Glycolysis

Glycolysis

The most common and important pathway for glucose catabolism is Embedded-Mayer Hoff pathway of glycolysis. This process occurs very widely and it is present in the microorganism as well as in animals and plant. the glycolysis which does not require oxygen and therefore it occurs in both aerobic and anaerobic cells because it can degrade glucose in presence or absence of oxygen. Aerobic cells degrade glucose by glycolysis and this process constitutes the preparatory stage for the aerobic phase of glucose oxidation. Thus,under aerobic conditions this situation prevails.

Glucose→Fermentation product

Glucose→Intermediate→CO₂+H₂O

This is the process in which, fructose 1, 6-diphosphateformed from glucose is divided into 3-carbon units (dihydroxyacetone phosphate and glyceraldehyde-3-phosphate), and they are consequently oxidised to pyruvic acid. In glycolysis glyceraldehyde-3-phosphate is oxidised, a pair of the electron may be used to reduce pyruvic acid to lactic acid or ethanol. ,this pairs of electrons may enter the respiratory chain. Many of reaction of glycolytic pathway are freely reversible and can be used for the synthesis of glucose as well as for its breakdown in the presence of oxygen. In glycolysis Only three of the reaction are non-reversible by the common enzyme, but the presence of another enzyme can reverse them for glucose synthesis to occur. Thus phosphoenolpyruvate is synthesised from pyruvate by the action of phosphoenol synthase and specific phosphatase hydrolyze fructose-1, 6-diphosphate and glucose-6-phosphate in the biosynthetic direction. The enzyme at these steps in the degradative direction are kinase and required ATP.For each molecule of glucose metabolised, two molecules of ATP are used up and four molecules of ATP are formed. Therefore for each molecule of glucose metabolised by glycolysis, there is a net yield of two ATP molecules. The overall reaction of glycolysis can be summarised as follows:

C₆H₁₂O₆+2NAD+2ADP+2P1→2CH₃COCOOH+2NADH₂+2ATP

Many chemoheterotrophs can dissimulate monosaccharides, especially the six-carbon glucose. Although different kinds of the pathway for glucose dissimilation may occur, the most common pathway is glycolysis. This pathway is found in many microorganisms as well as in animals and plants. The most important features of glycolysis are:

1. Two molecules of ATP are used for the conversion of glucose to fructose-1,6-diphosphate.
2. A total of four molecules of ATP is produced by substrate-level phosphorylation.

Two ATP molecules are produced during the conversion of two molecules of 1,3-diphosphoglyceric acid to two molecules of 3-phosphoglyceric acid. The other living organisms use one of two methods to regenerate NAD from NADH₂ –namely, fermentation and respiration.

Fermentation is an oxygen-independent process in which the NADH₂ that is produced during glycolysis or another dissimilatory pathway is used to reduce an organic electron acceptor made by the cell itself. For instance, when yeast cells are grown with glucose under anaerobic conditions, they carry out an alcoholic fermentation. After making pyruvic acid by glycolysis, they remove a molecule of CO₂ from the pyruvic acid to form acetaldehyde:

2 pyruvic acid →2 acetaldehyde +2 CO₂

Then the yeast cells use the acetaldehyde as an acceptor for the electrons of the NADH₂ that was produced during glycolysis. The acetaldehyde oxidises the NADH₂ from glycolysis and becomes reduced to ethanol (ethyl alcohol), thus regenerating NAD:

2 acetaldehyde + 2 NADH₂→ 2 ethanol + 2NAD

The ability of yeasts to carry out alcoholic fermentation is the basis for the alcoholic beverage industry. Other microorganisms use different fermentation to regenerate NAD. For instance, streptococcus lactic carries out a lactic acid fermentation by using pyruvic acid itself as the electron acceptor:

2 pyruvic acid + 2NADH₂ → 2 lactic acid +2 NAD

The ability of S. lactose to produce lactic acid as a fermentation product is of great importance in the dairy industry. Many other types of fermentations may be carried out by bacteria that lead to various end products. Knowledge of the kinds and amounts of the products made by a particular bacterium is often helpful in identifying that bacterium. Moreover, some fermentation products (such as acetone, isopropanol , butanol , propionic acid and butyric acid ) are useful in industry. However, fermentation products are toxic waste substances as far as the cells that they are concerned. For instance , the ethanol contains nature wine , seldom, exceeds 12% , because thus level up ethanol poison the yeast cell and prevents them from making them additional alcohol. fermentation is the very inefficient process for excepting energy because the end products still contain a great deal of chemical energy. respiration is the process of regenerating NAD by using NADH₂ as the electron donor for an electron transport system. if oxygen is the terminal electron acceptor for the electron transport system the process is called aerobic respiration. the dissimilation of glucose by aerobic organisms do not normally stop the production of pyruvic acid. further breakdown begins with the oxidation of pyruvic acid by NAD to acetyl CoA (a two carbon acid, acetic acid, linked to coenzyme A). each of the two resulting molecules of NADH₂ can serve as the electron donor for electron transport system with consequent ATP synthesis.

References

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.

Debey, RC and D K Maheshwari. A textbook of Microbiology. India: s.chand and company Ltd. 1999.