Page 161 - Color Atlas of Biochemistry
P. 161
152 Metabolism
Pentose phosphate pathway of the regenerative branch is to adjust the net
+
production of NADPH+H and pentose phos-
The pentose phosphate pathway (PPP, also phates to the cell’s current requirements. Nor-
+
known as the hexose monophosphate mally, the demand for NADPH+H is much
pathway) is an oxidative metabolic pathway higher than that for pentose phosphates. In
located in the cytoplasm, which, like glycoly- these conditions, the reaction steps shown
sis, starts from glucose 6-phosphate. It sup- first convert six ribulose 5-phosphates to
plies two important precursors for anabolic five molecules of fructose 6-phosphate and
pathways: NADPH+H+, which is required for then, by isomerization, regenerate five glu-
the biosynthesis of fatty acids and isopren- cose 6-phosphates. These can once again sup-
+
oids, for example (see p. 168), and ribose 5- ply NADPH+H to the oxidative part of the
phosphate, a precursor in nucleotide biosyn- PPP. Repeating these reactions finally results
thesis (see p. 188). in the oxidation of one glucose 6-phosphate
+
into six CO 2 .TwelveNADPH+H arise in the
same process. In sum, no pentose phosphates
A. Pentose phosphate pathway:
are produced via this pathway.
oxidative part
In the recombination of sugar phosphates
The oxidative segment of the PPP converts in theregenerativepart ofthe PPP, thereare
glucose 6-phosphate to ribulose 5-phosphate. two enzymes that are particularly important:
+
One CO 2 and two NADPH+H are formed in [5] Transaldolase transfers C 3 units from
the process. Depending on the metabolic sedoheptulose 7-phosphate, a ketose with
state, themuch morecomplex regenerative seven C atoms, to the aldehyde group of glyc-
part of the pathway (see B)can convertsome eraldehyde 3-phosphate.
of the pentose phosphates back to hexose [4] Transketolase, which contains thiamine
phosphates, or it can pass them on to glycol- diphosphate, transfers C 2 fragments from one
ysis for breakdown. In most cells, less than sugar phosphate to another.
10% of glucose 6-phosphate is degraded via The reactions in the regenerative segment
the pentose phosphate pathway. of the PPP are freely reversible. It is therefore
easily possible to use the regenerative part of
the pathway to convert hexose phosphates
B. Reactions
into pentose phosphates. This can occur
[1] The oxidative part starts with the oxida- when there is a high demand for pentose
tion of glucose 6-phosphate by glucose-6- phosphates—e. g., during DNA replication in
phosphate dehydrogenase. This forms theS phaseof the cell cycle(seep. 394).
NADPH+H + for the first time. The second
product, 6-phosphogluconolactone,is an in- Additional information
tramolecular ester (lactone) of 6-phospho-
gluconate. When energy in the form of ATP is required in
+
[2] A specific hydrolase then cleaves the addition to NADPH+H , the cell is able to
lactone, exposing the carboxyl group of channel the products of the regenerative
6-phosphogluconate. part of the PPP (fructose 6-phosphate and
[3] The last enzyme in the oxidative part is glyceraldehyde 3-phosphate) into glycolysis.
phosphogluconate dehydrogenase [3], which Further degradation is carried out via the tri-
releases the carboxylate group of 6-phospho- carboxylic acid cycle and the respiratory chain
gluconate as CO 2 and at thesame timeoxi- to CO 2 and water. Overall, the cell in this way
dizes the hydroxyl group at C 3 to an oxo obtains 12 mol NADPH+H + and around
+
group. In addition to a second NADPH+H , 150 mol ATP from 6 mol glucose 6-phos-
this also produces the ketopentose ribulose phate. PPP activity is stimulated by insulin
5-phosphate. This is converted by an isomer- (see p. 388). This not only increases the rate
ase to ribose 5-phosphate, the initial com- of glucose degradation, but also produces ad-
+
pound for nucleotide synthesis (top). ditional NADPH+H for fatty acid synthesis
The regenerative part of the PPP is only (see p. 168).
shown here schematically. A complete reac-
tion scheme is given on p. 408. The function
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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