Glycolysis is derived from the Greek words (glykys = sweet and lysis = splitting). It is a universal catabolic pathway in the living cells. Glycolysis can be defined as the sequence of reactions for the breakdown of Glucose (6-carbon molecule) to two molecules of pyruvic acid (3-carbon molecule) under aerobic conditions; or lactate under anaerobic conditions along with the production of small amount of energy. This pathway was described by Embden, Meyerhof and Parnas. Hence, it is also called as Embden-Meyerhof pathway (EM pathway).
Site of Glycolysis:
Glycolysis occurs in the cytoplasm of virtually all the cells of the body.
Types of Glycolysis:
There are two types of glycolysis.
- Aerobic Glycolysis: It occurs when oxygen is plentiful. Final product is pyruvate along with the production of Eight ATP molecules.
- Anaerobic Glycolysis: It occurs when oxygen is scarce. Final product is lactate along with the production of two ATP molecules.
Steps of Glycolysis:
Glycolysis is an extramitochondrial pathway and is carried by a group of eleven enzymes. Glucose is converted to pyruvate in 10 steps by glycolysis. The glycolytic patway can be divided into two phases:
Preparatory Phase :
This phase is also called glucose activation phase. In the preparatory phase of glycolysis, two molecules of ATP are invested and the hexose chain is cleaved into two triose phosphates. During this, phosphorylation of glucose and it’s conversion to glyceraldehyde-3-phosphate take place. The steps 1, 2, 3, 4 and 5 together are called as the preparatory phase.
Payoff Phase :
This phase is also called energy extraction phase. During this phase, conversion of glyceraldehyde-3-phophate to pyruvate and the coupled formation of ATP take place.
Because Glucose is split to yield two molecules of D-Glyceraldehyde-3-phosphate, each step in the payoff phase occurs twice per molecule of glucose. The steps after 5 constitute payoff phase.
Step 1 : Uptake and Phosphorylation of Glucose
- Glucose is phosphorylated to form glucose-6-phosphate.
- The reaction is catalysed by the specific enzyme glucokinase in liver cells and by non specific enzyme hexokinase in liver and extrahepatic tissue. The enzyme splits the ATP into ADP, and the Pi is added onto the glucose.
- Hexokinase is a key glycolytic enzyme. Hexokinase catalyses a regulatory step in glycolysis that is irreversible.
- Hexokinase, like many other kinases, requires Mg2+ for its activity.
Step 2 : Isomerization of Glucose-6-Phsphate to Fructose-6-Phosphate
- Glucose-6-phosphate is isomerised to fructose-6-phosphate by phosphohexose isomerase.
- This reaction involves an aldose-ketose isomerisastion catalysed by phosphohexose isomerase. There is opening of the glucopyranose ring of glucose-6-phosphate to a linear structure which then changes to the furanose ring structure of fructose-6-phosphate.
STEP 3 : Phosphorylation of F-6-P to Fructose 1,6-Biphosphate
- Fructose-6-phosphate is further phosphorylated to fructose 1,6-bisphosphate.
- The enzyme is phosphofructokinase-1. It catalyses the transfer of a phosphate group from ATP to fructose-6-phosphate.
- The reaction is irreversible.
- One ATP is utilised for phosphorylation.
- Phosphofructokinase-1 is the key enzyme in glycolysis which regulates breakdown of glucose.
Step 4 : Cleavage of Fructose 1,6-Biphosphate
- The 6 carbon fructose-1,6-bisphosphate is cleaved into two 3 carbon units; one glyceraldehyde-3-phosphate (GAP) and another molecule of dihydroxy acetone phosphate (DHAP).
- The enzyme which catalyses the reaction is aldolase. Since the backward reaction is an aldol condensation, the enzyme is called aldolase.
- The reaction is reversible.
Step 5 : Interconversion of the Triose Phosphates
- GAP is on the direct pathway of glycolysis, whereas DHAP is not. Hence Triose-phosphate isomerase converts DHAP into GAP useful for generating ATP. Thus net result
- is that glucose is now cleaved into 2 molecules of glyceraldehyde-3-phosphate.
- This reaction is rapid and reversible.
Step 6 : Oxidative phosphorylation of GAP to 1,3-Bisphosphoglycerate
- This reaction is catalyzed by glyceraldehyde 3-phosphate dehydrogenase.
- It is the energy-yielding reaction. Reactions of this type in which an aldehyde group is oxidised to an acid are accompanied by liberation of large amounts of potentially useful energy. During this reaction, NAD+ is reduced to NADH.
- This is a reversible reaction.
Step 7 : Conversion of 1,3-Biphosphoglycerate to 3-Phosphoglycerate
- The enzyme phosphoglycerate kinasetransfers the high-energy phosphoryl group from the carboxyl group of 1,3-bisphosphoglycerate to ADP, forming ATP and 3-phosphoglycerate.
- This is a unique example where ATP can be produced at substrate level without participating in electron transport chain. This type of reaction where ATP is formed at substrate level is called as Substrate level phosphorylation.
Step 8 : Conversion of 3-Phosphoglycerate to 2-Phosphoglycerate
- 3-phospho glycerate is isomerized to 2-phospho glycerate by shifting the phosphate group from 3rd to 2nd carbon atom.
- The enzyme is phosphogluco mutase.
- This is a readily reversible reaction.
- Mg2+ is essential for this reaction.
Step 9 : Dehydration of 2-Phosphoglycerate to Phosphoenolpyruvate
- 2-phosphoglycerate is converted to phosphoenol pyruvate by the enzyme enolase.
- One water molecule is removed.
- A high energy phosphate bond is produced. The reaction is reversible.
- Enolase requires Mg++.
Step 10 : Conversion of Phosphoenol Pyruvate to Pyruvate
- Phosphoenol pyruvate (PEP) is dephosphorylated to pyruvate, by pyruvate kinase.
- First PEP is made into a transient intermediary of enol pyruvate; which is spontaneously isomerized into keto pyruvate, the stable form of pyruvate.
- One mole of ATP is generated during this reaction. This is again an example of substrate level phosphorylation.
- The pyruvate kinase is a key glycolytic enzyme. This step is irreversible.