GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

11. MAIN MECHANISMS OF METABOLISM AND ENERGY CONVERSION IN MICROORGANISMS

11.5. PATHWAYS OF CATABOLISM OF GLUCOSE AND OTHER CARBOHYDRATES

11.5.4. Pentose phosphate pathway

Alternative names for this pathway include the phosphogluconate pathway and the hexose monophosphate shunt.

The Pentose phosphate pathway is not a primary route of glucose Cleavage. Instead, it serves as a supplier of NADPH for anabolic reactions and ribose for nucleotide synthesis.

The pentose phosphate pathway results in The oxidation of one of the glucose carbon atoms, yielding CO₂ and a pentose, which undergoes a series of rearrangements to regenerate the initial pathway intermediate, glucose-6-phosphate (Fig. 11.6).

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Fig. 11.6. Oxidative pentose phosphate pathway. Glucose is oxidized to 3CO₂ and glyceraldehyde-3-phosphate

In the pentose phosphate pathway, glucose-6-phosphate is converted into 6-phosphogluconate (catalyzed by glucose-6-phosphate dehydrogenase). 6-phosphogluconate is then oxidized by gluconate-6-phosphate dehydrogenase to ribulose-5-phosphate, which marks the completion of the oxidative Stages of the pentose phosphate pathway. The subsequent reactions involve The conversion of pentose phosphates into hexose phosphates and vice versa. Mediated by transketolase and transaldolase, ribulose-5-phosphate is converted into two molecules of fructose-6-phosphate and one molecule of glyceraldehyde-3-phosphate. Glyceraldehyde-3-phosphate can subsequently enter METABOLISM via Pyruvate. Fructose-6-phosphate is converted into glucose-6-phosphate (catalyzed by glucose phosphate isomerase), thus completing the cycle. The dehydrogenases involved in the functioning of the pentose phosphate pathway can be either NAD- or NADP-dependent.

It should be noted that the non-oxidative Reactions of the pentose phosphate pathway are involved in pentose degradation in many microorganisms. The General scheme of pentose Catabolism is as follows: phosphorylation of pentose at the C-5 position, isomerization and epimerization to Intermediates of the pentose phosphate cycle, conversion into fructose-6-phosphate and glyceraldehyde-3-phosphate, and subsequent degradation via the Embden-Meyerhof-Parnas pathway.

The overall net reaction of glucose degradation in the pentose phosphate pathway is: