Non-enzymic protein phosphorylation. Phosphorylation of 6-phosphogluconate dehydrogenase by acyl phosphates.

Incubation of 6-phosphogluconate dehydrogenase from Candida utilis with either acetyl phosphate, 1,3-diphosphoglycerate or carbamoyl phosphate results in the phosphorylation of the protein. The binding of one phosphate residue per enzyme subunit does not affect significantly the kinetic properties, but makes the enzyme less reactive toward thiol reagents, trypsin and pyridoxal 5'-phosphate. We suggest indicate that: (1) 6-phosphogluconate dehydrogenase from C. utilis is phosphorylated non-enzymically by physiological acyl phosphates and (2) the phosphorylation of the enzyme modifies the rate of protein inactivation.     

Purification and properties of 6-phosphogluconate dehydrogenase from rabbit mammary gland.

1. 6-Phosphogluconate dehydrogenase from rabbit mammary gland was purified to homogeneity by the criterion of polyacrylamide-gel electrophoresis in the presence of sodium dodecyl sulphate. The molecular weight of the subunit is 52 000. The enzyme was purified 150-fold with a final specific activity of 20 mumol of NADP+ reduced/min per mg of protein and overall yield of 3%. The molecular weight of the native enzyme is estimated to be 104 000 from gel-filtration studies. The final purification step was carried out by affinity chromatography with NADP+-Sepharose. 2. The Km values for 6-phosphogluconate and NADP+ are approx. 54 muM and 23 muM respectively. 3. Citrate and pyrophosphate are competitive inhibitors of the enzyme with respect to both 6-phosphogluconate and NADP+. 4. MgCl2 affects the apparent Km for NADP+ at saturating concentrations of 6-phosphogluconate.     

Human Brain 6-Phosphogluconate Dehydrogenase: Purification and Kinetic Properties

6-Phosphogluconate dehydrogenase has been purified from human brain to a specific activity of 22.8 U/mg protein. The molecular weight was 90,000. At low ionic strengths enzyme activity increased, due to an increase in Vmax and a decrease in Km for 6-phosphogluconate, and activity subsequently decreased as the ionic strength was increased (above 0.12). Both 6-phosphogluconate and NADP+ provided good protection against thermal inactivation, with 6-phosphogluconate also providing considerable protection against loss of activity caused by p-chloromercuribenzoate and iodoacetamide. Initial velocity studies indicated the enzyme mechanism was sequential. NADPH was a competitive inhibitor with respect to NADP+, and the Ki values for this inhibition were dependent on the concentration of 6-phosphogluconate. Product inhibition by NADPH was noncompetitive when 6-phosphogluconate was the variable substrate, whereas inhibition by the products CO2 and ribulose 5-phosphogluconate and NADP+ were varied. In totality these data suggest that binding of substrates to the enzyme is random. CO2 and ribulose 5-phosphate are released from the enzyme in random order with NADPH as the last product released.     

Bass liver 6-phosphogluconate dehydrogenase: inhibition by nucleoside phosphates and by fructose 1,6 bisphosphate

Nucleoside 5'-triphosphates, 5'-diphosphates and 5'-monophosphates are inhibitors of the 6-phosphogluconate dehydrogenase enzyme from bass liver. The 2'- and 3'-monophosphates of adenosine and guanosine are also inhibitory, the 2'-isomers being especially potent inhibitors. The catalytic activity of 6-phosphogluconate dehydrogenase has been found to be markedly inhibited by fructose 1, 6 bisphosphate. As the Km for 6-phosphogluconate, the Ki for fructose 1,6 bisphosphate and the concentration of both compounds in bass liver are all comparable, it appears that the inhibition of 6-phosphogluconate dehydrogenase by fructose 1,6 bisphosphate may be of significance in the regulation of carbohydrate metabolism in bass liver.     

