2-Keto-6 phosphogluconic acid

In a certain group of bacteria, still another pathway (Entner-Doudomff pathway) for the utilization of glucose has been studied. Here glucosc-6-phosphate is oxidized to 6-phosphogluconic acid which is dehydrated to 2-keto-3-deoxy-6-phusphogluconic acid. This substance is then split to pyruvic acid and glyceraldehyde-3-phosphatc l which alsu can be converted to pyruvic acid). 

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. 

The oxidative decarboxylation of 6-phosphogluconic acid occurs in a stepwise mechanism involving a )8-keto acid intermediate 64). However, it appears that no metal ion is required in the reaction (66). 2-Deoxy-6-phosphogluconate is also a substrate for this enzyme, but the reaction is about two orders of magnitude slower than reaction of the natural substrate. The keto acid produced in the initial oxidation is released into solution and subsequently undergoes slow enzyme-catalyzed decarboxylation (67). 

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. 

Note that in the oxidative decarboxylation of 6-phosphogluconate, oxidation occurs at the carbon [3 to the carboxyl group. A similar [3-oxidation occurs during the decarboxylation of isocitrate in the citric acid cycle. In both cases a (3-keto acid intermediate is formed. Since (3-keto acids are relatively unstable, they are easily decarboxylated. 

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). 

The oxidation of glucose to 5-p-ribulose involves in order phosphorylation by ATP oxidation of the sugar aldehyde to the lactone by TPN and finally oxidative decarboxylation of 6-p-gluconate to 5-p-ribulose again with TPN as specific electron acceptor. Phosphogluconate can be conceived of as a /3-hydroxy acid which serially is oxidized to the corresponding /3-keto acid and then decarboxylated to 5-p-ribulose. Both the oxidation and the corresponding decarboxylation appear to be catalyzed by the same enzyme. 

Keto-3-deoxy-6>phosphogluconic add was first isolated as a product of the action of a bacterial enzyme (Pseudomonas sauharophyla) on 6-phos-phogluconic acid . It is also formed from ATP and 2-keto-3-deoxy-D-gluconic add with an enzyme from E. coli. ... 

---