PENTOSE PHOSPHATE PATHWAY

· Also known as HEXOSE MONOPHOSPHATE SHUNT or PHOSPHOGLUCONATE PATHWAY.

FUNCTIONS:

· Produces NADPH in cytosol for reductive biosynthesis.

(NADPH is required for endergonic reductive biosynthesis whereas NADH participates in ATP synthesis via oxidative phosphorylation.)

· Converts hexoses to pentoses, particularly D-ribose-5-P required in the synthesis of nucleic acids, ATP, CoA, NAD(P)⁺, FAD, etc.

· Converts pentoses to hexoses, feeding into glycolysis.

· Part of the phosphogluconate pathway participates in the photosynthesis of glucose from CO₂.

THREE STAGES (Fig. 14-30)

· Stage I (Rxns 1-3):

Oxidation of G6P to ribulose-5-P and formation of NADPH.

6 G6P + 12 NADP⁺ + 6 H₂O ® 6 ribulose-5-P + 6 CO₂ + 12 NADPH + 12 H⁺

· Stage 2 (Rxns 4, 5):

6 ribulose-5-P ® 2 ribose-5-P + 4 xylulose-5-P

· Stage 3 (Rxns 6-8):

A series of C-C bond cleavage and formation.

6 pentose-5-P ® 4 fructose-6-P + 2 glyceraldehyde-3-P

Transketolase (transfer 2-C unit) and Transaldolase (transfer 3-C unit)

OVERALL: (p. 464)

3 G6P + 6 NADP⁺ + 3 H₂O ⇌ 2 F6P + 3 CO₂ + 6 NADPH + 6 H⁺ + GA3P

· Additional Reactions:

2 glyceraldehyde-3-P ® GA3P + dihydroxyacetone-3-P ® F-1,6-bisP

® F-6-P + Pi

↑H₂0

OVERALL:

6 G6P + 12 NADP⁺ + 7 H₂O ⇌ 5 F6P + 6CO₂ + 12 NADPH + 12 H⁺ + Pi