deaminated, that is, the amino group can be removed and converted to inorganic ammonia. Removal of an amino group from an amino acid produces a car- boxylic acid. The carboxylic acids produced in this manner can enter the glycolytic pathway or the Krebs cycle. Further catabolism results in decarboxylation with the production of C0₂. ATP is generated as a re­sult of the protonmotive force formed by the estab­lishment of a proton gradient across the plasma membrane of a prokaryotic cell or the mitochondrial membranes of eukaryotic cells.

Fermentation

In fermentation pathways the organic substrate mol­ecule is converted to another organic molecule that serves as the terminal electron acceptor. There is no net change in the oxidation state of the products rel­ative to the starting substrate molecule. The oxidized products are exactly counterbalanced by reduced products, and thus the required oxidation-reduction balance is always achieved. Such a metabolic path­way can occur in the absence of air because there is no requirement for oxygen or any other inorganic compound to serve as the terminal electron acceptor to balance a change in the oxidation state of the or­ganic molecule. Some microorganisms can carry out fermentation in the presence of air, simply ignoring the presence of oxygen, whereas others can carry out fermentation only in the absence of oxygen.

Fermentation is an anaerobic process that does not

require an external electron acceptor to balance the

oxidation-reduction reactions in the pathway.

In a fermentation pathway, substrate-level phos­phorylation converts ADP to ATP. Unlike respiration, fermentation does not involve an electron transport chain or chemiosmosis for generating ATP. Rather, the synthesis of ATP in fermentation is largely re­stricted to the amount formed during glycolysis. Fer­mentation yields far less ATP per substrate molecule than respiration. This is because during a fermenta­tion pathway the organic substrate molecule is not

completely oxidized to carbon dioxide. As a result, not as much energy can be released from the sub­strate molecule to drive the synthesis of ATP.

Fermentation generates ATP by substrate-level

phosphorylation.

Fermentation yields far less ATP per substrate mole­cule utilized than respiration.

The energy obtained from the complete oxidation of glucose to carbon dioxide and water by respiration is more than 10 times greater than that obtained when glucose is metabolized by fermentation. Be­cause more ATP can be generated per molecule of substrate, fewer substrate molecules must be metab­olized during respiration than during fermentation to achieve equivalent growth. From both the view­points of the energy needs of living organisms and conservation of available organic nutrient resources, respiration is more favorable than fermentation. Or­ganisms that have the metabolic capability to carry out both types of metabolism will generally use the energetically more favorable respiration pathway, when conditions permit, and will rely on fermenta­tion only when there is no available external electron acceptor.

The initial metabolic steps in the fermentation of a carbohydrate are identical to those in respiration. It begins with glycolysis. If the microorganism uses the Embden-Meyerhof glycolytic pathway, it generates two pyruvate molecules, two reduced coenzyme NADFI molecules, and two ATP molecules for each molecule of glucose as a result of the glycolytic breakdown of glucose.

In respiration the reoxidation of the coenzymes oc­curs in the electron transport chain and requires an external electron acceptor. In fermentation the reoxi­dation of NADH to NAD⁺ depends on the reduction of the pyruvate molecules formed during glycolysis to balance the oxidation-reduction reactions. Differ­ent microorganisms have developed different path­ways for utilizing the pyruvate to reoxidize the re­duced coenzyme. These pathways have different ter-

Intestinal Yeast Infections Cause Intoxication

Some people in Japan have been reported to suffer testines is carrying out an alcoholic fermentation,

from a strange form of alcoholism apparently related This fermentation produced sufficient alcohol to

to a yeast infection in their intestines. These people bring about this state of drunkenness. Reduction of

had not consumed any alcoholic beverages but nev- the yeast population in their intestines by treatment

ertheless frequently appeared to be inebriated. The with antibiotics eliminated this state of intoxication,

evidence indicates that yeast growing in their in- Thus there is a cure for such infectious intoxication.

174 Chapter 6 cellular metabolism

minal sequences and hence the various fermentation pathways result in the formation of varying end products (FIG. 6-16).

