Glycolysis via the Embden-Meyerhof-Parnas Glycolytic Pathway

What is glycolysis?

Glycolysis is the almost universal pathway that converts glucose into pyruvate along with the formation of nicotinamide adenine dinucleotide (NADH) and adenosine triphosphate (ATP). It primarily occurs in the cytoplasm of the cell.

Aerobic and anerobic glycolysis

Under aerobic conditions, the pyruvate passes into the mitochondria where it is completely oxidized by O₂ into CO₂ and H₂O and its chemical energy largely conserved as ATP. Pyruvate generated via aerobic glycolysis feeds into the TCA or Krebs cycle.

In the absence of sufficient oxygen, the pyruvate is reduced by NADH via anaerobic glycolysis or fermentation to a wide range of products, routinely lactate in animals and ethanol in yeasts.

Embden-Meyerhof-Parnas glycolytic pathway

The starting molecule for glycolysis is glucose, a simple and abundant sugar found in carbohydrates, which provides the energy for most cells. Carbohydrates synthesized during photosynthesis act as the main storage molecules of solar energy. When ingested, complex carbohydrates are enzymatically hydrolyzed to monosaccharides, such as starch to D(+)-glucose.

The catabolism of glucose is the primary energy source for short-term requirements.

The remainder of this article will focus on the glycolytic pathway known as the Embden-Meyerhof-Parnas (EMP) pathway, named for its discoverers, Gustav Embden, Otto Meyerhof, and Jakub Karol Parnas. The chemical steps of the pathway are illustrated in the image to the right and via the video below.

Click to download glycolytic pathway chart image.

10 steps in the glycolytic pathway and enzymes of glycolysis

The table below outlines the 10 individual reactions of the glycolytic pathway and the enzyme (with IUBMB Enzyme Commission number) that catalyzes each step.

Reaction	Enzyme	IUBMB EC Number
1. Phosphorylation of glucose. D(+)-Glucose is phosphorylated with ATP to give glucose-6-phosphate.	Hexokinase	EC 2.7.1.1
2. Isomerization of glucose-6-P to fructose-6-P. The isomerization of glucose-6-phosphate in the second reaction to fructose-6-phosphate occurs via ring-opening and subsequent keto-enol-tautomerization.	Glucose-6-phosphate isomerase	EC 5.3.1.9
3. Phosphorylation of fructose-6-P. The third reaction is another phosphorylation with ATP, whereby fructose-6-phosphate is converted to fructose-1,6-bisphosphate.	6-P-Fructokinase	EC 2.7.1.11
4. Fructose-1,6-bisphosphate to glyceraldehyde phosphate and dihydroxyacetone phosphate. A key branching reaction is the fourth reaction: a ring-opening reaction of fructose-1,6-bisphosphate, which is cleaved in a retro-aldol reaction into D-glyceraldehyde-3-phosphate, and dihydroxyacetone phosphate.	Fructose-bisphosphate aldolase	EC 4.1.2.13
5. Isomerization of dihydroxyacetone-P to glyceraldehyde-P. The branch via dihydroxyacetonephosphate is channelled back into D-glyceraldehyde-3-phosphate in the fifth reaction by an isomerization.	Triose-phosphate isomerase	EC 5.3.1.9
6. Glyceraldehyde phosphate oxidation & phosphorylation to 1,3-bisphosphoglycerate. In the sixth reaction, the combined D-glyceraldehyde- 3-phosphate from both routes is oxidized at the C1 to a carboxylic acid and then phosphorylated in the 1-position to yield 1,3-bisphospho-D-glycerate.	Glyceraldehyde phosphate dehydrogenase	EC 1.2.1.12
7. ATP formation. This phosphate group in the 1-position is transferred in the seventh reaction from the carboxyl group to ADP to give 3-phospho-D-glycerate.	Phosphoglycerate kinase	EC 2.7.2.3
8. 3-Phosphoglycerate to 2-phosphoglycerate. The eighth reaction is an isomerization of 3-phospho-D-glycerate to 2-phospho-D-glycerate.	Phosphoglycerate mutase	EC 5.4.2.1
9. 2-Phosphoglycerate to phosphonenolpyruvate. The next metabolite, phosphoenolpyruvate, is formed in a dehydration reaction from 2-phospho-D-glycerate.	Enolase	EC 4.2.1.11
10. Formation of pyruvate & ATP. The glycolysis pathway from D(+)-glucose to two molecules of pyruvate is concluded by the tenth reaction, which transfers a phosphate group from phosphoenolpyruvate to ADP, thereby giving ATP and pyruvate.	Pyruvate kinase	EC 2.7.1.40

Glycolysis animation - video

Enzymes of Glycolysis

2.7.1.1 - Hexokinase
5.3.1.9 - Glucose-6-Phosphase Isomerase
2.7.1.11 - 6-Phosphofructokinase
4.1.2.13 - Fructose-bisphosphate aldolase
1.2.1.12 - Glyceraldehyde-3-phosphate dehydrogenase
2.7.2.3 - Phosphoglycerate kinase
4.2.1.11 - Phosphopyruvate hydratase (enolase)
2.7.1.40 - Pyruvate kinase
1.1.1.27 - Lactate dehydrogenase

This article has been expanded from its original form, published in BioFiles 2007, 2.6, 4.