Introduction to Glycolysis Overview

Study Notes

Glycolysis Overview

Glycolysis is a fundamental metabolic pathway found in nearly all cells (prokaryotic and eukaryotic).
It takes place in the cell's cytosol.
The pathway converts one glucose molecule into two pyruvate molecules.
Glycolysis serves two key functions: ATP generation and the production of intermediates for other biomolecules (e.g., amino acids, fatty acids).
Glucose is the primary fuel for most organisms.
Glycolysis produces a net gain of 2 ATP molecules per glucose molecule.
Pyruvate can be further metabolized to produce more ATP.

Glucose Source

Most dietary carbohydrates are in the form of starch (a polymer of glucose).
Glucose is also obtained from lactose (milk sugar) and sucrose (table sugar).
Maltose, a product of starch breakdown, is a rich source in certain foods undergoing fermentation.
Glucose transport into the cell is facilitated by specific transporters.

Glycolysis Steps

Step 1: Phosphorylation of Glucose: Glucose is phosphorylated to glucose-6-phosphate. This step is irreversible and traps the glucose inside the cell. The enzyme responsible is hexokinase.
Step 2: Isomerization: Glucose-6-phosphate is converted to fructose-6-phosphate via isomerization. This step is reversible.
Step 3: Phosphorylation (second): Fructose-6-phosphate is phosphorylated to fructose-1,6-bisphosphate. This step is irreversible and is a key regulatory step in glycolysis. Catalyzed by phosphofructokinase.
Step 4: Cleavage: Fructose-1,6-bisphosphate is cleaved into two 3-carbon molecules: dihydroxyacetone phosphate and glyceraldehyde-3-phosphate. This is a reversible reaction that creates two 3-carbon molecules.
Step 5: Oxidation and Phosphorylation: Glyceraldehyde-3-phosphate is oxidized and phosphorylated, creating 1,3-bisphosphoglycerate. This reaction generates high-energy phosphate bonds. The enzyme is glyceraldehyde-3-P dehydrogenase.
Step 6: Phosphoryl Transfer: 1,3-bisphosphoglycerate transfers a phosphate group to ADP, forming ATP (substrate-level phosphorylation) and 3-phosphoglycerate. The enzyme is phosphoglycerate kinase.
Step 7: Isomerization: 3-phosphoglycerate is rearranged to 2-phosphoglycerate via the enzyme phosphoglycerate mutase.
Step 8: Dehydration: 2-phosphoglycerate loses a water molecule, forming phosphoenolpyruvate (PEP). Catalyzed by enolase.
Step 9: Phosphoryl Transfer (final): PEP donates its phosphate group to ADP, creating ATP (substrate-level phosphorylation) and pyruvate. The enzyme is pyruvate kinase.