EMP Pathway

The EMP pathway, or Embden-Meyerhof-Parnas pathway, is a metabolic pathway in cellular respiration and glycolysis. The EMP pathway is responsible for the breakdown of glucose into pyruvate, generating energy in the form of ATP and NADH. It occurs in the cytoplasm of cells. It is a common pathway for aerobic as well as anaerobic respiration. In glycolysis, a chain of ten reactions, under the control of different enzymes, takes place to produce pyruvate from glucose.

1.0Introduction

• The term glycolysis has originated from the Greek words, glycos for sugar, and lysis for splitting. It  was initially researched by three notable German scientists: Gustav Embden, Otto Meyerhof, and Jakub Karol Parnas, and is often referred to as the EMP pathway. In anaerobic organisms, it is the only process in respiration. This biochemical process is a vital component of cellular respiration, taking place within the cytoplasm of a cell and is present in all living organisms.
• Glycolysis serves as a fundamental pathway in both aerobic and anaerobic respiration, playing a central role in energy production for the cell.  In this process, glucose undergoes partial oxidation to form two molecules of pyruvic acid. In plants, this glucose is derived from sucrose, which is the end product of photosynthesis, or from storage carbohydrates. Sucrose is converted into glucose and fructose by the enzyme, invertase, and these two monosaccharides readily enter the glycolytic pathway. 

2.0Steps of EMP Pathway / Glycolysis

• To enable the carbon stored in carbohydrates to enter glycolysis, the glucose and fructose resulting from the breakdown of starch or sucrose must undergo conversion into hexose phosphates. Within the cytosol, the pool of hexose phosphates comprises glucose-1-phosphate, glucose-6-phosphate, and fructose-6-phosphate. Glycolysis is a 10 steps process completed in the following steps-

Step 1. Phosphorylation of Glucose: 

Glucose and fructose are phosphorylated to give rise to glucose-6- phosphate by the activity of the enzyme hexokinase. In this process one molecule of ATP is converted to ADP.

Image Source: Lehninger Principles of Biochemistry

Step 2. Isomerization:

This phosphorylated form of glucose then isomerises to produce fructose-6- phosphate  by the enzyme phosphoglucose isomerase. Subsequent steps of metabolism of glucose and fructose are the same.

Image Source: Lehninger Principles of Biochemistry

Step 3. Second Phosphorylation:

Another ATP molecule is used to phosphorylate fructose-6-phosphate, forming fructose-1,6-bisphosphate with the help of the enzyme phosphofructokinase.

Image Source: Lehninger Principles of Biochemistry

Step 4. Cleavage: 

Fructose-1,6-bisphosphate is split into two three-carbon molecules: Dihydroxyacetone phosphate (3-DHAP) and Glyceraldehyde-3-phosphate (3-PGAL) by the enzyme aldolase. 

Image Source: Lehninger Principles of Biochemistry

Step 5. Interconversion:

With the help of enzyme isomerase Dihydroxyacetone phosphate isomerized into another molecule of Glyceraldehyde-3-phosphate.

Image Source: Lehninger Principles of Biochemistry

Step 6. Oxidative Phosphorylation of Glyceraldehyde 3-phosphate

Glyceraldehyde-3-phosphate undergoes oxidation, generating NADH and producing 1,3-bisphosphoglycerate by the enzyme dehydrogenase. 

Image Source: Lehninger Principles of Biochemistry

Step 7. Substrate level phosphorylation

Subsequently, 1,3-bisphosphoglycerate donates a phosphate group to ADP, creating ATP and forming 3-phosphoglycerate by the enzyme phosphoglycerate kinase. At this stage, energy in the form of ATP is produced by substrate-level phosphorylation, forming ATP from ADP.

Image Source: Lehninger Principles of Biochemistry

Step 8. Isomerization of 3-phosphoglycerate

Mutase acts, transferring a phosphate group from 3-phosphoglycerate to ADP, creating ATP and forming 2-phosphoglycerate.

Image Source: Lehninger Principles of Biochemistry

Step 9. Dehydration 2-phosphoglycerate

The enzyme enolase facilitates the conversion of 2-phosphoglycerate to phosphoenolpyruvate.

Image Source: Lehninger Principles of Biochemistry

Step 10. Substrate level phosphorylation and formation of  pyruvate:

Lastly, phosphoenolpyruvate donates a phosphate group to ADP, yielding ATP and pyruvate through the action of pyruvate kinase.

Image Source: Lehninger Principles of Biochemistry

3.0Diagram of EMP Pathway

4.0Summary of EMP Pathway or Glycolysis 

• Glycolysis takes place inside the cytoplasm of all living cells. It is a chain process of ten chemical reactions, where 1, 3 and 10 reactions are irreversible. Glucose undergoes partial breakdown / oxidation to form two molecules of pyruvate. 
• Overall reaction of glycolysis : 

Glucose (6 carbons) + 2 NAD⁺ + 2 ATP + 4 ADP + 2 Pi → 2 Pyruvate (3 carbons) + 2 NADH + 4 ATP + 2 ADP 

5.0Significance of EMP Pathway or Glycolysis 

• Energy Production: Glycolysis is a central pathway for energy generation in cells. It breaks down glucose to produce ATP (adenosine triphosphate), the primary energy currency of cells. 
• Universal Pathway: It is a highly conserved pathway found in almost all organisms, from bacteria to humans, indicating its fundamental importance in cellular metabolism.
• Anaerobic Energy Production: Glycolysis is the primary energy-generating process in anaerobic respiration 
• Intermediary Metabolism: It serves as a crucial link between different metabolic pathways.
• Generation of Precursors: Besides energy production, glycolysis produces intermediate metabolites that serve as precursors for other biosynthetic pathways.