Oxidative phase of the pentose cycle

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In addition to AcCoA, propionyl-CoA is also formed by oxidation of odd-chain fatty acids. This is first carboxylated to methylmalonyl-CoA', which is converted to succinyl-CoA - an intermediate of the Krebs cycle'. Through conversion to oxaloacetate, it can participate in gluconeogenesis - glucose can be synthesized from these fatty acids. However, very few fatty acids with an odd number of carbon atoms are found in the body.

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Source
Fontana J., Trnka J., Maďa P., Ivák P. et al.: Transformation of substances and energy in the cell. In: Functions of cells and the human body : Multimedia scripts. Available online from:

The pentose cycle is a catabolic event that provides reduced cofactors NADPH and five-carbon saccharides, or pentoses. It is a metabolic conversion of glucose, the goal of which is not the creation of ATP.

Course of the oxidative phase of the pentose cycle
During the oxidative phase of the pentose cycle, the glucose-6-P molecule is oxidized to the ribulose-5-P molecule. At the same time, CO2 is released and two NADPH + H+ molecules are obtained.

Its course is summarized by the following equations:

'Glucose-6-phosphate + 2 NADP+ → CO2 + 2 NADPH+H+ + ribulose-5-phosphate '

Of the reactions of the first phase, the initial reaction catalyzed by ``glucose-6-phosphate dehydrogenase'' is important. This 'irreversible reaction is the main regulatory step of the pentose cycle.

The rate of the oxidative phase of the pentose cycle
The speed of the entire metabolic pathway depends on the activity of two dehydrogenation reactions, which depend on the availability of NADP+ (i.e. the oxidized form of the coenzyme). With a lack of NADP+, the rate of the pentose cycle decreases, in other words: an excess of NADPH "slows down" the oxidative phase of the pentose cycle.

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 * Pentose cycle
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