Pentose cycle

The pentose cycle (pentose phosphate pathway, hexose monophosphate shunt) is an oxidative catabolic pathway in the cytoplasm that provides the reduced cofactors NADPH and five-carbon carbohydrates (pentoses). It is a metabolic "transformation of glucose" whose "goal is not the creation of ATP".

NADPH is consumed in anabolic reactions, primarily in the biosynthesis of steroid substances and fatty acids. The formation of pentoses (ribose-5-phosphate) is necessary for the synthesis of nucleotides and nucleic acids. The pentose cycle takes place mainly in the cytosol of tissues producing steroid substances or fatty acids – liver, testes, ovaries, adrenal cortex, mammary gland, adipocytes and erythrocytes (reduction of glutathione). Furthermore, it is also important for providing NADPH for NADPH oxidase in the respiratory burst phase. [[File:Pentose cycle.png|thumb|400px|Pentose cycle diagram

Enzymes catalyzing individual reactions: 1 – hexokinase, 2 – glucose-6-phosphate dehydrogenase, 3 – 6-phosphogluconolactonase, 4 – phosphogluconate dehydrogenase, 5 – ribulose-5-phosphate epimerase, 6 – ribulose-5-phosphate isomerase, 7 – transketolase, 8 – transaldolase, 9 – transketolase]]

The pentose cycle is divided into two parts:


 * Oxidative - conversion of hexose to pentose and reduction of NADP+ to NADPH+H+.
 * Nonoxidative (regenerative) - individual monosaccharides (C3, C4, C5, C6, C7) are converted.

By repeating these reactions, the glucose-6-phosphate molecule is gradually oxidized to 6 CO2 and '12 NADPH is produced.

Oxidative phase

 * 1) oxidation of G-6-P' by glucose-6-phosphate dehydrogenase to 6-phosphogluconolactone and the first NADPH is formed
 * 2) Lactone is cleaved by hydrolase to form 6-phosphogluconate with free carboxyl ends.
 * 3) the enzyme ``phosphogluconate dehydrogenase decarboxylates 6-phosphogluconate to ``ribulose-5-phosphate (pentose), a second ``NADPH is formed and the carboxyl groups are split off as CO2
 * 4) ribulose-5-phosphate is changed to ribose-5-phosphate' by isomerization

During the oxidative phase, 2 molecules of NADPH+H+' are formed by the conversion of G-6-P to lactone' and 6-phosphogluconate to ribulose- 5-phosphate.

Nonoxidative phase
Adapts the frequent production of NADPH and pentose phosphates to the current consumption. Ribulose-5-phosphate is converted to fructose-6-phosphate' and 'glyceral-3-phosphate, both of which can then enter glycolysis. Or glucose-6-phosphate is regenerated from fructose-6-phosphate and can return to the cycle.

The graft donor is ketose in both, the acceptor is aldose. Graft transfer produces various intermediates:


 * sedoheptulose-7-phosphate;
 * erythrose-4-phosphate;
 * fructose-6-phosphate;
 * glyceraldehyde-3-phosphate.

Regulation
The speed is mainly determined by the activity of the enzyme ``glucose-6-phosphate dehydrogenase'', which is increased when NADP+ is supplied. An excess of NADPH slows down the reaction.

Related Articles

 * Glycolysis
 * Glucose
 * NADPH
 * Fatty acid synthesis
 * Glutathione

Source