Pentose phosphate pathway
- This pathway occurs in the cytosol
- Includes two irreversible reactions
- No ATP is consumed or produced in the pathway
- One glucose-6-phosphate is oxidized of its carbon 1, producing co2 and 2 reduced
NADPH, and ribulose 5-phosphate
The pathway's three-step oxidative portion is crucial in:
• Adipose tissue, lactating mammary glands, and the liver are all involved in the
NADPH-dependent production of fatty acids.
•The ovaries, placenta, and testes, which depend on NADPH to produce steroid
hormones
• Red blood cells, which need NADPH to maintain low glutathione levels
Steps of the pathway:
Glucose-6-phosphate dehydrogenase transforms glucose-6-phosphate into 6-
phosphogluconolactone.
The NADPH to NADP ratio controls this step, which uses NADP as the coenzyme.
This enzyme is activated by increasing NADP and insulin, while it is inhibited by high
NADPH.
6-phosphogluconolactone hydrolase hydrolyzes 6-phosphogluconolactone to 6-
phosphogluconate (irreversible).
, The irreversible oxidative decarboxylation of 6-phosphogluconate by 6-
phosphogluconate dehydrogenase yields ribulose 5-phosphate, CO2, and the second
NADPH.
DIAGRAM OF PATHWAY:
- https://encrypted-
tbn0.gstatic.com/images?q=tbn:ANd9GcSbcoksp6lE2KAT2cxdrdYq2qwGPko4
q0XoG3FBGydbsfX3kBPjFJlZHlubXiodhf-kZfs&usqp=CAU
- https://encrypted-
tbn0.gstatic.com/images?q=tbn:ANd9GcSj6RGll9tVEY44mxqsLBeGVBiqs69er
Gfj5CI4X2y4j8o33YKK7bahvB-nYXZC5g2gNMA&usqp=CAU
The non-oxidative reversible components:
These occur in all cell types that produce nucleic acids and nucleotides.
They enable the transformation of ribulose 5-phosphate into ribose 5-phosphate for
nucleotide synthesis or into glycolysis intermediates like fructose-6-phosphate and
glyceraldehyde-6-phosphate.
Transketolase and transaldolase convert ribulose to glycolysis intermediates when the
cell needs NADPH.
Glycolysis intermediates are transformed into ribose 5 phosphate through nonoxidative
processes if the cell requires ribose for nucleotide synthesis.
• Alpha ketoglutarate dehydrogenase, branched chain alpha ketoglutarate
dehydrogenase, and the PDH complex are examples of transketolases that require TPP
(thiamine pyrophosphate).
NADPH can serve as a reservoir of energy for reductive biosynthesis, which produces
fatty acids and steroids.
H2O2, which is detrimental to cells, is created when molecular oxygen is reduced.
In order for cells to use glutathione reductase to detoxify H2O2, NADPH keeps
glutathione reduced.
- This pathway occurs in the cytosol
- Includes two irreversible reactions
- No ATP is consumed or produced in the pathway
- One glucose-6-phosphate is oxidized of its carbon 1, producing co2 and 2 reduced
NADPH, and ribulose 5-phosphate
The pathway's three-step oxidative portion is crucial in:
• Adipose tissue, lactating mammary glands, and the liver are all involved in the
NADPH-dependent production of fatty acids.
•The ovaries, placenta, and testes, which depend on NADPH to produce steroid
hormones
• Red blood cells, which need NADPH to maintain low glutathione levels
Steps of the pathway:
Glucose-6-phosphate dehydrogenase transforms glucose-6-phosphate into 6-
phosphogluconolactone.
The NADPH to NADP ratio controls this step, which uses NADP as the coenzyme.
This enzyme is activated by increasing NADP and insulin, while it is inhibited by high
NADPH.
6-phosphogluconolactone hydrolase hydrolyzes 6-phosphogluconolactone to 6-
phosphogluconate (irreversible).
, The irreversible oxidative decarboxylation of 6-phosphogluconate by 6-
phosphogluconate dehydrogenase yields ribulose 5-phosphate, CO2, and the second
NADPH.
DIAGRAM OF PATHWAY:
- https://encrypted-
tbn0.gstatic.com/images?q=tbn:ANd9GcSbcoksp6lE2KAT2cxdrdYq2qwGPko4
q0XoG3FBGydbsfX3kBPjFJlZHlubXiodhf-kZfs&usqp=CAU
- https://encrypted-
tbn0.gstatic.com/images?q=tbn:ANd9GcSj6RGll9tVEY44mxqsLBeGVBiqs69er
Gfj5CI4X2y4j8o33YKK7bahvB-nYXZC5g2gNMA&usqp=CAU
The non-oxidative reversible components:
These occur in all cell types that produce nucleic acids and nucleotides.
They enable the transformation of ribulose 5-phosphate into ribose 5-phosphate for
nucleotide synthesis or into glycolysis intermediates like fructose-6-phosphate and
glyceraldehyde-6-phosphate.
Transketolase and transaldolase convert ribulose to glycolysis intermediates when the
cell needs NADPH.
Glycolysis intermediates are transformed into ribose 5 phosphate through nonoxidative
processes if the cell requires ribose for nucleotide synthesis.
• Alpha ketoglutarate dehydrogenase, branched chain alpha ketoglutarate
dehydrogenase, and the PDH complex are examples of transketolases that require TPP
(thiamine pyrophosphate).
NADPH can serve as a reservoir of energy for reductive biosynthesis, which produces
fatty acids and steroids.
H2O2, which is detrimental to cells, is created when molecular oxygen is reduced.
In order for cells to use glutathione reductase to detoxify H2O2, NADPH keeps
glutathione reduced.
