Pentose Phosphate Pathway Overview

Pentose Phosphate Pathway (PPP)

• Glucose is broken down via glycolysis into pyruvate, and then further oxidized in the citric acid cycle to produce ATP in animal tissues.
• The PPP is an alternative metabolic pathway.
• Also known as the Hexose Monophosphate Pathway (HMP shunt) or Phosphogluconate pathway.
• This pathway produces ribose sugar, ATP, and two NADPH.
• The PPP is a parallel metabolic pathway to glycolysis.
• It does not directly produce ATP.
• Occurs in the cytosol of cells.
• Key locations in the body include liver, adipose tissue, lactating mammary gland, and the cortex of the adrenal gland.

PPP Function

• Generates reducing equivalents (NADPH) for reductive biosynthesis reactions (e.g., fatty acid synthesis).
• Produces ribose 5-phosphate (R5P) for nucleotide and nucleic acid synthesis.
• Produces erythrose 4-phosphate (E4P) for aromatic amino acid synthesis.

PPP Phases

• Oxidative Phase (non-reversible):
  • Glucose 6-phosphate undergoes dehydrogenation and decarboxylation, producing ribulose 5-phosphate.
  • This phase generates NADPH.
• Non-oxidative Phase (reversible):
  • Ribulose 5-phosphate is converted back to glucose 6-phosphate through a series of reactions involving transketolase and transaldolase.
  • This phase is important for shuffling carbon atoms.

Key Enzymes in the Oxidative Phase

• Glucose-6-phosphate dehydrogenase (G6PD): Oxidizes glucose 6-phosphate to form 6-phosphoglucono-lactone and NADPH.
• Lactonase: Hydrolyzes 6-phosphoglucono-lactone to 6-phosphogluconate.
• 6-Phosphogluconate dehydrogenase: Oxidizes and decarboxylates 6-phosphogluconate to form ribulose 5-phosphate and CO2, while producing NADPH.

Key Enzymes in the Non-Oxidative Phase

• Phosphopentose isomerase: Converts ribulose 5-phosphate to ribose 5-phosphate.
• Phosphopentose epimerase: Converts ribulose 5-phosphate to xylulose 5-phosphate.
• Transketolase: Transfers a two-carbon unit from a ketose to an aldose.
• Transaldolase: Transfers a three-carbon unit from a ketose to an aldose.

Regulation of the PPP

• Glucose-6-phosphate dehydrogenase is the rate-limiting enzyme.
• The pathway is regulated allosterically by NADPH.
• It is induced by high glucose/insulin to glucagon ratios after carbohydrate meals.

This quiz explores the Pentose Phosphate Pathway (PPP), an alternative metabolic pathway that plays a crucial role in producing ribose sugar and NADPH. Understand its function, phases, and importance in various tissues like liver and adipose. Test your knowledge on how the PPP operates alongside glycolysis.