USMLE Guide: Biochemistry of Glycolysis

Introduction

Glycolysis is a fundamental metabolic pathway that occurs in the cytoplasm of all cells. It is the primary pathway for glucose metabolism and is essential for energy production. This USMLE guide aims to provide a comprehensive overview of the biochemistry of glycolysis, including its steps, regulation, and clinical relevance.

Glycolysis Steps

1. Step 1: Glucose Phosphorylation

   • Enzyme: Hexokinase (or glucokinase in the liver)
   • Glucose is phosphorylated to glucose-6-phosphate, consuming one ATP molecule.
   • This step serves to trap glucose within the cell and initiates its metabolism.

2. Step 2: Glucose-6-Phosphate Isomerization

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

3. Step 3: Phosphorylation of Fructose-6-Phosphate

   • Enzyme: Phosphofructokinase-1 (PFK-1)
   • Fructose-6-phosphate is phosphorylated to fructose-1,6-bisphosphate, consuming one ATP molecule.
   • This step is a key regulatory point and is the rate-limiting step of glycolysis.

4. Step 4: Cleavage of Fructose-1,6-Bisphosphate

   • Enzyme: Aldolase
   • Fructose-1,6-bisphosphate is cleaved into two triose phosphates: glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP).

5. Step 5: Interconversion of DHAP and G3P

   • Enzyme: Triose phosphate isomerase
   • DHAP is converted into G3P, which can continue through the glycolytic pathway.

6. Step 6: Oxidation of Glyceraldehyde-3-Phosphate

   • Enzyme: Glyceraldehyde-3-phosphate dehydrogenase
   • G3P is oxidized, generating NADH and high-energy phosphate (1,3-bisphosphoglycerate).

7. Step 7: Substrate-Level Phosphorylation

   • Enzyme: Phosphoglycerate kinase
   • High-energy phosphate from 1,3-bisphosphoglycerate is transferred to ADP, generating ATP and 3-phosphoglycerate.

8. Step 8: Conversion of 3-Phosphoglycerate to 2-Phosphoglycerate

   • Enzyme: Phosphoglycerate mutase
   • The phosphate group is shifted to convert 3-phosphoglycerate to 2-phosphoglycerate.

9. Step 9: Conversion of 2-Phosphoglycerate to Phosphoenolpyruvate

   • Enzyme: Enolase
   • Water is removed from 2-phosphoglycerate, forming phosphoenolpyruvate.

10. Step 10: Substrate-Level Phosphorylation and Pyruvate Formation

    • Enzyme: Pyruvate kinase
    • Phosphoenolpyruvate is dephosphorylated, generating ATP and pyruvate.

Regulation of Glycolysis

• Hexokinase regulation:

  • Inhibited by its product, glucose-6-phosphate.
  • Allosterically inhibited by glucose-6-phosphate in pancreatic beta cells, ensuring glucose is used for insulin synthesis.

• Phosphofructokinase-1 (PFK-1) regulation:

  • Allosterically activated by fructose-2,6-bisphosphate.
  • Allosterically inhibited by ATP and citrate.
  • Hormonal regulation by insulin and glucagon through the regulation of fructose-2,6-bisphosphate levels.

• Pyruvate kinase regulation:

  • Allosterically activated by fructose-1,6-bisphosphate.
  • Allosterically inhibited by ATP and alanine.
  • Hormonal regulation by insulin and glucagon affects pyruvate kinase activity.

Clinical Relevance

• Warburg effect: Cancer cells exhibit increased glycolysis even in the presence of oxygen (aerobic glycolysis). This is known as the Warburg effect and supports their rapid growth and proliferation.

• **Glycol