Amphibolic pathway

An amphibolic pathway is a metabolic pathway that has both catabolic and anabolic functions. It can both break down (catabolism) and synthesize (anabolism) molecules, depending on the cellular needs. Glycolysis is one such example of an amphibolic pathway as it serves both catabolic and anabolic roles in glucose metabolism.

Steps Involved in Glycolysis:

Glycolysis is a ten-step biochemical pathway that occurs in the cytoplasm of cells and is the first stage of both aerobic and anaerobic cellular respiration. Here are the steps involved in glycolysis:

Step 1: Phosphorylation (Energy-Investment Phase)

• Glucose (a six-carbon sugar) is phosphorylated by ATP to form glucose-6-phosphate.
• Enzyme: Hexokinase.
• ATP used: 1 ATP per glucose molecule.

Step 2: Isomerization (Energy-Investment Phase)

• Glucose-6-phosphate is converted to fructose-6-phosphate.
• Enzyme: Glucose-6-phosphate isomerase.

Step 3: Phosphorylation (Energy-Investment Phase)

• Fructose-6-phosphate is phosphorylated by ATP to form fructose-1,6-bisphosphate.
• Enzyme: Phosphofructokinase.
• ATP used: 1 ATP per glucose molecule.

Step 4: Cleavage (Energy-Investment Phase)

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

Step 5: Isomerization (Energy-Investment Phase)

• Dihydroxyacetone phosphate is converted into another molecule of glyceraldehyde-3-phosphate.
• Enzyme: Triose phosphate isomerase.

Step 6: Oxidation and ATP Formation (Energy-Payoff Phase)

• Each molecule of glyceraldehyde-3-phosphate is oxidized and generates two molecules of NADH while also forming 1,3-bisphosphoglycerate.
• Enzyme: Glyceraldehyde-3-phosphate dehydrogenase.
• ATP produced: 2 ATP per glyceraldehyde-3-phosphate molecule.
• NADH produced: 2 NADH per glyceraldehyde-3-phosphate molecule.

Step 7: Phosphorylation (Energy-Payoff Phase)

• 1,3-bisphosphoglycerate is converted to 3-phosphoglycerate.
• Enzyme: Phosphoglycerate kinase.
• ATP produced: 2 ATP per 1,3-bisphosphoglycerate molecule.

Step 8: Isomerization (Energy-Payoff Phase)

• 3-phosphoglycerate is converted to 2-phosphoglycerate.
• Enzyme: Phosphoglycerate mutase.

Step 9: Dehydration (Energy-Payoff Phase)

• 2-phosphoglycerate is dehydrated to form phosphoenolpyruvate (PEP).
• Enzyme: Enolase.

Step 10: Phosphorylation (Energy-Payoff Phase)

• PEP is converted into pyruvate, generating 2 ATP molecules per PEP molecule.
• Enzyme: Pyruvate kinase.
• ATP produced: 2 ATP per PEP molecule.

Summary of Glycolysis:

• Glycolysis begins with one molecule of glucose (a six-carbon sugar) and ultimately produces two molecules of pyruvate (a three-carbon compound).

• In the process, a net gain of 2 ATP and 2 NADH molecules is generated.

• Glycolysis also plays a critical role in providing intermediates for other metabolic pathways and can serve both catabolic and anabolic purposes in the cell, depending on the metabolic context.