Introduction to Glycolysis Overview

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.

Important Notes

• Kinases are enzymes that transfer phosphate groups.
• Isomerases rearrange atoms within a molecule, without changing its overall composition.

Net Reaction

• Glucose + 2 ADP + 2 NAD+ → 2 Pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O

• Glycolysis operates without oxygen requirement, making it anaerobic.

• Further energy production is possible with oxygen present.