What is Glycolsis ? Definition, Important Points

Glycolysis is a sequence of reaction which converts glucose and related hexose into two molecule of pyruvate with net production of 2 ATP molecules.It is the most important pathway in energy metabolism present in both aerobic and anaerobic organisms.The cycle is completed in 10 steps with the help of enzymes present in the cytoplasm.None of the reactions are oxygen dependent. in evolutionary terms it is regarded as a primitive pathway.

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Highlights of glycolysis

Glycolysis is also called EMP pathway as elucidated by the scientist namely Embden, Meyerhof and Parnas.
It is a major pathway for the production of ATP in tissues lacking mitochondria: example RBC cells and cornea of the eye
Glycolysis can occur in the absence of Oxygen anaerobic in such condition lactate instead of pyruvate ‚is a major end product.
Many of the intermediates of this part we provide branch point to other Pathways,Example dihydroxyacetone phosphate is a source of glycerol that combines with fatty acid to form triglycerides.
All the intermediates of the pathway are phosphorylated and what water soluble.
Only 3 reaction out of 10 are Irreversible in this pathway.

This pathway can be divided into three distinct phases

A. Energy investment phase

B. Splitting phase

C. Energy generation phase

A. Energy investment phase :

This phase involves reaction which require input of energy in the form of ATP. this energy is initially consumed by the cell to activate the glucose and form high energy intermediate in the form of phosphorylated products.The situation is similar to our day to day life experiences as we invest small amount of money to get better returns later. this phase involve following reactions:

Glucose is irreversibly phosphorylated to glucose 6 phosphate by hexokinase or glucokinase in the presence of magnesium ions .One molecule of ATP is utilised in the step.Glucokinase having (low affinity for glucose) present in liver catalyses phosphorylation of only glucose.Similarly hexokinase (having high affinity for glucose )present in almost all the tissues catalyzes phosphorylation of various hexose (fructose, glucose, mannose etc)
Glucose 6 phosphate undergoes isomerization to produce Fructose 6 phosphate in the presence of phosphohexose and magnesium ion.
Fructose-6-phosphate is irreversibly phosphorylated by phosphofructokinase in to Fructose 1, 6 bis phosphate. in this reaction one molecule of ATP is consumed. this reaction is regulatory step in glycolysis.

B. Splitting Phase:

This phase lead to splitting of high energy 6 carbon intermediate into two carbon products involving the following reactions:

Enzyme aldolase catalyzes the splitting of fructose 1 ‚6 bi phosphate ( 6 C) into glyceraldehyde 3 phosphate( 3C) and dihydroxyacetone phosphate( 3C).
The enzyme Phosphotriose isomerase catalyst interconversion of glyceraldehyde 3 phosphate and dihydroxyacetone phosphate. Thus, two molecules of glyceraldehyde 3 phosphate are produced from one molecules of glucose.

C. Energy generation phase:

This Phase includes reaction which finally return the principal energy initially consumed by cell with interest and involves the following reactions:

Enzyme glyceraldehyde 3 phosphate dehydrogenase converts glyceraldehyde 3‑phosphate into 1, 3 by phosphoglycerate. In this reaction ‚NADH + H ⁺ are produced and it in turns is oxidised via electron transport chain to synthesize ATP molecules.
Phosphoglycerate kinase catalyzed Irreversible conversion of 1,3 bi phosphoglycerate into three phosphoglycerate. In this process substrate level phosphorylation occurs at ATP is synthesized directly from the substrate without involving ETC.
3‑phosphoglycerate is isomerized into 2- phosphoglycerate by phosphoglycerate mutase.
Enzyme enolase catalyse the conversion of 2 — phosphoglycerate into phosphoenol pyruvate.This reaction requires Mg²⁺ or Mn²⁺ and is inhibited by fluoride.
The enzyme pyruvate kinase catalyzes Irreversible production of pyruvate from phosphoenol pyruvate.This reaction is also an example of substrate level phosphorylation where ATP is produced without involvement of ETC.

Energy investment phase involve reaction for conversion of glucose to fructose 1,6 bisphosphate with investment of two ATP molecules. Splitting phase results in splitting of 6 C molecule to two 3 C molecule which can interconvert . Energy generation phase. Proceeds with two 3 C molecules , therefore, all reaction will be doubled . it results in generation of energy rich molecules ; 4 ATP by substrate level phosphorylation and 2 NADH.

Overall Reaction

Glucose + 2ADP + 2 pi + 2NAD⁺——-> 2pyruvate + 2ATP + 2 NADH + 2 H ⁺

Sugars other than glucose such as fructose, galactose are converted into one of the intermediate of glycolysis.

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