Fermentation

• • • Defined as an energy yielding process Organic molecules serve as both electron donors and electron accepters.

The molecule being metabolized does not have all its potential energy extracted from it.

(not completely oxidized.)

• • NAD + is almost always reduced to NADH Remember that metabolism involves the oxidation of the substrate. These electrons are removed from the organic molecule and most often given to NAD. (This is true both in fermentation and respiration ).

• • Fermentation results in a excess of NADH Accumulation of NADH causes a problem for anaerobes. They have too much of it and it prevents further oxidation of substrate due to a lack of an NAD + pool to accept electrons. • In many fermentation pathways, the steps after energy generation are performed in part to get rid of the NADH: pyruvate + NADH + H + → lactate + NAD +

Glycolysis - Embden-Meyerhoff-Parnas pathway (EMP)

• • • The most commonly used series of reactions for oxidizing glucose to pyruvate . EMP is so ubiquitous. However, it is not the only method for the fermentation of glucose. It is an essential part of many organisms catabolism.

• • • A complete fermentation pathway begins with a substrate, includes glycolysis and results in various end-products . The different fermentation pathways typically are named for the end products that are formed.

yeast, convert NADH back to NAD + in a process called ethanol fermentation . In this process, the pyruvate is converted first to acetaldehyde and carbon dioxide , then to ethanol .

Pyruvate

• • • • Pyruvate is often an important intermediate Many of the reactions that we will look at eventually end up making pyruvate.

Pyruvate is a valuable intermediate because it can be used for cell synthesis.

Many different enzymes can act on it. It gives the microbe flexibility.

.

Fermentation Vs Aerobic oxidation

C 6 H 12 O 6 ------------>2C 2 H 5 OH + 2CO 2 (2ATP) ∆G = 33 Kcal ریمخت C 6 H 12 O 6 + 6O 2 ------------>6CO 2 سفنت + 6H 2 O (36ATP) ∆G = 686 Kcal_

Fermentation

Incomplete oxidation of glucose or other carbohydrates in the absence of oxygen Uses organic compounds as terminal electron acceptors Yields a small amount of ATP Production of ethyl alcohol by yeasts acting on glucose Formation of acid, gas & other products by the action of various bacteria on pyruvic acid

Glycolysis

Embden-Meyerhof-Parnas (EMP pathway)

Entner–Doudoroff pathway

• The Entner–Doudoroff pathway describes an alternate series of reactions that catabolize glucose to pyruvate using a set of enzymes different from those used in either glycolysis or the pentose phosphate pathway

Homolactic Fermentation

(Lactic acid)

- It is the sole end product.

- Pathway of the homolactic acid bacteria (Lactobacillus,

Lactococcus

and most streptococci).

- The bacteria are used to ferment milk and milk products in the manufacture of yogurt, buttermilk, sour cream, cottage cheese, cheddar cheese….

Mixed Acid Fermentations.

• • • Mainly the pathway of the Enterobacteriaceae. End products are a mixture of lactic acid, acetic acid, formic acid, succinate and ethanol.

Possibility of gas formation (CO

2

and H

2

) if the bacterium possesses the enzyme formate dehydrogenase , which cleaves formate to the gases.

Butanediol Fermentation

• • Mixed acids and gases but in addition, butanediol from the condensation of 2 pyruvate. The use of the pathway decreases acid formation (butanediol is neutral) and causes the formation of a distinctive intermediate, acetoin. Specific tests to detect low acid and acetoin in order to distinguish non fecal enteric bacteria (butanediol formers, such as Klebsiella and Enterobacter) from fecal enterics (mixed acid fermenters, such as E. coli, Salmonella and Shigella).

Butyric acid fermentations

(Butanol-acetone fermentation) • • • Butyric acid fermentations as well as the butanol acetone fermentation, are run by the clostridia, the masters of fermentation. In addition to butyric acid, the clostridia form acetic acid, CO 2 and H 2 from the fermentation of sugars. Small amounts of ethanol and isopropanol may also be formed.

Butanol and acetone were discovered as the main end products of fermentation by Clostridium

acetobutylicum

Propionic acid fermentation.

• • • • Carried out by the propionic acid bacteria which include corynebacteria, Propionibacterium and Bifidobacterium. Propionic acid bacteria ferment lactate to acetic acid, CO 2 and propionic acid. The formation of propionate is a complex and indirect process involving 5 or 6 reactions. The propionic acid bacteria are used in the manufacture of Swiss cheese , which is distinguished by the distinct flavor of propionate and acetate, and holes caused by entrapment of CO

2

.

Fermentation

Fermentations in bacteria that proceed through the Embden-Meyerhof pathway. Representive bacteria that utilize these pathways are in shown in BLUE.