Multiple Forms of Pseudomonas multivorans Glucose-6-Phosphate and 6-Phosphogluconate Dehydrogenases: Differences in Size, Pyridine Nucleotide Specificity, and Susceptibility to Inhibition by Adenosine 5′-Triphosphate

Two major species of glucose-6-phosphate dehydrogenase (EC 1.1.1.49) differing in size, pyridine nucleotide specificity, and susceptibility to inhibition by adenosine 5'-triphosphate (ATP) were detected in extracts of Pseudomonas multivorans (which has recently been shown to be synonymous with the species Pseudomonas cepacia) ATCC 17616. The large species (molecular weight ca. 230,000) was active with nicotinamide adenine dinucleotide (NAD) or nicotinamide adenine dinucleotide phosphate (NADP) and was markedly inhibited by ATP, which decreased its affinity for glucose-6-phosphate and for pyridine nucleotides. This form of the enzyme exhibited homotropic effects for glucose-6-phosphate. The small species (molecular weight ca. 96,000) was active with NADP but not with NAD, was not inhibited by ATP, and exhibited no homotropic effects for glucose-6-phosphate. Under certain conditions multiplicity of 6-phosphogluconate dehydrogenase (EC 1.1.1.43) activities was also noted. One form of the enzyme (80,000 molecular weight) was active with either NAD or NADP and was inhibited by ATP, which decreased its affinity for 6-phosphogluconate. The other form (120,000 molecular weight) was highly specific for NADP and was not susceptible to inhibition by ATP. Neither form of the enzyme exhibited homotropic effects for 6-phosphogluconate. The possible relationships between the different species of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase are discussed.     

Use of a cloned cDNA sequence to measure changes in 6-phosphogluconate dehydrogenase mRNA levels caused by thyroid hormone and dietary carbohydrate

A cDNA clone containing sequences complementary to the mRNA coding for rat hepatic 6-phosphogluconate dehydrogenase has been isolated and used to measure changes in specific mRNA levels during dietary and hormonal regulation of this enzyme. Hepatic mRNA was fractionated by sucrose gradient centrifugation to enrich for 6-phosphogluconate dehydrogenase mRNA sequences. A cDNA library was prepared from the fraction with maximal activity and then screened by differential colony hybridization using probes synthesized either from 6-phosphogluconate dehydrogenase mRNA enriched by polysome immunoadsorption or from unenriched hepatic mRNA. A single colony giving an appropriate differential signal was confirmed to contain sequences encoding 6-phosphogluconate dehydrogenase by specific immunoprecipitation of hybrid-selected translational products. 6-Phosphogluconate dehydrogenase mRNA contains about 2400 bases. The cloned cDNA comprises about 880 bases, or 35% of the mRNA. Southern analysis of restriction endonuclease digests of genomic DNA suggests that the major 6-phosphogluconate dehydrogenase gene is probably present in a single copy in the rat genome. Feeding a fat-free, high carbohydrate diet and administration of thyroid hormone increased the concentration of hybridizable 6-phosphogluconate dehydrogenase mRNA in liver. Thus, both dietary and hormonal regulation of 6-phosphogluconate dehydrogenase synthesis occurs at a pretranslational level.     

Characterization of Aspergillus niger phosphoglucose isomerase. Use for quantitative determination of erythrose 4-phosphate.

Phosphoglucose isomerase (PGI) was purified from Aspergillus niger and the in vitro kinetic properties of the enzyme were related to its functioning in vivo. A new assay method was developed to study the forward reaction making use of mannitol 1-P dehydrogenase as the coupling enzyme. In this simple assay system mannitol 1-P dehydrogenase converts fructose 6-P and NADH to mannitol 1-P and NAD+, respectively. At pH 7.5 the Km for glucose 6-P was 0.48 mM, whereas the Km for fructose 6-P was 0.32 mM. The pentose phosphate pathway intermediates 6-phosphogluconate and erythrose 4-P (E4P) were competitive inhibitors of PGI with Ki values of approximately 0.2 mM and 1 microM respectively. In citric acid producing A. niger mycelium inhibition by 6-phosphogluconate is of minor physiological significance (10% inhibition). Since E4P could not be detected by an existing procedure, a novel assay was developed based on the strong inhibition of PGI by E4P. Although the new assay is very sensitive (detection limit 25 pmol), E4P could still not be detected in metabolite extracts indicating that a very low level of E4P is present in the cells. Using in vitro kinetics and concentrations of intracellular metabolites the in vivo activity of PGI was calculated and closely matched the steady state glycolytic flux observed during citric acid production.     

