6-Phosphogluconate dehydrogenase, /3-keto

A variety of enzymes catalyze the oxidative decarboxylation of jS-hydroxy acids. Isotope effect studies of malic enzyme (29), isocitrate dehydrogenase (63), and 6-phosphogluconate dehydrogenase (64) indicate that all three of these oxidative decarboxylations occur by stepwise mechanisms in which hydride transfer occurs first, forming a j3-keto acid that then undergoes decarboxylation. Hydride transfer and decarboxylation are both partially rate determining. 

Like 6-phosphogluconate dehydrogenase and UDPglucuronate decarboxylase, this enzyme decarboxylates a /3-keto acid with an a-oxygen function without the involvement of a metal ion or other cofactor (70). Apparently the electronic properties of this oxygen function are sufficient to stabilize the enolate anion intermediate, so that no metal ion is needed. 

Pseudomonas Pathway. Certain strains of Pseudomonas have been found to possess different enzymes from those previously described. First, a glucose-6-phosphate dehydrogenase and a phosphogluconic dehydrogenase have been found that react with DPN in contrast to the enzymes from other sources that are specific for TPN. A novel pathway was found in studies in Doudoroff s laboratory. A dehydrase converts phosphogluconate to 2-keto-3-deoxygluconate. This enzyme has been separated from a second enzyme, which requires Fe++ and glutathione,... 

The four reactions involved in this conversion are shown in figure 12.31. The first oxidation, catalyzed by glu-cose-6-phosphate dehydrogenase at C-1, converts the hemi-acetal derivative of the aldehyde group to the lactone of the corresponding acid, 6-phosphogluconic acid. After hydrolysis of the lactone, the second oxidation at C-3, converts the secondary alcohol to a ketone. The expected product, 3-keto-6-phosphogluconic acid, is decarboxylated yielding ribulose-5-phosphate. 

Keto-6-phosphogluconate is a probable intermediate. The reaction is similar to those catalyzed by malic enzyme (in gluconeogenesis) and by isocitrate dehydrogenase (in the TCA cycle). 

Polyhydroxyalkanoates biosynthesis is regulated, on one hand, by the activity of 3-ketothiolase (EC 2.3.1.16), and on the other hand of acetoacetyl-CoA reductase (EC 1.1.1.36) intracellular PHA breakdown is dependent on the activity of 3-hydroxybutyrate dehydrogenase (EC 1.1.1.30). Besides these three enzymes, the following compounds can be pointed out as major factors responsible of the activities of the key enzymes acetyl-CoA, free CoA, NAD(P) + (or NAD(P)H2, respectively) and, to a lower extent, ATP, pyruvate and oxalacetate. In any case, acetyl-CoA can be considered as the central metabolite both for biomass formation and PHB biosynthesis. This compound stems from the catabolic break down of carbon substrates like sugars (mainly catabolized by the 2-Keto-3-desoxy-6-phosphogluconate pathway) or fatty acids (converted by 6-oxidation). 

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