Pentose Phosphate Pathway (Oxidative and Non-Oxidative Phases) 🧬

Overview

• The Pentose Phosphate Pathway (PPP) has two phases:
  • Oxidative Phase
  • Non-Oxidative Phase

Oxidative Phase

• Converts glucose 6-phosphate into:
  • 2 NADPH molecules
  • 1 Ribose 5-phosphate molecule
• NADPH is used for:
  • Generating fatty acids, cholesterol, nucleotides, neurotransmitters
  • Detoxifying various toxic agents
• Ribose 5-phosphate is used for:
  • Generating nucleotide-based molecules such as DNA, RNA, ATP, NADH, FAD, Coenzyme A

Need for NADPH vs Ribose 5-Phosphate

• Most cells require NADPH more than Ribose 5-phosphate
• Excess Ribose 5-phosphate is transformed into glycolytic intermediates to produce more NADPH

Non-Oxidative Phase

• Converts Ribose 5-phosphate into glycolytic intermediates
• Broken down into four processes:
  • Process 1
  • Process 2
  • Process 3
  • Process 4

Process 1

1. Ribose 5-phosphate → Ribulose 5-phosphate (catalyzed by phosphopentose isomerase)
2. Ribulose 5-phosphate → Xylulose 5-phosphate (catalyzed by phosphopentose epimerase)

• Stereochemistry change in the 3rd carbon
• Important for enzyme affinity in the next steps

Process 2

• Xylulose 5-phosphate + Ribose 5-phosphate → Sedoheptulose 7-phosphate + Glyceraldehyde 3-phosphate
  • Catalyzed by transketolase (requires Thiamine Pyrophosphate as cofactor)

Process 3

• Sedoheptulose 7-phosphate + Glyceraldehyde 3-phosphate → Erythrose 4-phosphate + Fructose 6-phosphate
  • Catalyzed by transaldolase
  • Generates glycolytic intermediates

Process 4

• Erythrose 4-phosphate + Xylulose 5-phosphate → Fructose 6-phosphate + Glyceraldehyde 3-phosphate
  • Catalyzed by transketolase (same enzyme as in Process 2)
• Generates additional glycolytic intermediates

Net Result

• 3 Ribose 5-phosphate → 2 Fructose 6-phosphate + 1 Glyceraldehyde 3-phosphate
• Glycolytic intermediates can be transformed into glucose 6-phosphate to undergo the oxidative phase and form more NADPH

Additional Insights

• Non-oxidative phase allows the ingestion of ribose sugars and their conversion into metabolizable intermediates
• Links glycolysis with the pentose phosphate pathway