Biochemistry and Molecular Biology - Belyasova N.A. 2002

Metabolism. Processes Leading to Energy Storage
Fermentation

Since Pasteur's time, Fermentation has been referred to as "life without air." This type of METABOLISM is one of the oldest on our planet, having emerged back when molecular oxygen was absent from the atmosphere. Earth's earliest inhabitants utilized this most primitive and energetically least efficient mode of metabolism to sustain life. In the course of evolution, newer and more promising energy-storage processes arose; however, fermentation is still utilized by many organisms because it enables them to survive and function in the absence of molecular oxygen.

Fermentation takes place in the Cells of Bacteria, Yeasts (for which fermentation is the predominant type of metabolism), Protozoans, Mollusks, certain Tissues of fish and birds, as well as higher animals, including humans (in Muscles, the lens, and the Cytology/practical/76.html">Cornea of the eye).

A wide variety of fermentation processes are known, differing from one another in their substrates, products, and chemical transformations. Several of the most frequently encountered Types of fermentation can be distinguished: alcoholic, lactic acid, propionic acid, formic acid, butyric acid, and acetone-butanol. These types of fermentation derive their names from the dominant or characteristic products formed As a result of a specific sequence of reactions. It is easy to see that organic acids and alcohols predominate among these products. Organic substances also serve as substrates for various types of fermentation—in most cases Pyruvate, as well as lactate, acetyl-CoA, certain glycolytic pathway intermediates, and Amino Acids. Thus, fermentation can be defined as a catabolic process occurring under conditions that do not require molecular oxygen, in which organic substances serve as both electron Donors and electron acceptors.

Fermentation reactions are always preceded by preliminary stages of substrate Catabolism: most commonly Glycolysis, and less frequently The pentose phosphate and phosphogluconate pathways. Therefore, the actual fermentation reactions are frequently considered together with the preceding stages, in which case energy storage can be said to occur during such processes. However, it should be remembered that energy in the form of high-energy ATP bonds is generally stored via substrate-level phosphorylation even before pyruvate is formed. In actual fermentation processes, energy is rarely stored; in fact, there are only three reactions in which ADP is phosphorylated: The formation of butyric, acetic, and acetoacetic acids. In Other types of fermentation, no additional energy is generated, and their primary objective is the regeneration of the oxidized form of the reducing equivalent carrier, NAD⁺. This is essential for the uninterrupted operation of glycolysis and other Catabolic pathways, during which substrates undergo breakdown accompanied by oxidation—a process that requires the continuous presence of reducing equivalent acceptors, such as NAD⁺, within The Cell.

A more detailed examination of several types of fermentation confirms that an equivalence is maintained between the number of NADH molecules generated during glycolysis and the number of NAD⁺ molecules formed during actual fermentation (provided that molecular oxygen does not participate in the metabolism). Consequently, under anaerobic conditions, cells employing a fermentative metabolism are forced to expend a substantial portion of two- and three-carbon organic substances to accept hydrogen, subsequently excreting them from the cell. This highly wasteful process results in cells Processing massive amounts of substrate under anaerobic conditions to extract the energy required for life. It is no coincidence that many facultatively anaerobic microorganisms have developed a mechanism to suppress fermentation in the presence of oxygen, switching instead to respiratory metabolism. Through Respiration, a cell can extract a maximum of 38 ATP molecules from a single glucose molecule, whereas fermentation yields only 4 (and in most cases, merely 2).