EMP Pathway or Embden-Meyerhof-Parnas Pathway

Introduction

EMP pathway is the other name of glycolysis. It is named after the three scientists Gustav Embden, Otto Meyerhof, and J. Parnas, who gave the scheme of glycolysis. It is the pathway of glucose catabolism. It occurs in the cytoplasm of all living cells, aerobic as well as anaerobic.

EMP pathway or glycolysis is the primary step of cellular respiration. Glucose is partially oxidised to pyruvate in this process. In aerobic organisms, it is followed by the Krebs cycle for the complete oxidation of glucose to CO₂ and water. In anaerobic organisms, glycolysis is followed by fermentation.

Let us learn in detail about each step of the EMP pathway.

Steps of EMP Pathway

The EMP pathway occurs in the cytoplasm of the cell. It doesn’t require oxygen. In plants, glucose is derived from sucrose formed during photosynthesis or from storage carbohydrates such as starch. The enzyme invertase converts sucrose into glucose and fructose that enter the EMP pathway.

• It is a series of ten enzyme-catalysed reactions, wherein a glucose molecule is broken down into two molecules of pyruvate.
• There is a net production of two ATP and 2 NADH also in this process.
• The first phase of the EMP pathway is the energy-requiring phase or preparatory phase, and the second half is the energy-yielding phase or pay-off phase.

The steps of glycolysis are as follows:

Step 1

Glucose is converted to Glucose 6-phosphate by the action of the enzyme hexokinase. An ATP is consumed in this process. The phosphate group of ATP is transferred to glucose.

Step 2

The second step is the conversion of G6P to Fructose 6-phosphate (F6P) by the enzyme phosphoglucoisomerase. It is an isomerisation reaction. Fructose can also enter the EMP pathway at this step by phosphorylation. After this step, all the steps are the same for glucose and fructose metabolism. It is a reversible reaction.

Step 3

F6P is converted to fructose 1,6-bisphosphate (FBP) by utilising another molecule of ATP. The enzyme phosphofructokinase catalyses the reaction. It is an irreversible reaction and is a rate-limiting step. During the gluconeogenesis process, which is an anabolic pathway, a different path is required for this step.

Step 4

The enzyme aldolase converts fructose 1,6-bisphosphate into two triose sugars, i.e. dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate, a ketose and aldose, respectively.

Step 5

The enzyme triosephosphate isomerase interconverts dihydroxyacetone phosphate (DHAP) with glyceraldehyde 3-phosphate (GADP) or 3-phosphoglyceraldehyde (PGAL).

Step 6

Now the pay-off phase starts. Since a molecule of glucose yields two molecules of triose sugar, each reaction from this step onwards occurs twice.

Here in this step, 3-phosphoglyceraldehyde (PGAL) is converted into 1, 3-bisphosphoglycerate (BPGA) by the action of the enzyme Glyceraldehyde 3-phosphate dehydrogenase (GAPDH). NAD⁺ is reduced to NADH + H⁺. In this step, dehydrogenation and phosphorylation take place.

Step 7

1, 3-bisphosphoglycerate (BPGA) is then converted into 3-phosphoglycerate (3-PGA). Phosphate is transferred from BPGA to ADP, forming ATP. This is a substrate-level phosphorylation. Two molecules of ATP are produced in this step. The enzyme phosphoglycerate kinase catalyses this reaction.

Step 8

3-PGA is then converted into 2-PGA by the action of the enzyme phosphoglycerate mutase. The enzyme transfers a phosphate group from C-3 to C-2.

Step 9

It is an elimination reaction, wherein a molecule of water is removed from 2-PGA to produce phosphoenolpyruvate (PEP). The enzyme enolase catalyses this dehydration reaction.

Step 10

It is the last step of the EMP pathway or glycolysis. Pyruvate kinase transfers a phosphate group from PEP to ADP, forming ATP and pyruvate or pyruvic acid. 2 ATP molecules and 2 pyruvate molecules are produced. This is also a substrate-level phosphorylation.

Summary

To sum up, in the EMP pathway, partial oxidation of glucose takes place. The overall reaction in the EMP pathway is:

Glucose + 2 NAD⁺ + 2 ADP + 2 Pi → 2 pyruvate + 2 NADH + 2 H⁺ + 2 ATP

It occurs in two phases:

• Preparatory Phase or Energy-requiring Phase: The first 5 steps of the EMP pathway are known as the investment phase. Energy is consumed in this process to produce two molecules of triose sugar phosphates.
• Pay-off Phase: The second half of glycolysis produces ATP and NADH and pyruvate.

Location: Cytoplasm of living cells

ATP Utilisation: ATP is utilised in two steps. First in the conversion of glucose to G6P and then when F6P is converted into fructose 1,6-bisphosphate. Total 2 ATP molecules are utilised.

ATP Synthesis: A total of 4 ATP molecules are synthesised. ATP is synthesised in two steps. First, when 1,3-bisphosphoglyceric acid is converted into 3-phosphoglyceric acid, and then in the conversion of phosphoenolpyruvate to pyruvate.

NADH Synthesis: 2 NAD⁺ is converted to 2 NADH + H⁺ in the sixth step when glyceraldehyde-3-phosphate is converted to 1,3-bisphosphoglyceric acid.

Rate-limiting Step: The conversion of fructose-6-phosphate to fructose-1,6-bisphosphate by the enzyme phosphofructokinase.

End Products of EMP Pathway: Two molecules each of pyruvate, ATP and NADH.

Significance of EMP Pathway

• The EMP pathway is the universal pathway of glucose degradation, whether energy is derived in aerobic respiration or fermentation.
• It is the first step of cellular respiration.
• It is required by all tissues to derive energy in the form of ATP.
• Cells that perform anaerobic respiration derive energy from this process.
• Cells that lack mitochondria derive energy from this process. Krebs cycle and oxidative phosphorylation occur in mitochondria, where most of the energy is produced during cellular respiration.
• The intermediates formed during the EMP pathway are utilised in other metabolic pathways.
• The EMP pathway is connected to the various other metabolic pathways such as the Pentose phosphate pathway, glycogen synthesis, formation of triglycerides, fatty acid synthesis, cholesterol synthesis, amino acid synthesis, etc.

This was all about the EMP Pathway. Explore notes on other important concepts related to NEET, only at BYJU’S.

Further reading:

• Krebs Cycle
• Significance Of Glycolysis
• MCQs On EMP Pathway
• Types Of Fermentation
• Flashcards Of Biology For NEET Respiration In Plants