Pentose phosphate pathway

The pentose phosphate pathway (PPP), also known as the phosphohexose pathway or hexose monophosphate shunt, is a metabolic route parallel to glycolysis that plays essential roles in cellular metabolism, particularly in producing reducing equivalents and pentoses necessary for nucleotide synthesis. The PPP is crucial for cellular biosynthesis, detoxification, and metabolic regulation, especially in tissues with high anabolic activity, such as the liver, bone marrow, and erythrocytes.​

Pathway Overview

The pathway consists of two distinct phases:

• Oxidative phase: generates NADPH and ribulose-5-phosphate

• Non-oxidative phase: interconverts sugars to produce ribose-5-phosphate and intermediates of glycolysis

Oxidative Phase

This phase begins with glucose-6-phosphate (G6P) and produces NADPH and ribulose-5-phosphate (Ru5P):

1. Glucose-6-phosphate dehydrogenase (G6PDH) reaction:

G6P + NADP⁺ → 6-phosphoglucono-δ-lactone + NADPH

This enzyme catalyzes the first, rate-limiting step, generating NADPH, essential for reductive biosynthesis and antioxidant defense.​

2. Hydrolysis of 6-phosphoglucono-δ-lactone:

6-phosphoglucono-δ-lactone + H₂O → 6-phosphogluconate

3. 6-Phosphogluconate dehydrogenase:

6-phosphogluconate + NADP⁺ → Ribulose-5-phosphate + CO₂ + NADPH

This step produces a second NADPH molecule and converts the sugar into ribulose-5-phosphate, a pentose sugar.​

Non-Oxidative Phase

This phase involves reversible interconversions of sugar phosphates, allowing the cell to adapt its nucleotide and glycolytic needs:​

Transketolase and transaldolase reactions:

Converts ribulose-5-phosphate into different sugars like xylulose-5-phosphate, glyceraldehyde-3-phosphate, and fructose-6-phosphate.

Example reaction catalyzed by transketolase:

Xylulose-5-P + Ribose-5-P → Glyceraldehyde-3-P + Fructose-6-P

Interconversion reactions:

These allow for the synthesis of ribose-5-phosphate for nucleotide biosynthesis and the rerouting of excess sugars into glycolysis.​

Regulation

G6PDH is the key regulatory enzyme, stimulated by NADP⁺ and inhibited by NADPH (feedback inhibition), ensuring balanced NADPH production according to cellular demand.

The pathway adjusts dynamically depending on the cell's reductive needs versus biosynthetic requirements.​

The pentose phosphate pathway provides cells with:

• NADPH for reductive biosynthesis and protection against oxidative stress

• Ribose-5-phosphate for nucleic acid synthesis

• Metabolic flexibility through non-oxidative sugar interconversions

It plays vital roles in maintaining cellular redox balance and biosynthesis, especially in rapidly dividing or metabolically active tissues.