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PENTOSE PHOSPHATE PATHWAY:
Nonoxidative Branch 
In 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...

 
        Glucose 6-phosphate  +  2 NADP1+  +  H2
                Ribose 5-phosphate  +  2 NADPH  +  2 H1+  +  CO2
The net result is the production of NADPH for biosynthesis and ribose 5-phosphate for nucleotide synthesis.

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:

Transketolase
C5  +  C5  C7  +  C3
Transaldolase
C7  +  C3  C4  +  C6
Transketolase
C5  +  C4  C6  +  C3
In 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


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