Glycolysis is a term used to describe the sequential series of reactions present in a wide variety of tissues that start with a hexose sugar (glucose commonly) and end with pyruvic acid. The usual reaction steps are known as Embden – Meyerhof-Parnas (E.M.P) pathway after names of the three German scientists who traced them.
Carbohydrates are not utilized is respiration as such. As a result of inter- conversions and phosphorylation the carbohydrates first produce various types of hexose phosphates. The phosphorylation of hexose sugars is catalyzed by enzymes of transphosphorylase group with the help of ATP- the universal biological currency of energy transaction in a living cell.
1. Glucose is the respiratory substrate and is converted to Glucose-6-phosphate by reaction with ATP. This reaction is brought about by the enzyme known as hexokinase.
2. Glucose-6-phosphate changes, in the next step, to its isomer fructose-6- phosphate under the influence of the enzyme phosphohexose isomerase”
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3. Further phosphorylation of ‘Fructose-6-phosphate with the help of ATP results in the formation of Fructose-1-6-diphosphate. This reaction is catalysed by the enzyme phosphohexokinase which requires Mg++ as cofactor.
4. Under the influence of desmolytic enzyme aldolase, Fructose 1,6 diphosphate splits up to produce two molecules of triosephosphates viz. 3-phosphoglyceraldehyde (3-PGAL) and dihidroxy acetone phosphate. These two molecules are chemical isomers and are interconvertible by the enzyme triosephosphate isomerase.
5. In the next step 3-PGAL is converted into 1, 3-phosphoglyceric acid. The reaction involves the incorporation of inorganic phosphate and the oxidation of aldehyde into acid by the enzyme phosphoglyceraldehyde dehydrogenase. The co-enzyme NAD is reduced to NADH2.
6. In the next reaction, 1-3, diphosphoglyceric acid reacts with ADP and loses a phosphate group forming 3-PGA. ADP is converted to ATP. This reaction is catalysed by the enzyme phosphoglyceric transphosphorylase.
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7. In the next step of glycolysis, 3-PGA is converted to 2-PGA by the enzyme phosphoglucomutase. This is accomplished by an intramolecular shift in the position of phosphate group.
8. The last compound loses a molecule of water and changes to Phosphoenol pyruvic acid. This reaction is catalysed by the enzyme enolase.
9. The final reaction of glycolysis consists in the dephosphorylation of P.E.P. by the enzyme phosphopyruvic transphosphorylase in the presence of the co-enzyme and ADP resulting in the formation of pyruvic acid and ATP. This reaction requires Mg++ and K+ as cofactors.
It may be emphasised that the above reactions of glycolysis do not involve atmospheric oxygen. This explains how the first phase of respiration can be the same in both anaerobic and aerobic respiration.
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Pyruvic acid is a key compound in respiration. Its further breakdown proceeds in two different ways depending on the availability of oxygen.