Anaerobic Respiration Overview

Anaerobic Respiration

• Cellular respiration is typically linked to the cytoplasmic membrane
• In anaerobic respiration, inorganic compounds other than oxygen act as the final electron acceptor
• The electron transport chain in anaerobic respiration is similar to aerobic respiration, but the final electron acceptor differs
• Nitrate (NO3-) and sulfate (SO42-) are common electron acceptors in anaerobic respiration
• E. coli can utilize nitrate as an electron acceptor during anaerobic respiration, resulting in nitrate derivatives (NO2-) and nitrogen gas (N2) production
• Desulfovibrio sulfuricans is an anaerobic bacteria that reduces sulfate (SO42-) to sulfide ions (S) in the form of hydrogen sulfide (H2S) or atomic sulfur.
• The reduction of sulfates to hydrogen sulfide can cause mud blackening, particularly in environments like the Black Sea, due to the reaction of hydrogen sulfide with ferrous ions to form ferrous sulfide.
• Diagram of the process indicating various components involved in the electron transport chain

Fermentation

• Fermentation is an anaerobic pathway used by many microorganisms to break down glucose
• Microbes may lack or inhibit electron transport chains in anaerobic conditions
• Pyruvate produced in glycolysis doesn't continue through the citric acid or electron transport chain in fermentation
• Pyruvate or its derivative acts as an electron acceptor to reoxidize NADH during fermentation
• Different types of fermentation exist, each with specific end products

1- Lactic Acid Fermentation

• This type of fermentation occurs in some Bacillus and Lactobacillus species
• NADH directly transfers electrons to pyruvate, producing lactate as a byproduct
• This process is important in cheese production

2- Alcohol Fermentation

• Yeast and some bacteria participate in ethanol fermentation, breaking down pyruvate into ethanol and carbon dioxide
• NADH donates electrons to a pyruvate derivative, producing ethanol in a two-step process
• i. Carboxyl group removal from pyruvate, forms carbon dioxide, and acetaldehyde.
• ii. Acetaldehyde is reduced to ethanol in presence of NADH..

3- Heterolactic Acid Fermentation

• Anaerobic conditions, besides lactic acid, produces ethanol, acetic acid, and carbon dioxide, and glycerin
• This is produced in addition to lactic acid as a product in anaerobic conditions.

4- Butyric Fermentation

• Clostridium bacteria (including C. butyricum) are involved, producing carbon dioxide (CO2), butyrate, and gaseous acids during fermentation.

5- Propionic Fermentation

• Propionic acid, trace amounts of carbon dioxide and acetic acid, are produced by Propionibacterium

6- Butanediol Fermentation

• This is an anaerobic fermentation of glucose using 2,3-butanediol, along with ethanol, lactic acid, and formic acid, as byproducts
• This process occurs in Klebsiella and Enterobacter genera. Detected using the Voges-Proskauer test

Catabolism of Carbohydrates (other than glucose)

• The catabolic pathways for monosaccharides like glucose, fructose, mannose and galactose involve interconversion to glucose or glucose derivatives.
• Glucose, fructose, and mannose are phosphorylated by ATP, enabling entry into the Embden-Meyerhof pathway.
• Galactose needs uridine diphosphate galactose conversion followed by glucose 6-phosphate production after initial phosphorylation

Disaccharides

• Common disaccharides (maltose, sucrose, lactose) are cleaved into monosaccharides via hydrolysis
• Maltose, cellobiose, and sucrose can also be cleaved via phosphorolysis, a process utilizing phosphate attack on the bond linking the two sugars.

Polysaccharides

• Polysaccharides like disaccharides are cleaved by both hydrolysis and phosphorolysis
• The enzymes that break down polysaccharides are secreted by prokaryotes and fungi, cleaving into smaller components that can be absorbed
• Starch and glycogen are hydrolyzed by amylases to glucose and maltose
• Many fungi and some types of bacteria produce cellulase enzymes that hydrolyze cellulose into cellobiose and glucose
• Soil bacteria, like Azotobacter, hydrolyze poly-β-hydroxybutyrate (PHB) into 3-hydroxybutyrate, then to acetoacetate
• Soil bacteria and bacterial phytopathogens degrade pectin into galacturonic acid; certain fungi can degrade lignin
• Some actinomycetes and certain members of Cytophaga excrete an enzyme called agarase, which degrades agar
• Enzymes like amylase and cellulase break down stored sugar polymers in cells, like starch and cellulose.

Explore the key concepts of anaerobic respiration, including its processes and the role of electron acceptors like nitrate and sulfate. Understand how specific bacteria, such as E. coli and Desulfovibrio sulfuricans, function in these environments and the implications for ecosystems, like the Black Sea. Test your knowledge on this vital biological process through engaging questions.