Glycolysis Lecture Notes

Definition of Glycolysis

• Glycolysis is the process of oxidizing glucose, a six-carbon monosaccharide (sugar).
• Converts glucose into two molecules of pyruvate (three-carbon).

Transport of Glucose into Cells

• Glucose is water-soluble and cannot diffuse through cell membranes.
• Requires specialized transporters known as GLUT transporters.

Types of GLUT Transporters

• GLUT1: Found in red blood cells, fetus, and blood-brain barrier.
• GLUT2: Present in the kidney, liver, pancreas, and gastrointestinal tract.
• GLUT3: Found in the placenta, neurons, and kidneys.
• GLUT4: Muscle and adipose tissue; insulin-dependent, increases number and efficiency in the presence of insulin.

Steps of Glycolysis

1. Glucose to Glucose-6-Phosphate

   • Enzyme: Hexokinase or Glucokinase (liver only).
   • ATP is used to add a phosphate group, converting ATP to ADP.

2. Glucose-6-Phosphate to Fructose-6-Phosphate

   • Enzyme: Phosphohexose Isomerase.
   • Isomerization process.

3. Fructose-6-Phosphate to Fructose-1,6-Bisphosphate

   • Enzyme: Phosphofructokinase-1 (PFK-1).
   • This is an irreversible and heavily regulated step; ATP is used.

4. Fructose-1,6-Bisphosphate Cleavage

   • Enzyme: Aldolase.
   • Splits into two 3-carbon fragments: Dihydroxyacetone phosphate (DHAP) and Glyceraldehyde-3-phosphate (G3P).

5. Interconversion of DHAP and G3P

   • Enzyme: Triose Phosphate Isomerase.
   • Primarily produces G3P for further glycolysis.

6. G3P to 1,3-Bisphosphoglycerate (1,3-BPG)

   • Enzyme: Glyceraldehyde-3-Phosphate Dehydrogenase.
   • Produces NADH and adds an inorganic phosphate.

7. 1,3-BPG to 3-Phosphoglycerate (3-PG)

   • Enzyme: Phosphoglycerate Kinase.
   • ATP is generated in this step.

8. 3-PG to 2-Phosphoglycerate (2-PG)

   • Enzyme: Phosphoglycerate Mutase.

9. 2-PG to Phosphoenolpyruvate (PEP)

   • Enzyme: Enolase.

10. PEP to Pyruvate

    • Enzyme: Pyruvate Kinase.
    • Produces ATP; this step is irreversible.

End Products of Glycolysis

• Net Gain: 2 ATP (4 ATP produced, 2 used).
• NADH Production: 2 NADH molecules generated.
• End Product: 2 Pyruvate molecules.

Anaerobic vs. Aerobic Conditions

• Anaerobic: Without oxygen, pyruvate is converted into lactic acid via lactate dehydrogenase; can lead to metabolic acidosis due to decreased pH.
• Aerobic: In presence of oxygen, pyruvate will enter the mitochondria for further processing.

Clinical Relevance

• Elevated levels of lactate dehydrogenase (LDH) can indicate conditions like myocardial infarction or necrotic tissues due to lack of oxygen.

Summary

• Glycolysis occurs in the cytoplasm.
• Starts with glucose and ends with two pyruvate molecules.
• Produces a net gain of 2 ATP and 2 NADH.
• Key processes are regulated, and glycolysis can divert based on aerobic or anaerobic conditions.