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Saturday, January 4, 2020

Glycolysis


Glycolysis is derived from greek words Glyks and Lysis. Glyks = Sweet and Lysis = Splitting.
Glycolysis: Glyco = Glucose, Lysis = Breakdown.

Glycolysis is the metabolic pathway involves the splitting / breakdown of Glucose (6 carbon compound) systematically to two 3 carbon compound, Pyruvate. Glycolysis takes place in the cytosol of the cell.

Overview of Glycolysis

In Glycolysis, Glucose, which is a 6 carbon compound is broken down to 2 molecules of pyruvate, pyruvate is a 3 carbon compound. During this process energy is released in the form of ATP and NADH. The pyruvate generated from glycolysis enter other metabolic pathways to generate more energy.

The process of glycolysis can be explained in Two Phases: Phase 1 & Phase 2.

In Phase 1, Glucose is converted to two molecules of Glyceraldehyde 3 Phosphate, energy is utilized in this step as there is addition of phosphate.

In Phase 2, Glyceraldehyde - 3- phosphate is converted to Pyruvate.

Glycolysis

Step1: Glucose is converted to Glucose – 6 – Phosphate. This reaction is catalysed by the enzyme hexokinase. Kinases are the group of enzymes that catalyses phosphorylation. Hexokinase transfers phosphate from ATP to glucose converting it to Glucose 6 Phosphate. Since ATP is converted to ADP it is an energy utilizing reaction.


Step2: Glucose – 6- phosphate is converted to Fructose – 6- Phosphate. This is a simple rearrangement reaction. This is catalysed by the enzyme Isomerase.



Step3: Fructose – 6- Phosphate is converted to Fructose – 1, 6- bisphosphate. Addition of phosphate group to the 1st carbon of fructose ring. The reaction is catalysed by phosphor Fructokinase. Since ATP is converted to ADP it is an energy utilizing reaction.


Step4 & 5: Fructose – 1, 6- bisphosphate is converted to glyceraldehyde 3 phosphate and dihydroxyacetone phosphate.  This reaction is catalysed by the enzyme aldolase.


Glyceraldehyde 3 phosphate and dihydroxyacetone phosphates are isomers and are interconvertible by the enzyme isomerase. Since glyceraldehyde 3 phosphate enters the glycolysis pathway, the equilibrium of isomerization reaction is always towards glyceraldehyde 3 phosphate.



Every molecule of glucose is split into 2 molecules of pyruvate, so every reaction of second phase happens twice and produce twice the products.


Step6: Glyceraldehyde – 3 – phosphate is converted to 1, 3-bisphosphoglycerate. This reaction is catalysed by the enzyme glyceraldehyde 3 phosphate dehydrogenase. Phosphate group is added to the 1st position of the phosphoglycerate. In this step NAD is reduced to NADH and which enters the electron transport chain to generate more energy. Also this step utilizes a molecule of inorganic phosphate.


Step7: 1, 3-bisphosphoglycerate is converted to 3 phosphoglycerate. This reaction is catalysed by the enzyme phosphoglycerate kinase. The kinase enzyme transfers the phosphate group at the first position of phosphoglycerate to adenosine di phosphate and an ATP molecule is generated in this process.



Step8:  3 phosphoglycerate is converted to 2 phosphoglycerate. This is a rearrangement reaction catalysed by phosphoglycerate mutase.




Step9: 2 phosphoglycerate is converted to Phosphoenol pyruvate. This reaction is catalysed by the enzyme enolase. Mg2+ is utilized in this reaction.
  


Step10: Phosphoenol pyruvate to Pyruvate. This reaction is catalysed by the enzyme pyruvate kinase. The phosphate group of the Phosphoenol pyruvate is transferred to ADP to generate a molecule of ATP.



Net Reaction of Glycolysis

1 molecule of 6 carbon glucose converted to 2 molecules of 3 carbon pyruvate.
2 ADP, 2 NAD+ and 2 Inorganic phosphates are utilized in net and 2 ATP & 2 NADH are produced.