Purification and kinetic characterization of 6-phosphogluconate dehydrogenase from Schizosaccharomyces pombe.

6-Phosphogluconate dehydrogenase is the pivotal enzyme that links the gluconate route and the oxidative phase of the pentose phosphate pathway in Schizosaccharomyces pombe. The enzyme differs from the known 6-phosphogluconate dehydrogenases of other sources in that the Schizosaccharomyces enzyme is tetrameric having a subunit mass of 38 kDa, that it requires NADP+ obligatorily for activity, and that it can be activated by divalent metal ions such as Co2+ and Mn2+. Steady-state kinetic studies were undertaken. Initial rate and product inhibition results suggest that 6-phosphogluconate dehydrogenase from Schizosaccharomyces pombe catalyzes NADP(+)-linked oxidative decarboxylation of 6-phosphogluconate by an equilibrium random mechanism with two independent binding sites, namely one site for the nicotinamide coenzyme, NADP+/NADPH, and another site for 6-phosphogluconate-D-ribulose-5-phosphate and for CO2. Studies of pH dependence implicated a basic residue with a pK value of 7.4 in the binding of 6-phosphogluconate and an acidic residue with a pK value of 6.7 in the cation-mediated interaction of NADP+ with the enzyme.     

Identification and sequencing of a cDNA encoding 6-phosphogluconate dehydrogenase from a fungus, Cunninghamella elegans and expression of the gene in Escherichia coli

The fungus, Cunninghamella elegans has been widely used in bioremediation and microbial models of mammalian studies in many laboratories. Using the polymerase chain reaction to randomly amplify the insert directly from the single non-blue plaques of a C. elegans cDNA library, then partly sequencing and comparing with GenBank sequences, we have identified a clone which contains C. elegans 6-phosphogluconate dehydrogenase gene. The polymerase chain reaction product was cloned into a plasmid, pGEM-T Easy vector for full insert DNA sequencing. The 6-phosphogluconate dehydrogenase gene (1458 bases) and the deduced protein sequence were determined from the insert DNA sequence. The gene was found by open reading frame analysis and confirmed by the alignment of the deduced protein sequence with other published 6-phosphogluconate dehydrogenase sequences. Several highly conserved regions were found for the 6-phosphogluconate dehydrogenase sequences. The 6-phosphogluconate dehydrogenase gene was subcloned and over-expressed in a plasmid–E. coli system (pQE30). The cell lysate of this clone has a very high 6-phosphogluconate dehydrogenase enzyme activity. Most of the recombinant protein in this system was formed as insoluble inclusion bodies, but soluble in high concentration of urea-buffer. Ni-NTA resin was used to purify the recombinant protein which showed 6-phosphogluconate dehydrogenase enzyme activity. The recombinant protein has a predicted molecular size correlating with that revealed by sodium dodecylsulfate-polyacrylamide gel electrophoresis analysis. The C. elegans 6-phosphogluconate dehydrogenase was in a cluster with yeast' 6-phosphogluconate dehydrogenase in the phylogenetic tree. Bacterial 6-phosphogluconate dehydrogenase and higher organisms' 6-phosphogluconate dehydrogenase were found in different clusters.     

