganisms.
• Microorganisms lacking or repressing electron transport chain components perform fermentation.
• In fermentation, pyruvate (from glycolysis) bypasses the citric acid cycle and the electron transport chain.
• Microbes use pyruvate or its derivatives to re-oxidize NADH (a product of glycolysis.)
• There are multiple types of fermentation pathways for sugar breakdown.

Specific Types of Fermentation

Lactic Acid Fermentation

• Occurs in some Bacillus and Lactobacillus species.
• This leads to cheese production.
• In lactic acid fermentation, NADH directly transfers electrons to pyruvate creating lactate as a byproduct.
• The process converts glucose to lactic acid, and involves the transfer of electrons from NADH to pyruvate, producing lactate.

Alcohol Fermentation

• Performed by yeast and selected bacteria.
• Pyruvate from glucose metabolism is converted into ethanol and carbon dioxide.
• NADH donates electrons in a two-step process.
• Step one: removing the carboxyl group from pyruvate releases carbon dioxide, making a two-carbon molecule called acetaldehyde.
• Step two: acetaldehyde is reduced to ethanol using NADH.

Heterolactic Fermentation

• Produces ethanol, acetic acid, carbon dioxide and glycerol besides lactic acid under anaerobic conditions.

Butyric Fermentation

• Clostridium bacteria, including C. butyricum, participate in this.
• The end products are carbon dioxide, butyrate and gas (acid).

Propionic Fermentation

• It involves the production of propionic acid, carbon dioxide and acetic acid by Propionibacterium.

Butanediol Fermentation

• Using 2,3-butanediol as a terminal electron acceptor.
• It can also produce ethanol, lactic acid and formic acid.
• This fermentation type is tested using the Voges-Proskauer test.

Catabolism of Carbohydrates other than Glucose

Monosaccharides

• Glucose, fructose, mannose and galactose are converted into glucose or its derivates.
• First three are phosphorylated using ATP and easily enter the Embden-Meyerhof pathway.
• Galactose requires conversion to UDP-galactose then glucose-6-phosphate before entering glycolysis.

Disaccharides

• Common Disaccharides like maltose, sucrose, and lactose break down to monosaccharides.
• Hydrolysis: These disaccharides can be directly hydrolyzed to constituent monosaccharides by the respective enzymes.
• Phosphorolysis: Maltose, cellobiose, and sucrose are cleaved by a phosphate attack on the bond between sugar monomers.

Polysaccharides

• Polysaccharides are broken down into smaller molecules usable by prokaryotes or fungi by enzymes.
• Starch and glycogen are hydrolyzed by amylases to monosaccharides, such as glucose, maltose, and others.
• Cellulose is digested by cellulases from some fungi into cellobiose and glucose.
• Pectin and lignin are degraded by soil bacteria, fungi.
• Agar breakdown is performed by certain actinomycetes and Cytophaga through an enzyme called agarase.

Reserve Polymers

• Glycogen and starch are degraded by phosphorolysis, forming glucose-1-phosphate, which enters glycolysis.
• Poly-β-hydroxybutyrate (PHB) is hydrolyzed to 3-hydroxybutyrate by the soil bacterium Azotobacter which is then oxidized to acetoacetate.

This quiz covers the processes of anaerobic respiration and fermentation, highlighting the role of different electron acceptors like nitrate and sulfate. It discusses the mechanisms in bacteria such as E. coli and Desulfovibrio sulfuricans. Test your knowledge on how microorganisms metabolize glucose without oxygen.