Embden-Meyerhof pathway

Embden-Meyerhof pathway (EMP), also known as the glycolytic pathway, is a series of biochemical reactions used by all cells for the metabolic breakdown of glucose to pyruvate, generating ATP (energy) and NADH (reducing power) in the process. This pathway is named after Gustav Embden and Otto Meyerhof, two of the primary scientists who elucidated its components and steps.

Overview

The Embden-Meyerhof pathway is a central metabolic pathway that is the most widely used method of glucose catabolism. It is a ten-step process, divided into two phases: the preparatory phase and the payoff phase. In the preparatory phase, glucose is converted into two molecules of glyceraldehyde 3-phosphate (G3P), consuming two ATP molecules. The payoff phase generates four ATP molecules and two NADH molecules per glucose, resulting in a net gain of two ATP and two NADH molecules. The final product, pyruvate, can further undergo fermentation or enter the citric acid cycle depending on the cellular environment and the presence of oxygen.

Steps of the Embden-Meyerhof pathway

1. Glucose is phosphorylated to glucose-6-phosphate by Hexokinase or Glucokinase in liver cells, using ATP.
2. Glucose-6-phosphate is isomerized to Fructose 6-phosphate by phosphoglucose isomerase.
3. Fructose 6-phosphate is phosphorylated to fructose-1,6-bisphosphate by Phosphofructokinase-1, using another ATP.
4. Fructose-1,6-bisphosphate is split into two three-carbon sugars, dihydroxyacetone phosphate and glyceraldehyde 3-phosphate, by aldolase.
5. Dihydroxyacetone phosphate is converted to glyceraldehyde 3-phosphate by triose phosphate isomerase.
6. Glyceraldehyde 3-phosphate is oxidized and phosphorylated to 1,3-bisphosphoglycerate by glyceraldehyde 3-phosphate dehydrogenase, producing NADH.
7. 1,3-Bisphosphoglycerate donates a phosphate to ADP, forming ATP and 3-phosphoglycerate, catalyzed by phosphoglycerate kinase.
8. 3-Phosphoglycerate is converted to 2-phosphoglycerate by phosphoglycerate mutase.
9. 2-Phosphoglycerate is dehydrated to phosphoenolpyruvate by enolase.
10. Phosphoenolpyruvate donates a phosphate to ADP, forming ATP and pyruvate, catalyzed by Pyruvate kinase.

Regulation

The Embden-Meyerhof pathway is tightly regulated at three key enzymatic steps: hexokinase/glucokinase, phosphofructokinase-1, and pyruvate kinase. These regulatory points are influenced by the energy needs of the cell, allowing the pathway to adapt to varying metabolic demands. For instance, high levels of ATP inhibit phosphofructokinase-1, slowing down glycolysis when the cell has ample energy.

Significance

The Embden-Meyerhof pathway is not only crucial for energy production but also for the generation of metabolic intermediates required for other biochemical pathways, such as nucleotide, amino acid, and lipid synthesis. It is a universal pathway found in nearly all organisms, highlighting its evolutionary importance.

See also

• Gluconeogenesis
• Pentose phosphate pathway
• Citric acid cycle
• Fermentation