Lipid metabolism by rat lung in vitro. Effect of starvation and re-feeding on utilization of [U-14C]glucose by lung slices

1. The incorporation of [U-(14)C]glucose into several lipid components of lung and liver slices, and the activities of glucose 6-phosphate dehydrogenase (EC 1.1.1.49), 6-phosphogluconate dehydrogenase (EC 1.1.1.44), ;malic' enzyme (EC 1.1.1.40) and NADP-isocitrate dehydrogenase (EC 1.1.1.42) of the cell cytosol were examined in normal, starved and re-fed rats. 2. Lipogenesis and the activities of these enzymes in liver were decreased markedly in rats starved for 72h. Re-feeding starved rats on a fat-free diet for 72h resulted in the well documented hyperlipogenic response in liver, particularly in its ability to convert glucose into neutral lipid, and increased activities of glucose 6-phosphate dehydrogenase, ;malic' enzyme and 6-phosphogluconate dehydrogenase to values approx. 700, 470 and 250% of controls respectively. 3. Approx. 70% of the total label in lung lipids was present in the phospholipid fraction. Hydrolysis of lung phospholipids revealed that lipogenesis from glucose was considerable, with approx. 40% of the total phospholipid radioactivity present in the fatty acid fraction. 4. Incorporation of glucose into total lung lipids was decreased by approx. 40% in lung slices of starved rats and was returned to control values on re-feeding. Although phospholipid synthesis from glucose was decreased in lung slices of starved rats, the decrease proportionally was greater for the fatty acid fraction (approx. 50%) as compared with the glycerol fraction (approx. 25%). 5. The activities of lung glucose 6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase and NADP-isocitrate dehydrogenase were not affected by the dietary alterations. ;Malic' enzyme activity was not detected in lung cytosol preparations. 6. The results are discussed in relation to the surface-active lining layer (surfactant) of the lung.     

Retrograde alteration of 6-phosphogluconate dehydrogenase in axotomized superior cervical ganglia of the rat.

Mechanisms underlying increased activity of 6-phosphogluconate dehydrogenase (6-phospho-D-gluconate: NADP oxidoreductase [decarboxylating] EC 1.1.1.44) in axotomized rat superior cervical ganglia were explored using a highly sensitive micro-immunochemical assay employing antibodies raised in rabbits against the purified enzyme. 6-Phosphogluconate dehydrogenase was purified from rat brain more than 1700-fold by salt fractionation, anion exchange, and immunoaffinity chromatography. The purified enzyme consisted of identical subunits having molecular weights of about 48,800 which could aggregate to catalytically active isomers of various sizes; however, only one form of the enzyme was detected in freshly prepared homogenates of rat neural tissue. Physical and immunological properties of the enzyme from rat brain were similar to those from superior cervical ganglia and liver. Augmented 6-phosphogluconate dehydrogenase activity noted in superior cervical ganglia 2 days after transection of major postganglionic nerve trunks was accompanied by a parallel increase in immunoreactive protein. Michaelis constants of the enzyme were the same in control and axotomized ganglia, and the presence of activators and inhibitors was not detected. It is concluded that increases in 6-phosphogluconate dehydrogenase subsequent to axotomy can be accounted for entirely by an increase in the steady state concentration of this protein.     

Enzymes of glucose metabolism in Frankia sp.

Enzymes of glucose metabolism were assayed in crude cell extracts of Frankia strains HFPArI3 and HFPCcI2 as well as in isolated vesicle clusters from Alnus rubra root nodules. Activities of the Embden-Meyerhof-Parnas pathway enzymes glucokinase, phosphofructokinase, and pyruvate kinase were found in Frankia strain HFPArI3 and glucokinase and pyruvate kinase were found in Frankia strain HFPCcI2 and in the vesicle clusters. An NADP+-linked glucose 6-phosphate dehydrogenase and an NAD-linked 6-phosphogluconate dehydrogenase were found in all of the extracts, although the role of these enzymes is unclear. No NADP+-linked 6-phosphogluconate dehydrogenase was found. Both dehydrogenases were inhibited by adenosine 5-triphosphate, and the apparent Km's for glucose 6-phosphate and 6-phosphogluconate were 6.86 X 10(-4) and 7.0 X 10(-5) M, respectively. In addition to the enzymes mentioned above, an NADP+-linked malic enzyme was detected in the pure cultures but not in the vesicle clusters. In contrast, however, the vesicle clusters had activity of an NAD-linked malic enzyme. The possibility that this enzyme resulted from contamination from plant mitochondria trapped in the vesicle clusters could not be discounted. None of the extracts showed activities of the Entner-Doudoroff enzymes or the gluconate metabolism enzymes gluconate dehydrogenase or gluconokinase. Propionate- versus trehalose-grown cultures of strain HFPArI3 showed similar activities of most enzymes except malic enzyme, which was higher in the cultures grown on the organic acid. Nitrogen-fixing cultures of strain HFPArI3 showed higher specific activities of glucose 6-phosphate and 6-phosphogluconate dehydrogenases and phosphofructokinase than ammonia-grown cultures.     

Degradation of maltose by proliferating cells of Desulfovibrio desulfuricans 2198.

Desulfovibrio desulfuricans 2198 can grow on maltose-based medium only in the presence of yeast extract. The results of kinetic measurements of maltose consumption by the cells show that there is no marked difference in Km and Vmax values for this bacterium versus other carbohydrate-utilizing microorganisms. The determination of some enzymes of sugar metabolism in D. desulfuricans 2198 suggests that maltose degradation occurs by the Embden--Meyerhof pathway. The cell extract also contains glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase. 2-Keto-3-deoxy-6-phosphogluconate aldolase, the key enzyme of the Entner--Doudoroff pathway, is not found in D. desulfuricans 2198. In the bacterium grown on [U-14C]maltose-containing medium, a portion of the labeled carbon is incorporated into biomass. As degradation products, labeled acetate and carbon dioxide are found.     

Purification and Characterization of the Two 6-Phosphogluconate Dehydrogenase Species from Pseudomonas multivorans

The two species of 6-phosphogluconate dehydrogenase (EC 1.1.1.43) from Pseudomonas multivorans were resolved from extracts of gluconate-grown bacteria and purified to homogeneity. Each enzyme comprised between 0.1 and 0.2% of the total cellular protein. Separation of the two enzymes, one which is specific for nicotinamide adenine dinucleotide phosphate and the other which is active with nicotinamide adenine dinucleotide or nicotinamide adenine dinucleotide phosphate was facilitated by the marked difference in their respective isoelectric points, which were at pH 5.0 and 6.9. Comparison of the subunit compositions of the two enzymes indicated that they do not share common peptide chains. The enzyme active with nicotinamide adenine dinucleotide was composed of two subunits of about 40,000 molecular weight, and the nicotinamide adenine dinucleotide phosphate-specific enzyme was composed of two subunits of about 60,000 molecular weight. Immunological studies indicated that the two enzymes do not share common antigenic determinants. Reduced nicotinamide adenine dinucleotide phosphate strongly inhibited the 6-phosphogluconate dehydrogenase active with nicotinamide adenine dinucleotide by decreasing its affinity for 6-phosphogluconate. Guanosine-5'-triphosphate had a similar influence on the nicotinamide adenine dinucleotide phosphate-specific 6-phosphogluconate dehydrogenase. These results in conjunction with other data indicating that reduced nicotinamide adenine dinucleotide phosphate stimulates the conversion of 6-phosphogluconate to pyruvate by crude bacterial extracts suggest that in P. multivorans, the relative distribution of 6-phosphogluconate into the pentose phosphate and Entner-Doudoroff pathways might be determined by the intracellular concentrations of reduced nicotinamide adenine dinucleotide phosphate and purine nucleotides.     

Purification and properties of an NADP-specific 6-phosphogluconate dehydrogenase from Streptococcus faecalis.

A procedure is described for the purification of 6-phosphogluconate dehydrogenase (6-phospho-D-gluconate:NADP oxidoreductase (decarboxylating) EC 1.1.1.44) from cell extracts of Streptococcus gaecalis. A 180-fold purification was achieved with an over-all yield of about 12% and an average specific activity of 14. The enzyme was homogeneous as determined by polyacrylamide gel electrophoresis, immunoelectrophoresis, and sedimentation equilibrium, studies. Its weight average molecular weight, as measured by sedimentation equilibrium, was 108,000 +/- 3,600. Other methods employed for molecular weight determinations gave values that ranged between 106,000 and 115,000. An analysis of the enzyme by sodium dodecyl sulfate polyacrylamide gel electrophoresis showed it to be a dimer composed of subunits having equal molecular weight. The amino acid composition of the streptococcal enzyme is reported. The apparent Km values for NADP and 6-phosphogluconate were calculated from kinetic data and found to be 0.015 mM and 0.024 mM, respectively. Kinetic studies also indicated that the binding of one substrate did not affect the apparent affinity of the enzyme for the other substrate.     

Effect of gluconic acid as a secondary carbon source on non-growing L-lysine producers cells of Corynebacterium glutamicum. Purification and properties of 6-phosphogluconate dehydrogenase

We studied the production of L-lysine in Corynebacterium glutamicum ATCC 21543 non growing cells obtained by nutrient limitation. Statistical analysis revealed significant differences in the L-lysine titers of glucose, gluconic acid or glucose-gluconic acid cultures. Higher L-lysine titer obtained in batch cultures with mixed carbon sources or gluconic acid alone were found to be associated with a high 6-phosphogluconate dehydrogenase activity (6PGDH, E.C.1.1.1.44). This enzyme is a pivotal enzyme within the hexose monophosphate pathway, and thus of importance for L-lysine production. 6PGDH was purified and characterized. The purified enzyme migrates as a single band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis with a molecular mass of 52.5 kDa. The molecular mass of the native enzyme was estimated to be 120 kDa by molecular exclusion chromatography, thus suggesting a homodimeric structure. The amino terminal sequence shows a strong similarity (a match of 86% of the first 20 amino acid) to the 6PGDH from other microorganisms such as, E. coli and B. subtilis. The pI of the dimeric native enzyme and the optimum pH were 6.2 and 8.0, respectively. For the oxidative decarboxylation of 6-phosphogluconate, K_m of 71 μM and 43 μM were obtained for 6-phosphogluconate and NADP⁺, respectively.     

Changes in the rates of synthesis and messenger RNA levels of hepatic glucose-6-phosphate and 6-phosphogluconate dehydrogenases following induction by diet or thyroid hormone

The lipogenic capacity of rat liver is increased in animals fed a high carbohydrate, fat-free diet or by the administration of 2,2',5'-triiodo-L-thyronine. Underlying this change is a generalized induction of the enzymes involved in lipogenesis, including glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, and malic enzyme, which together serve to generate the additional NADPH required for increased fatty acid synthesis. This report presents evidence indicating that induction of the hexose-shunt dehydrogenases involves increased enzyme synthesis secondary to elevated enzyme specific mRNA levels, as has previously been shown for malic enzyme. Activities of specific mRNAs, estimated by cell-free translation of hepatic poly(A)-containing RNA in the mRNA dependent rabbit reticulocyte lysate, were compared with enzyme specific activities and relative rates of specific enzyme synthesis. The 2-fold increase in glucose-6-phosphate dehydrogenase specific activity in hyperthyroid rats and the 13-fold increase in rats fed a high carbohydrate, fat-free diet, relative to euthyroid, chow-fed controls were paralleled by comparable increases in the synthetic rates and mRNA levels of this enzyme. Similarly, consonant changes in the rate of enzyme synthesis and concentration of 6-phosphogluconate dehydrogenase mRNA accompanied the 2.5- and 3-fold increases in specific activity of this enzyme observed in response to hormonal and dietary induction, respectively. Thus, both thyroid hormone and carbohydrate feeding appear to induce glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase primarily by increasing the effective cellular concentrations of their respective mRNAs and, consequently, their rates of synthesis.     

Regulation of alternate peripheral pathways of glucose catabolism during aerobic and anaerobic growth of Pseudomonas aeruginosa.

