General characteristics of fermentative metabolism
Fermentation allows bacteria to generate energy without oxygen. In this process, hydrogen cleaved from an organic substrate is transferred to other organic compounds rather than inorganic ones, as occurs in respiration.
The products of such metabolism include various alcohols, organic acids, and gases. Carbohydrates, polyols, purines, pyrimidines, and most amino acids can undergo fermentation. 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. Here, the tricarboxylic acid cycle does not serve an energy-producing function, but rather an exclusively biosynthetic one, supplying precursors for plastic metabolism.
- ATP formation: 2 ATP molecules are synthesized via substrate-level phosphorylation (during the oxidation of triose phosphate to pyruvate).
- Fate of hydrogen: hydrogen from reduced NAD is transferred to pyruvate itself or its derivatives, converting them into ethanol, acids, or gases.
Main types of fermentation
There are several classical pathways of carbohydrate fermentation, each distinguished by its enzyme systems and end products:
- 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 is replaced by energetically more favorable respiration (Pasteur effect).
- Lactic acid fermentation. Divided into homofermentative (production of lactic acid exclusively via the glycolytic pathway) and heterofermentative (breakdown via the pentose phosphate pathway yielding lactic acid, acetic acid, and ethanol). Characteristic of lactobacilli, streptococci, and bifidobacteria.
- Mixed acid fermentation. Characteristic of enterobacteria and vibrios. Divided into acid pathway (yielding acids and potential gas production) and butanediol pathway (synthesis of acetoin and 2,3-butanediol).
- Butyric acid fermentation. Typical for strict anaerobes (saccharolytic clostridia). Products include butyric acid and other volatile fatty acids, as well as solvents (butanol, acetone).
Protein fermentation (peptolytic metabolism)
Some microorganisms do not utilize carbohydrates and instead obtain energy through an alternative pathway: the fermentation of amino acids and hydrolysis of proteins. Such bacteria are called peptolytic. They include certain members of the genus Clostridium (e.g., Clostridium botulinum and Clostridium histolyticum).
Briefly: answers to topical questions
Which clostridial species undergo butyric acid fermentation?
Butyric acid fermentation is carried out by strict anaerobes, which include members of the genus Clostridium. This type of energy generation is typical for the following species:
- Clostridium botulinum (C. botulinum) is a strict anaerobe for which molecular oxygen is toxic.
- Clostridium tetani (C. tetani) is a strict anaerobe that dies or halts growth in the presence of oxygen.
During the anaerobic breakdown of carbohydrates, these chemoorganotrophs produce butyric acid and various gases.
Which enzymes catalyze 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 catalyzed by two enzymes. This process ensures ATP synthesis by transferring a high-energy phosphate group to ADP.
- Phosphoglycerate kinase catalyzes the first substrate-level phosphorylation reaction, in which 1,3-bisphosphoglycerate is converted into 3-phosphoglycerate.
- Pyruvate kinase catalyzes the second reaction, ensuring the conversion of phosphoenolpyruvate into pyruvate.
Both reactions are crucial stages of bioenergetics, allowing the cell to generate energy without the participation of the respiratory chain.
Class and exam questions
How does fermentation differ from respiration?
In fermentation, organic compounds act as hydrogen acceptors, and the process proceeds without oxygen. In respiration, the acceptor is an inorganic substance.
What is the role of the Krebs cycle in fermentation?
During fermentation, the tricarboxylic acid cycle functions not for energy production, but for biosynthesis—supplying precursors for the construction of cellular structures.
What is the Pasteur effect?
This phenomenon occurs when oxygen availability inhibits alcoholic fermentation in yeast, shifting metabolism to respiration because it yields more energy.
What else to study on this topic
- Mechanism of substrate-level phosphorylation
- Differences between homo- and heterofermentative lactic acid fermentation
- Diagnostic methods: methyl red test and Voges-Proskauer test
- Identification of anaerobes by gas-liquid chromatography (GLC)
- Establishment of colonization resistance by lactobacilli and bifidobacteria
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