It is the initial sequence of catabolic chemical reactions in which six carbon glucose molecules is broken down into two molecules of three carbon compound called pyruvate of pyruvic acid with net production of two molecules of ATP when glucose is completely burned in a test tube it will give about 690,000 calories of energy per mole in the form of heat. In the cell some of this energy is not lost as heat but is retained in the form of ATP.
Steps involved in the process of glycolysis:
(1) Glucose is converted to glucose – 6 – phosphate with the help of an enzyme hexokinase in the presence of ATP.
(2) Glucose – 6 – phosphate is rearranged to form its isomes fructose – 6 – phosphate with the help of an enzyme phosphogluco isomerase.
(3) Fructose – 6 – phosphate reacts with another molecule of ATP to form fructose – 1, 6 – diphosphate or hexose diphosphate with the help of an enzyme phosphor fructokinase.
(4) Fructose – 1, 6 diphosphate is then either converted into 3 – phosphoglyceral dehyde by dihydroxy acetone phosphate under the enzyme aldolase. There is established equilibrium between these compound by inter converting into one another through an enzyme isomerase – 3 – phosphoglyceral dehyde is utilized at a faster rate and when there is deficiency of this compound, then dihydroxy acetone phosphate is converted into 3 – phosphoglyceral dehyde which is processed further by glycolysis.
(5) 3 – phosphoglyceral dehyde is initially oxidized by NAD and then the inorganic phosphate present in the cytoplasm combines to form 1, 3 – phosphoglyceric acid in the presence of enzyme triose phosphate dehydrogenase.
(6) 1, 3 – phosphoglyceric acid is then converted to 3 – phosphoglyceric acid or 3 – phosphoglycerate along with the release two ATP molecules by the enzyme phosphoglycerokinase.
(7) 3 – phosphoglyceric acid or 3 – phosphoglycerate is then converted into 2 – phosphoglyceric acid or 2 – phosphoglycerate in the presence of phosphoglyceromutase.
(8) 2 – phosphoglyceric acid or 2 – phosphoglyceratase is converted into phosphoenol pyruvic acid or phosphoenopyruvate in the presence of enolase.
(9) Finally phosphoenol pyruvic acid or phosphoenylpyruvate is converted into pyruvic acid or pyruvate along with the production of two more ATP molecules by the enzyme pyruvate kinase.
Energy yielding steps are:
(1) During process of glycosis two ATP molecules were used as starter energy to convert Glycosets fructose 1 – 6 biphosphate.
(2) Two ATP molecules are formed during the conversion of 1, 3 – diphosphoglycerate to 3 – phosphoglycerate.
(3) Two additional ATP molecules are released when phosphenolpurate is converted into pyrute the end – product of glycolysis.
(4) In addition to four ATP molecules produced during glycolysis, energy rich compounds NADH2 is also produced which is used to make the ATP by oxidative phosphorylation.
End result of Glycolysis: All the reactions of glycolysis are performed by soluble enzymes that are present in the cytosol and are not found in mitochondrion. Two moles of ATP are needed to start of glycolysis of glucose and during the oxidative reaction of glycolysis these two moles of ATP are regained. The NADH2 formed in above reactions is oxidized via the electron transport chain to form three more ATP molecules. The end result of glycolysis is the formation of two moles of pyruvic acid from each mole of glucose together with two moles of ATP. Although glycolysis does not efficiently harvest all the available energy from glucose, it was the only way most organisms could harvest energy and generate ATP molecules for hundreds of millions of years during the anaerobic stages of early life on earth.
Evolutionary Perspectives on Glycolysis:
All forms of animal life including man carry on glycolysis within their cells, a metabolic memory of animals’ evolutionary past – if glycolysis is such an inefficient method of harvesting energy, why has it persisted?
One reason might be that evolution is slow, incremental process involving change based on past events when glycolysis first evolved the cells possessing it had competitive advantage over these that did not.
Importance of Glycolysis as observed through biochemistry:
Biochemistry of contemporary organisms indicates that only those organisms capable of glycolysis survived the early competition of life on earth. Later on the evolutionary changes in catabolism build on this success. During this building process, glycolysis was not discarded but used as a stepping stone for the evolution of another process for complete breakdown of glucose.
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