Anaerobic Respiration and Fermentation

Study Notes

Cellular respiration, typically linked to cytoplasmic membranes, utilizes inorganic electron acceptors (not oxygen) in anaerobic respiration.
The electron transport chain resembles aerobic respiration, differing only in the final electron acceptor.
Nitrate (NO₃⁻) and sulfate (SO₄²⁻) commonly function as electron acceptors in anaerobic respiration.
E. coli bacteria can respire anaerobically using nitrate as the terminal electron acceptor, producing nitrate derivatives (NO₂⁻) and nitrogen gas (N₂).
Desulfovibrio sulfuricans bacteria respire anaerobically, reducing sulfate (SO₄²⁻) to sulfide (H₂S) or atomic sulfur (S).
The production of hydrogen sulfide (H₂S) blackens mud, such as in the Black Sea, due to the reaction of H₂S with ferrous ions (+Fe), creating black ferrous sulfide salts. (FeS).

Fermentation is an anaerobic pathway for glucose breakdown in various microorganisms.
Microorganisms lacking or inactivated electron transport chains, or those maintaining anaerobic conditions, employ fermentation.
Fermentation bypasses the citric acid (Krebs) cycle and electron transport chain.
Pyruvate, or derivatives, are employed as electron acceptors for the reoxidation of NADH.
Key fermentation pathways include lactic acid and alcohol fermentation.

Occurs in some Bacillus and Lactobacillus species, involved in cheese production.
NADH directly donates electrons to pyruvate, generating lactate (lactic acid) as a byproduct.
The overall reaction involves glucose becoming lactic acid with the release of energy.

Yeast and some bacteria use this pathway.
Pyruvate is converted to ethanol and carbon dioxide in a two-step process.
In the first step, the carboxyl group of pyruvate releases as carbon dioxide creating acetaldehyde.
In the second step, acetaldehyde is reduced to ethanol using electrons from NADH.

Mixed-acid fermentation produces a variety of products like ethanol, acetic acid, CO2, and glycerol in addition to lactic acid.
Butyric fermentation is carried out by Clostridium bacteria, producing CO₂, butyrate, and gases (acids).
Propionic fermentation involves the production of propionic acid, with minor amounts of CO₂ and acetic acid formed by Propionibacterium.
Butanediol fermentation involves the production of 2,3-butanediol, as well as some ethanol, lactic acid, and formic acid.
Specific genera, such as Klebsiella and Enterobacter, exhibit this type of fermentation, which can be detected using Voges-Proskauer test.

Catabolism of monosaccharides like fructose, mannose, and galactose often involves conversion to glucose or glucose derivatives.
Glucose, fructose, and mannose enter the Embden-Meyerhof pathway after phosphorylation with ATP.
Galactose, however, is initially phosphorylated, then converted to UDP-galactose before being converted to glucose-6-phosphate.

Common disaccharides (maltose, sucrose, and lactose) are hydrolyzed to their monosaccharide components.
Some disaccharides (maltose, cellobiose, and sucrose) are also broken down through phosphorolysis involving phosphate attack.
Specific enzymes (maltase, sucrase, lactase, cellobiose phosphorylase) catalyze the hydrolysis and phosphorolysis.

Polysaccharides, including starch and glycogen, are hydrolyzed by amylases to yield glucose and maltose.
Cellulose is hydrolyzed by cellulases, producing cellobiose and glucose.
Pectin, a component of plant cell walls, is broken down by soil bacteria and bacterial phytopathogens into galacturonic acid.
Lignin is degraded by some fungi.
Agar is degraded by agarase.

Microbes catabolize intracellular glycogen, starch, poly-β-hydroxybutyrate (PHB), and other carbon/energy reserves in the absence of external nutrients.
Glycogen and starch are degraded by phosphorolysis, a process catalyzed by phosphorylases.
This process produces glucose-1-phosphate from the polysaccharide.
Poly-β-hydroxybutyrate (PHB) is hydrolyzed by Azotobacter bacteria into 3-hydroxybutyrate, which is oxidized to acetoacetate.