A complete fermentation pathway (1) begins with a substrate, (2) includes glycolysis, and (3) terminates with the formation of end products. There is no net change in the oxidative state of the coenzymes dur­ing the overall fermentation pathway, and the coen­zyme does not appear in the overall fermentation equation. There are several different fermentation pathways that are generally named for the character­istic end products that are formed (see Table 6-3). Many of the products of fermentation pathways have commercial value. As will become apparent from the discussion of the specific fermentation pathways, nu­merous foods, beverages, and other products we fre­quently use are the products of microbial fermenta­tions.

Ethanolic Fermentation

In the ethanolic fermentation, or alcoholic fermen­tation, pyruvate is converted to ethanol and carbon dioxide. Ethanolic fermentation is carried out by many yeasts, such as Saccharomyces cerevisiae, but by relatively few bacteria. The ethanolic fermentation pathway can be written as:

glucose + 2 ADP + 2 Pj —> 2 ethanol + 2 CO_z + 2 ATP

This fermentation pathway is very important in food and industrial microbiology. It is used to pro­duce beer, wine, and distilled spirits (FIG. 6-17). For example, the carbohydrates in grains are converted to ethanol by yeasts in the production of beer and spir­its. The sugars in grapes are the substrates for ethanol production by the wine-producing yeasts. Saccha­romyces cerevisiae, also known as baker's yeast, is used in bread making; the carbon dioxide released during ethanolic fermentation causes bread dough to rise in a process called leavening (FIG. 6-18). The ethanol is driven from the bread during baking, which, accord­ing to the United States Environmental Protection Agency, is an important source of air pollution that must be controlled. The ethanol produced by S. cere­visiae is also used as a fuel to augment gasoline in a product known popularly as gasohol.

Lactic Acid Fermentation

Lactic acid fermentation is carried out by bacteria that—by virtue of these metabolic reactions—are classified as the lactic acid bacteria. This fermentation pathway produces lactic acid as an end product. When the Embden-Meyerhof scheme of glycolysis is used in the lactic acid fermentation pathway, the overall pathway is a homolactic fermentation be­cause the only end product formed is lactic acid. Ho­molactic fermentation is carried out by Streptococcus

FIG. 6-18 Bread is made by an alcoholic fermentation.The C0₂ from the ethanolic fer­mentation causes the bread to rise.

and Lactobacillus species (FIG. 6-19). The homolactic acid fermentation pathway can be written as:

Streptococci, even though they are metabolically obligate anaerobes, live on human tooth surfaces and produce lactic acid that is held against the tooth by dental plaque. The acid can gradually eat through the enamel of the tooth and create cavities. The lactic acid produced by Lactobacillus species in the vaginal tract helps protect against sexually transmitted pathogens. Lactobacillus species are the initial colo­nizers of the human intestinal tract and occur in the human digestive tract, aiding in the digestion of milk and dairy products. Many adults cannot digest the carbohydrates in milk and so L. acidophilus is added to various commercial milk products, such as aci­dophilus milk, to aid those who cannot tolerate milk. The enzymes produced by L. acidophilus convert milk sugars to products that do not accumulate and cause gastrointestinal upset. The homolactic acid fermenta­tion pathway is important in the dairy industry be­cause it is responsible for the souring of milk. It is used in the production of many types of cheese, yo­gurt, and other dairy products. Buttermilk and sour cream are made by using different strains of lactic

acid bacteria as starter cultures and different parts of whole milk as the starting substrate.

In some bacteria, however, a different pathway of glycolysis—the Entner-Doudoroff pathway—is used. Such bacteria are termed heterolactic because both ethanol and carbon dioxide are produced in addition to lactic acid. The ethanol and carbon dioxide come from the glycolytic portion of the pathway. This fer­mentative pathway is carried out by Leuconostoc and various Lactobacillus species and is responsible for the production of sauerkraut (FIG. 6-20). Several types of sausage are produced by allowing meat to undergo heterolactic acid fermentation during curing. The overall reaction for the heterolactic fermentation can be written as:

Propionic Acid Fermentation

Another fermentation pathway of interest is the pro­pionic acid fermentation pathway. This metabolic sequence carried out by the propionic acid bacteria produces propionic acid and carbon dioxide. The bacterial genus Propionibacterium, which contributes to acne, is defined as Gram-positive rods that pro­duce propionic acid from the metabolism of carbohy- |

176 CHAPTER 6 CELLULAR METABOLISM