Glycolysis Overview and Key Steps

Oct 8, 2024

Glycolysis Lecture Notes

Introduction to Glycolysis

  • Glycolysis definition: Oxidation of glucose.
  • Glucose: A six-carbon monosaccharide obtained from diet.
  • Process: Glucose is oxidized through 10 steps to become two molecules of pyruvate (each three carbons).

Glucose Transport into Cells

  • Glucose: Water-soluble, cannot diffuse through membrane.
  • GLUT Transporters: Specialized bidirectional transporters for glucose.
    • GLUT1: Found in red blood cells, fetus, blood-brain barrier.
    • GLUT2: Present in kidney, liver, pancreas, and GI tract.
    • GLUT3: Located in placenta, neurons, kidneys.
    • GLUT4: Found in muscle and adipose tissue; insulin-dependent.

Initial Steps of Glycolysis

  1. Glucose Formation: Glucose enters cells via GLUT transporters.
  2. Glucose Phosphorylation:
    • Enzymes: Hexokinase (muscles) & Glucokinase (liver).
    • Phosphate source: ATP is converted to ADP.
    • Product: Glucose-6-phosphate.
  3. Isomerization: Conversion of glucose-6-phosphate to fructose-6-phosphate.
    • Enzyme: Phosphohexose isomerase.

Key Steps in Glycolysis Pathway

  1. Fructose Phosphorylation:
    • Enzyme: Phosphofructokinase 1 (PFK1).
    • Product: Fructose-1,6-bisphosphate.
  2. Cleavage into 3-Carbon Molecules:
    • Products: Dihydroxyacetone phosphate (DHAP) & Glyceraldehyde-3-phosphate (GA3P).
    • Enzyme: Aldolase (conversion by triose phosphate isomerase).
  3. Oxidation and Phosphate Addition:
    • Enzyme: Glyceraldehyde-3-phosphate dehydrogenase.
    • Products: 1,3-bisphosphoglycerate, NADH.
  4. Phosphate Transfer to Form ATP:
    • Enzyme: Phosphoglycerate kinase.
    • Product: 3-phosphoglycerate, ATP.
  5. Mutation of Phosphate Position:
    • Enzyme: Phosphoglycerate mutase.
    • Product: 2-phosphoglycerate.
  6. Conversion to Enol Form:
    • Enzyme: Enolase.
    • Product: Phosphoenolpyruvate (PEP).
  7. Final Phosphate Transfer:
    • Enzyme: Pyruvate kinase.
    • Product: Pyruvate, ATP.

Fate of Pyruvate

  • Anaerobic Conditions: Pyruvate converts to lactic acid via lactate dehydrogenase.
    • NADH donates hydrides, regenerating NAD+.
    • Clinical relevance: Indicator of oxygen deficiency conditions.
  • Aerobic Conditions: Leads to the formation of acetyl-CoA.

Summary of Outputs

  • Location: Glycolysis occurs in the cytoplasm.
  • Starting Substrate: Glucose.
  • End Products: 2 Pyruvate, 2 net ATP (4 total produced minus 2 used), 2 NADH.
  • Anaerobic Process: Generates lactic acid if oxygen is low.

Future Lecture

  • Transition step under aerobic conditions: Conversion to acetyl-CoA for entry into the Krebs cycle.