Pentos Phosphate Pathway (PPP)

Overview

• Final concept from Chapter 12
• Also known as "PPP"
• Converts monosaccharide (glucose 6-phosphate) into nucleotides and other metabolites
• Takes place entirely in the cytoplasm
• Main functions:
  1. Generate NADPH (main reducing agent of the cell)
  2. Produce ribose-5-phosphate (for DNA and RNA synthesis)

Pathway Accomplishments

• Oxidative Phase: Generates NADPH
• Non-Oxidative Phase: Generates ribose-5-phosphate and regulates glucose-6-phosphate

Net Reaction

• Inputs: 6 glucose, 12 NADP+, water
• Outputs: 5 glucose, 12 NADPH, hydrogen ions, CO2

Key Enzymes

• Glucose-6-phosphate dehydrogenase: Committed and regulated step
• Transketolases & Transaldolases: Perform carbon shuffling

Phases of PPP

Oxidative Phase

• Converts glucose-6-phosphate to ribulose-5-phosphate
• Generates CO2 and NADPH
• Performed by glucose-6-phosphate dehydrogenase

Non-Oxidative Phase

• Carbon shuffling to produce various intermediates (ribose-5-phosphate, fructose-6-phosphate, etc.)
• Can enter glycolysis or nucleotide synthesis pathways

Regulation

• Regulated based on cell's need for NADPH, nucleotides, or ATP
• High NADP+: Activates glucose-6-phosphate dehydrogenase → More NADPH
• High NADPH: Inhibits glucose-6-phosphate dehydrogenase → Redirects glucose-6-phosphate to glycolysis
• Cellular demand determines if the pathway focuses on NADPH regeneration, nucleotide production, or ATP generation