Glycolysis Pathway And Sequence Of Reactions Explained

INTRODUCTION:

Glycolysis is derived from the Greek words: Glycose=sweet or sugar and Lysis=splitting or dissolution. It is a pathway in all the living cells. The complete pathway was elucidated in the year 1940. This pathway is also known as Embden-Meyerhof pathway (E.M.pathway) in honour of the two biochemists who made major contribution to the process of Glycolysis.

DEFINITION:

Glycolysis is defined as the sequence of reactions converting glucose(or glycogen) to pyruvate or lactate with the production of ATP.

PURPOSE:

Conversion of glucose to pyruvate. Pyruvate is further processed for the generation of ATP. (ATP- Adenosine Tri- Phosphate, it is the energy currency of the cell. Splitting of one mole of ATP gives energy of 7.3 calories. This energy is used by the cell for various purposes).

SITE:

Glycolysis occurs in the cytoplasm of the cell.

SALIENT FEATURES:

1. It occurs in presence of oxygen as well as absence of oxygen.

a. Aerobic Glycolysis-

It occurs in the presence of oxygen.

End product is Pyruvate.

Energy produced- 8 ATP

b. Anaerobic Glycolysis-

It occurs in the absence of oxygen.

End product is Lactate.

Energy produced- 2 ATP.

2. It is a major pathway for ATP synthesis in tissues and cells lacking Mitochondria. Examples: RBC, Cornea, Lens.

3. Brain uses nearly two-thirds of the total blood glucose. Here, glycolysis plays a major role in energy production for the cells in the Brain.

4. Intermediates of glycolysis are used by the cell for the synthesis of amino Acid and Fat.

GLYCOLYSIS PATHWAY:

The pathway (sequence of reactions) are divided into 3 distinct phases:

Phase 1: Energy Investment Phase.

Phase 2: Splitting Phase.

Phase 3: Energy Generation Phase.

These three phases together constitute 10 reactions.

AN OVERALL PICTURE OF GLYCOLYSIS:

GLUCOSE

1 ↓ HEXOKINASE OR GLUCOKINASE

GLUCOSE 6-PHOSPHATE

2 ↓ PHOSPHOHEXOSE ISOMERASE

FRUCTOSE 6-PHOSPHATE

3 ↓ PHOSPHOFRUCTOKINASE

FRUCTOSE 1, 6-BISPHOSPHATE

4 ↓ ALDOLASE↓

5 DHAP ↔ GLYCERALDEHYDE 3-PHOSPHATE

PHOSPHOTRIOSE ISOMERASE

6 ↓ GLYCERALDEHYDE 3-PHOSPHATE DEHYDROGENASE

1, 3 -BISPHOSPHOGLYCERATE

7 ↓ PHOSPHOGLYCERATE KINASE

3-PHOSPHOGLYCERATE

8 ↓ PHOSPHOGLYCERATE MUTASE

2-PHOSPHOGLYCERATE

9↓ ENOLASE

PHOSPHOENOL PYRUVATE

10 ↓ PYRUVATE KINASE

PYRUVATE

DHAP (Dihydroxy Acetone Phosphate)

Reactions 1, 2, 3, 4 constitute energy Investment Phase

Reaction 5 constitutes Splitting Phase

Reaction 6, 7, 8, 9, 10 constitute Energy generation phase

(The names of the enzymes catalyzing the respective reactions are given on

the arrows)

REACTIONS IN DETAIL:

1. PHOSPHORYLATION: Glucose is phosphorylated to glucose 6-phosphate by hexokinase or glucokinase (both are Iso-Enzymes). This is an irreversible reaction. ATP and Magnesium ion are needed for the reaction to proceed.

2. ISOMERISATION: Glucose 6-phosphate is isomerised to fructose 6-phosphate in the presence of enzyme phosphohexose isomerase and Magnesium ion.

3. PHOSPHORYLATION: Fructose 6-phosphate is phosphorylated to fructose 1,6-bisphosphate by phosphofructokinase enzyme. This is an irreversible and regulatory step of glycolysis.

4. CLEAVAGE: The six carbon fructose 1, 6- bisphosphate is split into glyceraldehyde 3- phosphate and dihydroxy acetone phosphate(DHAP). Both of them are 3 carbon compounds. Aldolase is the enzyme involved in the reaction.

5. ISOMERISATION: DHAP is isomerised to Glyceraldehyde 3-phosphate by enzyme Phosphotriose Isomerase. So, now there are 2 molecules of glyceraldehyde 3-phosphate available.

6. OXIDATION: Glyceraldehyde 3-phosphate is oxidised to 1,3 bisphosphoglycerate by the enzyme glyceraldehyde 3-phosphate dehydrogenase(G-3-P dehydrogenase). Here one molecule of NADH is generated from one molecule of NAD+. This NADH participates in Electron Transport Chain to produce ATP.

7. DEPHOSPHORYLATION: 1,3 Bisphosphoglycerate is converted to 3-phosphoglycerate by the enzyme phosphoglycerate kinase. Here one molecule of phosphate from the substrate is lost. This phosphate is taken up by a molecule of ADP to produce one molecule of ATP. This is a SUBSTRATE LEVEL PHOSPHORYLATION. This is a rare example of a reversible kinase reaction. Magnesium ion is also required in this reaction.

