Anaerobic Metabolism Of Pyruvate

Anaerobic metabolism of pyruvate:

In the absence of O₂ (or in absence of aerobic respiration), cellular metabolism of pyruvate is conducted by different fermentative pathways (or simply by fermentation) during which different organic end products are formed. Unlike in aerobic metabolism, during fermentation, the substrate is partially oxidized in which there is no need of oxygen and ATP is formed only during glycolysis.

Definition of Fermentation:

Fermentation can be defined as the process that:

1. releases energy from the partial oxidation of sugars or other organic molecules, such as amino acids, organic acids, purines and pyrimidines.
2. does not require oxygen ( but sometimes may be conducted in the presence of O₂)
3. does not undergo the path of TCA cycle and electron transport chain.
4. uses endogenous organic molecules as the final electron accepters.
5. produces only few amounts of ATP (only one or two ATPs per molecule of starting material) because much of the energy in glucose remains in the chemical bonds of organic end products, such as lactic acids or ethanol.

Shortly fermentation can be defined as the utilization and partial oxidation of the organic compounds in absence of extra electron accepters.

Fermentation is carried out in two major steps:

Step I: electrons are transferred long with the protons to NAD⁺ from the catabolism of glucose.

Step II: electrons are then transferred along with protons from reduced coenzyme (NADH) to pyruvic acid or its derivatives, the final electron acceptors because no extra electron acceptors are available those final electron carriers are reduced to end products.

The second step of fermentation serves to ensure the steady supply of NAD⁺ so that glycolysis can be constantly continued. Here we mainly talk two end products and their fermentation; Ethanol and Lactic acid as these are commercially most important fermentative products.

Ethanol/ Ethanol Fermentation

In alcohol fermentation, the substrate, sugar is partially broken down into ethanol and CO₂. The process is totally catalyzed by fermentative enzymes. These enzymes are biologically produced in microorganisms like bacteria, yeasts, algae and protozoa which ferment sugar to ethanol and CO₂ through the process of alcohol fermentation.

At first, the substrate, glucose is first broken down to pyruvate by normal glycolysis process. Then after there is the occurrence of actual fermentative reactions. Pyruvate obtained from glycolysis then undergoes two-step reactions for its conversion into ethanol. Firstly, pyruvate there is the occurrence of decarboxylation reaction which is catalyzed by the enzyme, pyruvate decarboxylase which removes CO₂ from pyruvate producing aldehyde (acetaldehyde) as the intermediate compound. In the second step reaction, acetaldehyde is reduced to ethanol with the aid of the enzyme, alcohol dehydrogenase. In this course of reaction, NADH is reoxidized as one molecule of NADH donates electron pairs and itself is converted into NAD⁺. Industrially, yeasts are utilized as ferments. The end products of this fermentation are alcohol, which is of versatile use and CO₂, which is used to cause bread to rise.

Step I: Pyruvate + pyruvate decarboxylase → acetaldehyde + CO₂

Step II: Acetaldehyde + NADH + Alcohol dehydrogenase → Ethanol + NAD⁺

Lactic acid fermentation

In lactic acid fermentation, the substrate, pyruvate is partially oxidized into lactic acid or simply lactate. Lactic acid fermentation is mainly occurred in bacteria like lactic acid bacteria, algae (chlorella), some water molds, protozoa and even in animal skeletal muscles.

Lactic acid fermentation is mainly categorized into two groups, they are as follows:

1) Homolactic acid fermentation

As per homolactic acid fermentation, the fermenting microorganisms produce pyruvate from glycolysis process and then lactic acid as the sole product by reducing pyruvate. This overall process is known as homolactic acid fermentation. This is one step reaction process. In this process, pyruvate is reduced into lactate by the aid of enzyme, Lactate dehydrogenase. The fermenting microorganisms use the glycolytic pathway until the carbohydrates are totally converted into pyruvate then after use fermentative pathway in which almost all pyruvate is directly reduced to lactate with the aid of enzyme, lactate dehydrogenase.

Glucose → → →Pyruvate + lactate dehydrogenase + NADH → Lactate + NAD⁺

2) Heterolactic acid Fermentation

When the fermenting microorganisms produce the substantial amount of end products like ethanol and CO₂ along with lactic acid, such process of pyruvate reduction is known as heterolactic acid fermentation. In this process, Glucose is not broken down into pyruvate directly, insted the fermenting organisms follow the phospho ketolase pathway of glucose breakdown to produce ethanol, CO₂ and lactic acid.

Like in pentose phosphate pathway, in phospho ketolase pathway, there is the formation of xylulose. Xylulose thus formed is broken down to acetyl phosphate and glyceraldehyde 3-phosphate. enzyme, phosphoketolase catalyzes the reaction. The heterolactate fermenters convert acetyl phosphate to ethanol via acetyl CoA and glyceraldehyde 3 phosphate to lactate via pyruvate (reactions of EMP pathway)

Lactic acid bacteria

Those bacteria which carry out lactic acid fermentation (homo or hetero lactic acid fermentation) are called lactic acid bacteria (LAB). Such groups of bacteria are collectively assigned to the family lactobacillaceae. Lactobacillaciae, the lactic acid fermenting group of bacteria have following general characteristics:

- LABs are long or short rods or cocci.

- All LABs are gram positive.

- All are non-sporulating (except sporolactobacillus).

- Mostly these are non-motile and are catalase positive.

- This family is aerobic in nature and is highly aerotolerante.

Occurrence: They mostly occur in the nutrition rich regions as they are energy demanding group of organisms. They mostly occur in milk and the place where milk is produced and processed, dead and rotten plant bodies. And more importantly they are found in the intestinal tracts and mucosal membrane of human bodies.

Examples of LAB:

Homoermentative rods: Lactobacillus sps. eg. Lactobacillus delbreukii, L. bulgaricus, L. acidophilus.

Homofermentative cocci: Streptococcus ps. eg. S. fecalis, S. saloiovarus, S. pyogenes.

Heterofermentative rods: Lactobacillus sps. eg. L. brevis, L. fermentum, L. bifermantans.

Heterofermentative rods: Leuconostoc sps. eg. L mesenteroides,L. cremoris, L. lactis,

Lactic acid bacteria are very important ferments in dairy industries. They are used in souring of milk, clotting etc. They are dominantly used microorganisms in the production of dairy products like cheese, yoghurt etc.

Difference between lactic acid fermentation and alcohol fermentation: