Cloning, Expression, and Characterization of a Psychrophilic Glucose 6-Phosphate Dehydrogenase from Sphingomonas sp. PAMC 26621

Biochemical and Kinetic Characterization of the Glucose-6-Phosphate Dehydrogenase from Helicobacter pylori Strain 29CaP

Helicobacter pylori (H. pylori) has been proposed as the foremost risk factor for the development of gastric cancer. We found that H. pylori express the enzyme glucose-6-phosphate dehydrogenase (HpG6PD), which participates in glucose metabolism via the pentose phosphate pathway. Thus, we hypothesized that if the biochemical and physicochemical characteristics of HpG6PD contrast with the host G6PD (human G6PD, HsG6PD), HpG6PD becomes a potential target for novel drugs against H. pylori. In this work, we characterized the biochemical properties of the HpG6PD from the H.pylori strain 29CaP and expressed the active recombinant protein, to analyze its steady-state kinetics, thermostability, and biophysical aspects. In addition, we analyzed the HpG6PD in silico structural properties to compare them with those of the HsG6PD. The optimal pH for enzyme activity was 7.5, with a T_1/2 of 46.6 °C, at an optimum stability temperature of 37 °C. The apparent K_m values calculated for G6P and NADP⁺ were 75.0 and 12.8 µM, respectively. G6P does not protect HpG6PD from trypsin digestion, but NADP⁺ does protect the enzyme from trypsin and guanidine hydrochloride (Gdn-HCl). The biochemical characterization of HpG6PD contributes to knowledge regarding H. pylori metabolism and opens up the possibility of using this enzyme as a potential target for specific and efficient treatment against this pathogen; structural alignment indicates that the three-dimensional (3D) homodimer model of the G6PD protein from H. pylori is different from the 3D G6PD of Homo sapiens.

Unconventional biochemical regulation of the oxidative pentose phosphate pathway in the model cyanobacterium Synechocystis sp. PCC 6803

Metabolite production from carbon dioxide using sugar catabolism in cyanobacteria has been in the spotlight recently. Synechocystis sp. PCC 6803 (Synechocystis 6803) is the most studied cyanobacterium for metabolite production. Previous in vivo analyses revealed that the oxidative pentose phosphate (OPP) pathway is at the core of sugar catabolism in Synechocystis 6803. However, the biochemical regulation of the OPP pathway enzymes in Synechocystis 6803 remains unknown. Therefore, we characterized a key enzyme of the OPP pathway, glucose-6-phosphate dehydrogenase (G6PDH), and related enzymes from Synechocystis 6803. Synechocystis 6803 G6PDH was inhibited by citrate in the oxidative tricarboxylic acid (TCA) cycle. Citrate has not been reported as an inhibitor of G6PDH before. Similarly, 6-phosphogluconate dehydrogenase, the other enzyme from Synechocystis 6803 that catalyzes the NADPH-generating reaction in the OPP pathway, was inhibited by citrate. To understand the physiological significance of this inhibition, we characterized succinic semialdehyde dehydrogenase (SSADH) from Synechocystis 6803 (SySSADH), which catalyzes one of the NAD(P)H generating reactions in the oxidative TCA cycle. Similar to isocitrate dehydrogenase from Synechocystis 6803, SySSADH specifically catalyzed the NADPH-generating reaction and was not inhibited by citrate. The activity of SySSADH was lower than that of other bacterial SSADHs. Previous and this studies revealed that unlike the OPP pathway, the oxidative TCA cycle is a pathway with low efficiency in NADPH generation in Synechocystis 6803. It has, thus, been suggested that to avoid NADPH overproduction, the OPP pathway dehydrogenase activity is repressed when the flow of the oxidative TCA cycle increases in Synechocystis 6803.

Effect of active-site aromatic residues Tyr or Phe on activity and stability of glucose 6-phosphate dehydrogenase from psychrophilic Arctic bacterium Sphingomonas sp

Cold-adapted enzymes maintain correct conformation at their active sites despite their intrinsically flexible structures. The psychrophilic Arctic bacterium Sphingomonas sp. PAMC 26621 has two glucose 6-phosphate dehydrogenase (G6PD) isozymes, SpG6PD1 involved in the Entner-Doudoroff pathway and SpG6PD2 in the oxidative pentose phosphate pathway. Structural modeling of SpG6PD1 showed that the hydroxyl group of Tyr¹⁷⁷ participates in substrate binding by forming a hydrogen bond with the phosphate group of glucose 6-phosphate, whereas in SpG6PD2, a Phe residue is present in the corresponding position of Tyr¹⁷⁷. In this study, we investigated how subtle differences in aromatic residues in the substrate-binding pocket of SpG6PD1 affect enzymatic activity and stability. Mutations of Tyr¹⁷⁷ to Ala, His, Phe, and Trp caused increases in the rigidity of the SpG6PD1 structure. Particularly, mutants Y177F and Y177W showed increased thermal stabilities compared to wild-type (WT) but 3- and 15-fold lower catalytic efficiencies, respectively. However, mutants Y177A and Y177H became heat-labile at moderate temperatures. These results indicate that an aromatic residue (Tyr or Phe) is necessary for the substrate-binding pocket of SpG6PD1; Tyr with its hydroxyl group is preferred for enzymatic activity, whereas the more hydrophobic Phe is preferred for thermal stability. Substitutions of bulky Trp for Tyr or Phe at this position resulted in substantial loss of activity. Our study suggests that delicate adjustment of aromatic residues can regulate the activity and stability of psychrophilic G6PD isozymes involved in different metabolic pathways.