Glucose may be converted to 6-phosphogluconate by alternate pathways in Pseudomonas aeruginosa. Glucose is phosphorylated to glucose-6-phosphate, which is oxidized to 6-phosphogluconate during anaerobic growth when nitrate is used as respiratory electron acceptor. Mutant cells lacking glucose-6-phosphate dehydrogenase are unable to catabolize glucose under these conditions. The mutant cells utilize glucose as effectively as do wild-type cells in the presence of oxygen; under these conditions, glucose is utilized via direct oxidation to gluconate, which is converted to 6-phosphogluconate. The membrane-associated glucose dehydrogenase activity was not formed during anaerobic growth with glucose. Gluconate, the product of the enzyme, appeared to be the inducer of the gluconate transport system, gluconokinase, and membrane-associated gluconate dehydrogenase. 6-Phosphogluconate is probably the physiological inducer of glucokinase, glucose-6-phosphate dehydrogenase, and the dehydratase and aldolase of the Entner-Doudoroff pathway. Nitrate-linked respiration is required for the anaerobic uptake of glucose and gluconate by independently regulated transport systems in cells grown under denitrifying conditions.     

Ribulose 1,5-bisphosphate carboxylase/oxygenase from Pseudomonas oxalacticus.

Ribulose 1,5-bisphosphate carboxylase/oxygenase was purified by a rapid, facile procedure from formate-grown Pseudomonas oxalaticus. The electrophoretically homogeneous enzyme had specific activities of 1.9 mumol of CO2 fixed per min per mg of protein and 0.15 mumol of O2 consumed per min per mg of protein. The amino acid composition was similar to that of other bacterial sources of the enzyme. The molecular weights determined by sedimentation equilibrium and by gel filtration were 421,000 and 450,000, respectively. Upon sodium dodecyl sulfate electrophoresis of enzyme purified under conditions which would limit proteolysis, two types of large (L) subunits and two types of small (S) subunits were observed with apparent molecular weights of 57,000, 55,000, 17,000 and 15,000. By densitometric scans at two different protein concentrations the stoichiometry of the total large to total small subunits was 1:1, implying an L6S6 structure. Electron micrographs of the enzyme revealed an unusual structure that was inconsistent with a cubical structure. The enzyme had an unusually high Km for ribulose 1,5-bisphosphate (220 microM) and was strongly inhibited by 6-phosphogluconate in the ribulose 1,5-bisphosphate carboxylase assay (Ki = 270 microM). One, 5, and 12 days after purification the enzyme was half-maximally activated at 0.13 microM, 0.23 mM, and 0.70 mM CO2, respectively, at saturating Mg2+. At saturating CO2, enzyme 1 day afer purification responded sigmoidally to Mg2+ and was half-maximally activated by 0.85 mM Mg2+ in the absence of 6-phosphogluconate (Hill coefficient, h = 2.0) and by 0.19 mM Mg2+ in the presence of mM 6-phosphogluconate (h = 1.7).     

Purification and Characterization of 6-Phosphogluconate Dehydrogenase from Phormidium sp.

the native enzyme was 104,000 by gel filtration, and SDS-polyacrylamide gel electrophoresis showed that the enzyme consisted of two subunits with an identical molecular weight of 52,000. The optimum pH of the reaction was 8.0. The Km values for 6-phosphogluconate and NADP were 3.6×10^?5m and 1.3 × 10^?5m, respectively. The enzyme showed no Mg^2𠀫 requirement for the activity, but was activated by Mn^2𠀫 and Ca^2𠀫. The enzyme was inhibited by sulfhydryl reagents, indicating that a sulfhydryl group may be involved in the active site of the enzyme. The enzyme was also inhibited by NADPH₂, ATP, and the intermediates formed during photosynthesis. The substrate 6-phosphogluconate and cofactor NADP partially protected the enzyme from inactivation. The enzyme had enzymological and physicochemical properties similar to enzymes isolated from other sources.