Degradation of Glucose to Pyruvate
Glycolysis definition:- In all living organisms, the initial phase of respiratory metabolism involves degradation of glucose in to pyruvate. The steps involved in the degradation were studied and explained by three biochemists namely Emden, Meyerhof and Parnas, therefore, it is also called EMP pathway. The enzymes for breakdown of glucose are present in the cytosol (the soluble part of cytoplasm is called cytosol) mainly. Thus, glycolysis is not associated with any particular organelle. The oxygen is not required to convert glucose to pyruvate and some energy is also produced during the process. Therefore, the glycolysis serves as primary means of energy production in plant tissues where oxygen levels are low; for example, in roots growing in water saturated soils (flooded soil).
Glycolytic Pathway – Biochemistry of Glycolysis
It is an oxidation reduction process in which one glucose molecule is broken down in to two molecules of pyruvate acid and net two ATP are produced.
The glucose and fructose can readily enter the pathway. These are derived from the breakdown of either sucrose or starch. Thus, following steps are involved in breakdown of glucose to pyruvate.
Phosphorylation of Glucose
The glucose is phosphorylated to glucose 6-phosphate in the presence of ATP and hexokinase.
Isomerization of Glucose 6-P
The glucose 6-phosphate is isomerized to fructose 6-phosphate by hexose-phosphate isomerase. Phosphorylation of fructose 6-P: Fructose 6-P is phosphorylated to fructose 1,6-bisphospahte in the presence of ATP and enzyme phosphofructokinase.
Cleavage of Fructose 1,6-bisP
Fructose 1,6-bisP is cleavage by two molecules of 3-C isomers which are interconvertible, glyceraldehyde 3-phosphate (phosphoglyceraldehyde) and dihydroxyacetone phosphate.
Dehydrogenation of Glyceraldehyde 3-P
In the presence of ATP and NAD+, the aldehyde group of (-CHO) of glyceraldehyde 3-P is oxidized to carboxyl group (-COOH) to produce 1,3-bisphosphoglyceric acid. The H+ released reduces NAD+ to NADH2.
Dephosphorylation of 1, 3-bisPGA & Formation of ATP
1,3-bisphosphoglyceric acid is converted to 3-phosphoglyceric acid by phosphoglycerate kinase, generating two ATP molecules.
Formation of 3-Phosphoglyceric Acid
1,3-phosphoglyceric acid mutates two 2-phosphoglyceric acid under the catalytic activity of phosphoglycerate mutase.
Formation of Phosphoenolpyruvate
Enolase converts 2-phosphoglyceric acid to phosphoenolpyruvate.
Formation of pyruvate & ATP Generation
Phosphoenolpyruvate (PEP) is converted into pyruvate by pyruvate kinase. Two molecules of ATP are generated during this step.
Glycolyses Products are following:-
- 2 molecules of pyruvate
- 2 molecules of NADH2
- 4 ATP molecules Total
two molecules of ATP are consumed during the reaction, therefore, there is net gain of 2 ATP molecules.