Sees pentoses phosphates

The Way of pentoses phosphates, or sees of Warburg-Dickens-Horecker is one of the four main roads, with the Glycolyse, the way of Entner-Doudoroff) of degradation of the Glucose in Pyruvate and sees it méthylglyoxal.

The crucial roles of this way are:

production of molecules of NADPH, H⁺, used during the synthesis of the Fatty-acids, the Cholesterol and for the reduction of the Glutathion
production of ribose-5-phosphate used during the synthesis of the Nucleotide S;
production of érythrose-4-phosphate, precursor of amino-acid aromatic: Phenylalanine, Tyrosin and Tryptophan.

This way exists at all the Eucaryote S and the near total of the Bactérie S. It is independent of oxygen. It is done as well in aerobiosis as in anaérobiose, in the Cytoplasme.

Stages of the way of pentoses phosphates

The way of pentoses is made up of two phases. The first is a oxydative phase irreversible which allows the formation of NADPH. The second is a nonoxydative phase reversible which allows the synthesis of the Ribose.

Oxydative phase

During this phase, two molecules of NADP⁺ are reduced in NADPH by using the energy of conversion of the Glucose-6-phosphate (G6P) into Ribose-5-phosphate.

The first stage is catalyzed by the Glucose-6-phosphate déshydrogénase (G6PDH). This stage of oxidation allows the production of the first NADPH and product de la' 6-phosphoglucono-δ-lactone' . This lactone is then hydrolized by a lactonase to give 6-phosphogluconate . The 6-phsophogluconate undergoes an oxydative decarboxylation catalyzed by the 6-phosphogluconate déshydrogénase. This produced a pentose phosphates, the ribulose-5-phosphate and generates one second molecule of NADPH.

The ribulose-5-phospate (a Cétose) undergoes then a isomerization catalyzed by the phosphopentose isomérase which gives the ribose-5-phosphate (a Aldose), precursor of the synthesis of nucleotides.

Nonoxydative phase (reversible)

This phase forms two fructose-6-phosphate molecules (F6P) and a glycérealdéhyde-3-phosphate molecule (G3P) starting from ribose phosphates generated at the time of the first phase. The F6P can be used to reform G6P in order to start again the first oxydative phase.

This phase is catalyzed by two enzymes, the transaldosase and the trancétolase, which act on sugar-phosphate (aldoses and ketosis) with 3,4,5,6 or 7 carbons and catalyze reactions of transfer of one or two carbons, respectively. They have like substrates and products:

the glycéraldéhyde-3-phosphate (Triose)
the érythrose-4-phosphate (Tetrose)
ribose-5-phopshate and the xylulose-5-phosphate (Pentose S)
the fructose-6-phosphate (Hexose)
the sédoheptulose-7-phosphate (Heptose)

These various sugar-phosphates are used in various metabolic ways: Fructose and glycéraldéhyde in the Glycolysis, the érythrose in the synthesis of the aromatic amino-acids, ribose in that of nucleotides and the sédoheptulose in that of the Lipopolysaccharide of the bacterial Wall.

Assessment

3 G6P + 6 NADP⁺ + 3:00 ₂O - > 2 F6P + G3P + 3 CO₂ + 6 NADPH + 6:00 ⁺

See too

Other ways of degradation of glucose:

the Glycolysis;
the way of Entner-Doudoroff.
Deficit of glucose-6-phosphate déshydrogénase (favism) - a hereditary disease which stops the way of pentoses.

External bonds

the chemical logic of the way of pentoses phosphates (English)

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