Fermentation is a word that is difficult to precisely define. It is a word that has long been used [local] in science and industry. For our purposes, we will limit consideration to the catabolism of sugars in the absence of oxygen. This makes fermentation distinct from respiration and, consequently, the yield of energy will be much less. Perhaps the best known type of fermentations are those used to produce alcoholic beverages, but there are a wide variety of possible types of fermentaion.
Two further generalizations will also be useful for our understanding. The first is that the sugar is glucose and it is processed by glycolysis (also known as the Embden-Meyerhoff Pathway) to pyruvate. The second is that this pathway results in the conversion of the bulk of the NAD+ in the cell to NADH, limiting further flux through the glycolytic pathway. The real purpose for fermentation is to find a way to regenerate NAD+ from NADH in the absence of O2. This is summarized [local].
The most simple case is the reduction of pyruvate to lactate by lactate dehydrogenase.
Pyruvate + NADH <--> Lactic acid + NAD+ + H+
The net effect is to regenerate NAD+ so that glycolysis can continue and energy (ATP) can continue to be produced by substrate level phosphorylations. The ultimate limitation on being able to use this process is that it is inefficient compared to respiration and it acidifies the medium, limiting growth even if the sugar supply is not limiting. Organisms that produce lactate as the sole product of fermentation are said to do homolactic fermentation.
A second important fermentation is alcoholic fermentation. This involves the enzymes pyruvate decarboxylase and alcohol dehydrogenase.
Pyruvate CO2 + Acetaldehyde
Acetaldehyde + NADH Ethanol + NAD+ + H+
Again, the net effect is to regenerate NAD+ for continued energy production via glycolysis. Because the ethanol does not deprotonate, this fermentative lifestyle causes less acidification of the medium. However, the ethanol is still toxic, which limits most fermentations to about 12% ethanol yield as commonly found in wines.
Another type of fermentation is mixed-acid fermentation. In this case, several acids can be formed from pyruvate, principally lactic, acetic and succinic acids, and formic acid and ethanol.
Notice that in addition to organic acids, this can produce formate and Acetyl CoA from pyruvate using the enzyme pyruvate-formate lyase. The formate can then be converted to H2 and CO2 by the enzyme formate-hydrogen lyase. Bacteria that carry out this type of fermentation include the Enterobacteria that inhabit the human gut. This ability to produce H2 serves as the basis for clinical diagnosis of lactose intolerance. If your intestines do not have the lactase enzyme, it can be consumed by the Enterobacteria and H2 is produced. This is absorbed into the blood and can be detected in the breath. If you are able to absorb lactose, no hydrogen is produced and you are not lactose intolerant.
Another type of fermentation is the production of butanediol from two molecules of pyruvate.
Notice that, like ethanol fermentation, the production of butanediol results in the formation of CO2 without respiration.
Not all fermentations utilize the glycolytic pathway to form pyruvate from glucose. Some utilize the Hexose Monophosphate Shunt (Entner-Doudoroff Pathway) which oxidizes glucose-6-P to 6-P-gluconate, which is then metabolized to pyruvate. There is a variation called Heterolactic Fermenation or the phosphoketolase pathway in which the 6-P-gluconate is further metabolized to both lactate and acetate. The latter product can be further metabolized to ethanol.
Fermentation is also important in the production of a variety of foods, beverages and seasonings [local].
