2. Pentose Phosphate Pathway
Other names:
Phosphogluconate Pathway
Hexose Monophosphate Shunt
Direct oxidative pathway
The linear part of the pathway carries out oxidation
and decarboxylation of the 6-C sugar (glucose-6-
P), producing the 5-C sugar (ribulose-5-P) and
others
3. Glucose-6-phosphate 6-Phospho- O O−
Dehydrogenase glucono- 1C
6CH2OPO32− 6 CH OPO 2− lactonase
+ 2 3
NADPH + H HC OH
H
5 O OH NADP
+
H
5 O H2O H+ 2
H H HO 3CH
4 OH H 1 4
OH H O
1 HC OH
4
OH H OH
3 2 3 2 HC OH
5
H OH H OH CH2OPO32−
6
glucose-6-phosphate 6-phoshogluconolactone 6-phosphogluconate
Glucose-6-phosphate Dehydrogenase catalyzes
oxidation at C1 of glucose-6-phosphate, to a
carboxylic acid, in ester linkage (lactone)
NADP+ serves as electron acceptor
4. Glucose-6-phosphate 6-Phospho- O O−
Dehydrogenase glucono- 1C
6CH2OPO32− 6 CH OPO 2− lactonase
+ 2 3
NADPH + H HC OH
H
5 O OH NADP
+
H
5 O H2O H+ 2
H H HO 3CH
4 OH H 1 4
OH H O
1 HC OH
4
OH H OH
3 2 3 2 HC OH
5
H OH H OH CH2OPO32−
6
glucose-6-phosphate 6-phoshogluconolactone 6-phosphogluconate
6-Phosphogluconolactonase catalyzes hydrolysis of the
ester linkage, resulting in ring opening
The product is 6-phosphogluconate
Although ring opening occurs in the absence of a catalyst,
6-Phosphogluconolactonase speeds up the reaction,
decreasing the lifetime of the highly reactive and
potentially toxic, 6-phosphogluconolactone
5. O O− Phosphogluconate
1C Dehydrogenase
HC OH CH OH
2 NADP+ NADPH + H+ 1 2
HO 3CH C O
2
HC OH HC OH
4 3
HC OH CO2 HC OH
5 4
CH2OPO32− CH2OPO32−
6 5
6-phosphogluconate ribulose-5-phosphate
Phosphogluconate Dehydrogenase catalyzes
oxidative decarboxylation of 6-phosphogluconate, to
yield the 5-C ketose ribulose-5-phosphate
The OH at C3 (C2 of product) is oxidized to a ketone
This promotes loss of the carboxyl at C1 as CO2
NADP+ serves as oxidant
6. CH2OH
Epimerase inter- C O
converts stereoisomers Epimerase
HO C H
ribulose-5-P and
xylulose-5-P CH2OH H C OH
CH2OPO32−
Isomerase converts the C O
xylulose-5-
ketose ribulose-5-P to H C OH phosphate
the aldose ribose-5-P H C OH HC O
Both reactions are CH2OPO32− H C OH
reversible ribulose-5- H C OH
phosphate Isomerase
H C OH
CH2OPO32−
ribose-5-
phosphate
7. CH2OH
Transketolase
C O
CH2OH HC O HO C H
C O H C OH H C OH
HO C H H C OH HC O H C OH
H C OH
+ H C OH H C OH
+ H C OH
CH2OPO32− CH2OPO32− CH2OPO32− CH2OPO32−
xylulose- ribose- glyceraldehyde- sedoheptulose-
5-phosphate 5-phosphate 3-phosphate 7-phosphate
Transketolase transfers a 2-C fragment from
xylulose-5-P to either ribose-5-P or erythrose-4-P
8. CH2OH
Transketolase
C O
CH2OH HC O HO C H
C O H C OH H C OH
HO C H H C OH HC O H C OH
H C OH
+ H C OH H C OH
+ H C OH
CH2OPO32− CH2OPO32− CH2OPO32− CH2OPO32−
xylulose- ribose- glyceraldehyde- sedoheptulose-
5-phosphate 5-phosphate 3-phosphate 7-phosphate
Transfer of the 2-C fragment to the 5-C aldose
ribose-5-phosphate yields sedoheptulose-7-phosphate
Transfer of the 2-C fragment instead to the 4-C aldose
erythrose-4-phosphate yields fructose-6-phosphate
9. CH2OH
Transaldolase
C O H2C OH
HO CH C O
HC OH HC O HO CH
HC OH HC O HC OH HC OH
HC OH + HC OH HC OH + HC OH
H2C OPO32− H2C OPO32− H2C OPO32− H2C OPO32−
sedoheptulose- glyceraldehyde- erythrose- fructose-
7-phosphate 3-phosphate 4-phosphate 6-phosphate
Transaldolase catalyzes transfer of a 3-C
dihydroxyacetone moiety, from sedoheptulose-7-phosphate
to glyceraldehyde-3-phosphate