8. ISOMERISATION: 3-phosphoglycerate is converted to 2-phosphoglycerate by the enzyme phosphoglycerate mutase. This is an isomerisation reaction.

9. DEHYDRATION: 2-phosphoglycerate is converted to Phosphoenol Pyruvate by removal of one molecule of Water by the action of the enzyme Enolase. For this reaction Magnesium ion or Manganese ion are needed. Phosphoenol Pyruvate is a high energy compound.

10. DEPHOSPHORYLATION: Phosphoenol Pyruvate is converted to Pyruvate by the removal of one molecule of Phosphate(Pi) which is taken up by one molecule of ADP to produce one molecule of ATP. This is also a SUBSTRATE LEVEL PHOSPHORYLATION. Pyruvate Kinase is the enzyme involved. The enzyme needs Potassium ion and either Magnesium ion or Manganese ion. The reaction is irreversible.

ENERGETICS OF GLYCOLYSIS:

1. 1 molecule of NADH participates in ETC(Electron Transport Chain) to liberate 3 ATP. There are 2 NADH produced from 2 molecules of Glyceraldehyde 3-phosphate so liberating 6 ATP.

2. 1 molecule of ATP is produced by Substrate Level Phosphorylation in reaction 7. Total 2 ATP are produced since there are 2 Glyceraldehyde 3-Phosphate molecules.

3. In reaction 10, again there is a Substrate Level Phosphorylation from Phosphoenol Pyruvate. As there are 2 Phosphoenol Pyruvate molecules produced from 2 molecules of Glyceraldehyde 3- Phosphate, 2 ATP are liberated.

4.In Energy investment phase 2 ATP are used. 1 ATP in reaction 1 and 1 ATP in reaction 3.

So, Net ATP Synthesis in glycolysis is 8 ATP

(2NADH = 6 ATP

Reaction 7 = 2 ATP

Reaction 10 = 2ATP

Total = 10 ATP

ATP used = 2 ATP (in energy investment phase)

Net Production= 10-2=8 ATP)

NOTE:

ANAEROBIC GLYCOLYSIS:

Here pyruvate is converted to lactate by lactate dehydrogenase. Here one NADH is converted to NAD. That means the NADH produced in reaction 6 is not used for ATP production. Hence here only 2 ATP are produced.

(2 NADH =0 ATP

Reaction 7=2 ATP

Reaction 10=2ATP

ATP Used = 2 ATP (in energy investment phase)

Net Production=4-2=2ATP)

FATE OF PYRUVATE: The Pyruvate produced as the end product of glycolysis undergoes oxidative decarboxylation and thereby forms Acetyl Co-A which is used in Citric Acid Cycle to generate ATP.

INHIBITORS OF GLYCOLYSIS:

1. Iodoacetate and Arsenate inhibit the enzyme Glyceraldehyde 3-Phosphate Dehydrogenase of reaction 6.

2.Flouride inhibits Enolase of reaction 9.

GLYCOLYSIS RELATED DETAILS:

1.Lactic Acidosis: Accumulation of Lactic acid is seen in its excess production due to anaerobic glycolysis, example: In skeletal muscle during strenuous exercise. Lactic acidosis may also be due to its decreased elimination. In Lactic Acidosis ATP production is reduced. Here the reason is lack of Oxygen supply. It may cause pain in muscles during severe exercise. Normal plasma lactic acid concentration- 4-15 mg/dl.

2.Oxygen Debt: It is the extra amount of Oxygen required to recover from anaerobic glycolysis.

3.Cancer and Glycolysis: During cancer there is excessive proliferation of cells. The excess number of cells show an increased uptake of glucose and thereby glycolysis. As the tumor grows in size there is increased demand of Oxygen by the tumor, which the blood vessels are unable to supply. So, Hypoxic conditions get established in the tumor. So, anaerobic glycolysis increases.

Later the tumor cells get accustomed to hypoxia by involvement of transcription factor named Hypoxia Inducible Transcription Factor(HIF). HIF increases the synthesis of glycolytic enzymes and glucose transporters.

However the tumors cannot survive for long under these hypoxic conditions. So, a method of treatment of cancer is to reduce the vascularisation to the tumor so that hypoxia prevails and the cancers can be eliminated.

4.Pasteur Effect: The inhibition of Glycolysis by Oxygen (aerobic condition) is known as Pasteur effect. It is due to inhibition of the enzyme Phosphofructokinase of reaction 3 by the effect of ATP produced in the presence of Oxygen via Glycolysis.

5.Crabtree Effect: The phenomenon of inhibition of Oxygen uptake by the addition of glucose to the tissues having high aerobic glycolysis. It is because when glucose is added to a tissue having high aerobic. Glycolysis more glycolysis occurs leading to production of more ATP. So, the need of Oxygen for producing ATP via Citric Acid Cycle is reduced and hence, Oxygen consumption is decreased.