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Fermentation

For medical students2 min readUpdated 2026-10-10

Fermentation is an anaerobic pathway for energy generation in which hydrogen is transferred from a substrate to organic compounds. This process yields acids, alcohols, and gases without the participation of inorganic electron acceptors.

SubstratesCarbohydrates, polyols, non-aromatic amino acids, purines, and pyrimidines
Non-fermentableFatty acids, steroids, carotenoids, and aromatic hydrocarbons remain stable in the absence of oxygen
Energy yield2 molecules of ATP per glucose molecule via substrate-level phosphorylation

Overview of Fermentative Metabolism

Fermentation enables bacteria to generate energy in the absence of oxygen. In this process, hydrogen removed from an organic substrate is transferred to other organic compounds rather than inorganic compounds, which occurs during respiration.

The end products of this metabolism include various alcohols, organic acids, and gases. Fermentation substrates can include carbohydrates, polyols, purines, pyrimidines, and most amino acids. However, aromatic compounds, fatty acids, and steroids remain stable under anaerobic conditions and are not fermented.

Biochemistry of Glucose Fermentation

A key feature of fermentation is the partial oxidation of pyruvate. The tricarboxylic acid (TCA) cycle does not serve an energetic function here, but rather a strictly biosynthetic one, supplying precursors for anabolic metabolism.

Major Types of Fermentation

There are several classic pathways of carbohydrate fermentation, each distinguished by its enzyme systems and end products:

  1. Alcoholic fermentation. Carried out by yeasts under anaerobic conditions via the glycolytic pathway. End products are ethanol and carbon dioxide. In the presence of oxygen, this process shifts to the more energetically favorable respiration (Pasteur effect).
  2. Lactic acid fermentation. Divided into homofermentative (producing only lactic acid via glycolysis) and heterofermentative (breaking down via the pentose phosphate pathway to produce lactic acid, acetic acid, and ethanol). Typical for lactobacilli, streptococci, and bifidobacteria.
  3. Mixed acid fermentation. Characteristic of Enterobacteriaceae and vibrios. Divided into acid pathway (producing mixed acids with possible gas evolution) and butanediol pathway (synthesizing acetoin and 2,3-butanediol).
  4. Butyric acid fermentation. Characteristic of strict anaerobes (saccharolytic clostridia). Products include butyric acid, other volatile fatty acids, and solvents (butanol, acetone).

Protein Fermentation (Peptolytic Metabolism)

Some microorganisms do not utilize carbohydrates, obtaining energy through an alternative pathway via amino acid fermentation and protein hydrolysis. These bacteria are called peptolytic. They include certain members of the genus Clostridium (e.g., Clostridium botulinum and Clostridium histolyticum).

Frequently asked questions

Which clostridial species perform butyric acid fermentation?

Butyric acid fermentation is carried out by strict anaerobes belonging to the genus Clostridium. This type of energy metabolism is characteristic of the following species:

  • Clostridium botulinum — a strict anaerobe for which molecular oxygen is toxic.
  • Clostridium tetani — a strict anaerobe whose growth is inhibited or killed in the presence of oxygen.

During the anaerobic breakdown of carbohydrates, these chemoorganotrophs produce butyric acid and various gases.

Which enzymes mediate substrate-level phosphorylation during the oxidation of triose phosphate to pyruvate?

Substrate-level phosphorylation during the oxidation of triose phosphate (glyceraldehyde-3-phosphate) to pyruvate is mediated by two enzymes. This process ensures ATP synthesis via the transfer of a high-energy phosphate group to ADP.

  • Phosphoglycerate kinase — catalyzes the first substrate-level phosphorylation reaction, converting 1,3-bisphosphoglycerate into 3-phosphoglycerate.
  • Pyruvate kinase — catalyzes the second reaction, ensuring the conversion of phosphoenolpyruvate into pyruvate.

Both reactions are critical steps in cell bioenergetics, allowing the cell to generate energy without a respiratory chain.

How does fermentation differ from respiration?

In fermentation, organic compounds act as hydrogen acceptors, and the process occurs without oxygen. In respiration, the final electron acceptor is an inorganic substance.

What is the role of the Krebs cycle during fermentation?

The tricarboxylic acid cycle during fermentation does not function for energy production, but rather for biosynthesis—providing precursors for building cellular structures.

What is the Pasteur effect?

It is the phenomenon where the presence of oxygen inhibits alcoholic fermentation in yeast, switching metabolism to respiration because it yields more energy.

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