The Pathway And Key Events In Glycolysis Process: Pathway, Steps & Products

What Is Glycolysis?

This is the metabolic pathway by which the six-carbon sugar glucose is broken down into two molecules of the three-carbon compound, pyruvate. Glycolysis is a metabolic pathway that takes place in the cytoplasm of a cell and is also the first stage of cellular respiration. This process is important in that it yields a net gain of two ATP molecules and two NADH molecules to the cell and meanwhile injects energy and reduces the power that these end products can easily be converted to other metabolic processes.

The first stage is glycolysis of cellular respiration. It takes place under both aerobic and anaerobic conditions. In the presence of oxygen, through glycolysis and then into a second pathway in the mitochondria called the citric acid cycle, followed by oxidative phosphorylation, in which glucose is fully oxidised with a high yield of ATP; under anaerobic conditions, glycolysis leads to fermentation, where the cells can produce ATP without oxygen. This flexibility makes glycolysis a central pathway to energy production in an extraordinarily large number of organisms.

Steps Of Glycolysis

The detailed steps of glycolysis are given below:

Step 1: Glucose to Glucose-6-Phosphate

• Enzyme: Hexokinase

• Catalyses the phosphorylation of glucose to glucose-6-phosphate.

• ATP investment: 1 ATP

• ATP donates a phosphate group, converting it to ADP.

Step 2: Glucose-6-Phosphate to Fructose-6-Phosphate

• ATP investment: 1 ATP

• ATP is spent to phosphorylate a molecule and it is regenerated back to ADP.

• Regulation point: Key regulatory step in glycolysis

• This reaction step is tightly regulated by intracellular levels of ATP.

Step 3: Fructose-6-Phosphate to Fructose-1,6-Bisphosphate

• Enzyme: Phosphofructokinase

• Adds a second phosphate group to fructose-6-phosphate, forming fructose-1,6-bisphosphate.

• ATP investment: 1 ATP

• ATP is used to add a phosphate group, converting it to ADP.

• Regulation point: Key regulatory step in glycolysis

• This step is highly regulated by cellular energy levels.

Step 4: Fructose-1,6-Bisphosphate to DHAP and G3P

• Enzyme: Aldolase

• Fructose-1,6-bisphosphate is cleaved into two three-carbon molecules: dihydroxyacetone phosphate and glyceraldehyde-3-phosphate.

• Products: Dihydroxyacetone

• Converts DHAP to G3P, so that two G3P passes through glycolysis.

Step 5: Conversion of DHAP to G3P

• Enzyme: Triose phosphate isomerase

• Converts DHAP to G3P, ensuring that two molecules of G3P proceed through glycolysis.

Step 6: G3P to 1,3-Bisphosphoglycerate

• Enzyme: glyceraldehyde-3-phosphate dehydrogenase

• Catalyzes the oxidation and phosphorylation of G3P to produce 1,3-bisphosphoglycerate

• NADH production: 1NADH per G3P

• NAD+ is reduced to NADH at this step

Step 7. 1,3-Bisphosphoglycerate To 3-Phosphoglycerate

• Enzyme: Phosphoglycerate kinase

• Transfers a phosphate group from 1,3-bisphosphoglycerate to ADP, forming ATP and 3-phosphoglycerate.

• ATP generation: 1 ATP per G3P

• This step produces one molecule of ATP for each G3P.

Step 8: 3-Phosphoglycerate To 2-Phosphoglycerate

• Enzyme: Phosphoglycerate mutase

• Converts 3-phosphoglycerate reversibly to 2-phosphoglycerate by shifting the position of a phosphate group from the third to the second carbon.

Step 9: 2-Phosphoglycerate To Phosphoenolpyruvate

• Enzyme: Enolase

• Dehydrates 2-phosphoglycerate to form phosphoenolpyruvate (PEP), removing a molecule of water.

• Water molecule removal: 1 H2O per G3P

• Water is released as a byproduct.

Step 10: Phosphoenolpyruvate To Pyruvate

• Enzyme: Pyruvate kinase

• Transfers a phosphate group from PEP to ADP, forming ATP and pyruvate.

• ATP generation: 1 ATP per G3P

• This step produces one molecule of ATP for each G3P.

• Regulation point: Second key regulatory step

• This step is regulated to ensure efficient energy production.

Key Events And Regulation In Glycolysis

Glycolysis is closely regulated to ensure that the generation of energy is highly efficient and integrated with the cell.

Regulatory Enzymes

Hexokinase: The enzyme catalysing the phosphorylation of glucose to glucose-6-phosphate is inhibited by the product of the reaction, glucose-6-phosphate.

Phosphofructokinase: This enzyme controls the step of conversion from fructose-6-phosphate to fructose-1,6-bisphosphate; a major regulatory step affected by ATP, AMP, and citrate levels.

Pyruvate kinase: This is the final step in the conversion of phosphoenolpyruvate to pyruvate and is regulated by the levels of ATP and alanine, while it is activated by fructose-1,6-bisphosphate.

Mechanisms Of Regulation

• Allosteric regulation: Enzymes like phosphofructokinase are regulated through binding at places other than the active site of the enzyme, thereby modifying its activity.

• Feedback inhibition: The end products like ATP will inhibit the early steps of glycolysis so that overproduction can be avoided.

• Hormonal regulation: Insulin enhances glycolysis by increasing the expression of most glycolytic enzymes, while glucagon inhibits glycolysis to enhance gluconeogenesis.

Energetics Of Glycolysis

Glycolysis generates energy in the form of ATP and NADH through its activity and is, therefore an important aspect of cellular metabolism.

ATP Yield

In glycolysis, while 4 ATP molecules per glucose are produced, 2 ATP are consumed; hence a net gain of 2 ATP.

NADH Production

The NADH produced during glycolysis in each glucose molecule passes high-energy electrons into the electron transport chain in the mitochondria, which leads to additional production of ATP.

Overall Energy Efficiency

Part of the energy from the glucose is captured and stored as ATP and NADH during glycolysis while still having some energy remaining in pyruvate, which can be further oxidised in the presence of oxygen.

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