PENTOSE PHOSPHATE PATHWAY:
Nonoxidative BranchIn the first step of the nonoxidiatve branch, D-ribulose 5-phosphate has two possible fates: conversion to xylulose 5-phosphate or conversion to ribose 5-phosphate.
Conversion to xylulose 5-phosphate occurs in a reaction catalyzed by ribulose 5-phosphate epimerase (also called phosphopentose epimerase). Conversion to ribose 5-phosphate occurs in a reaction catalyzed by ribose 5-phosphate isomerase (also called phosphopentose isomerase).
If large amounts of both NADPH and nucleotides are needed, all the ribulose 5-phosphate can be isomerized to ribose 5-phosphate, and the pathway is completed. CO2 is evolved, and in these tissues the pathway ends at this point, and the overall reaction is given by...The net result is the production of NADPH for biosynthesis and ribose 5-phosphate for nucleotide synthesis.
Glucose 6-phosphate + 2 NADP1+ + H2O
Ribose 5-phosphate + 2 NADPH + 2 H1+ + CO2
If NADPH is needed for reductive biosynthesis, ribulose 5-phosphate is converted to glyceraldehyde 3-phosphate and fructose 6-phosphate through a complex series of reactions beginning with xylulose 5-phosphate. Interconversions of the carbon frameworks are catalyzed by transketolase and transaldolase:
C5 + C5 C7 + C3
C7 + C3 C4 + C6
C5 + C4 C6 + C3In the first step, xylulose 5-phosphate and ribose 5-phosphate react to produce glyceraldehyde 3-phosphate and sedoheptulose 7-phosphate. The reaction is catalyzed by transketolase.
Transketolase is a thiamine pyrophosphate(TPP)-dependent enzyme, that transferes a two-carbon keto group. The keto group must have an S configuration at C-3.
Once formed, sedoheptulose 7-phosphate reacts with glyceraldehyde 3-phosphate to produce erythrose 4-phosphate and fructose 6-phosphate. The reaction is catalyzed by transaldolase.
A Schiff base intermediate with a lysine residue is involved, and the three-carbon group transferred from a ketone phosphate to an aldose phosphate must have the S configuration at C-3. Erythrose 4-phosphate produced by the reactiong subsequently reacts with xylulose 5-phosphate to produce glyceraldehyde 3-phosphate and fructose 6-phosphate. This reaction is catalyzed by transketolase.
Glyceraldehyde 3-phosphate and fructose 6-phosphate may "recycle" to glucose 6-phosphate and reenter the pentose phosphate pathway. The glyceraldehyde 3-phosphate and fructose 6-phosphate produced are used by most cells to re-synthesize glucose 6-phosphate which re-enters the pentose phosphate pathway.
To completely oxidize a glucose 6-phosphate to CO2 takes six (6) cycles through the pentose phosphate pathway. If six glucose 6-phosphates are oxidized to six ribulose 5-phosphates, the ribulose 5-phosphates can be "rearranged" by the pathway to form 5 glucose 6-phosphates. The net stoichiometry is...
6 glucose 6-phosphate + 12 NADP1+
5 glucose 6-phosphate + 12 NADPH + 6 CO2 + Pi