Fluorescent labelling of 6-phosphogluconate dehydrogenase from Bacillus stearothermophilus.

The thermophilic 6-phosphogluconate dehydrogenase from Bacillus stearothermophilus was inhibited upon specific modification of the -SH group of cysteine residues by 7-chloro-4-nitrobenzo-2-oxa-1, 3-diazole (NBD-Cl) at pH 7.0. By using 20-100-fold molar excess of NBD-CL the reaction occurs slowly at pH 7.0 as a first order process. Partial protection from inactivation was observed when the substrate 6-phosphogluconate or the coenzyme NADP was added to the reaction mixture. Complete inactivation was achieved upon modification of 1.9 of the six cysteine residues per mole of enzyme, which corresponds to nearly one residue per enzyme subunit. Circular dichroism measurements suggest that the gross structure of the protein molecule is practically unchanged upon reaction of the enzyme with NBD-Cl. Melting profile experiments revealed a single transition occurring at about 65 degrees C. Analogously, the profile of intensity of the fluorescence emission at 520 nm of the enzyme-bound S-NBD groups versus temperature indicated a midpoint of transition near 65 degrees C. Since this melting temperature corresponds closely to that observed with the native enzyme, these results would indicate that the molecular organizations of the native and modified enzyme are similar and stabilized by similar interactions within the polypeptide chain.     

Half-site reactivity of an essential thiol group of D-beta-hydroxybutyrate dehydrogenase

D-beta-Hydroxybutyrate dehydrogenase is a lipid-requiring enzyme, which is a tetramer both in the mitochondrial inner membrane and as the purified enzyme reconstituted with phospholipid. For the active enzyme-phospholipid complex in the absence of ligands, we previously found that reaction with N-ethylmaleimide (at 5 mol/mol of enzyme subunit) resulted in progressive loss of enzymic activity with an inactivation stoichiometry of 1 equiv of sulfhydryl derivatized per mole of enzyme and a maximum derivatization of 2 equiv [Latruffe, N., Brenner, S. C., & Fleischer, S. (1980) Biochemistry 19, 5285-5290]. We now find, in the presence of nucleotide or substrate, that the rate of inactivation is significantly reduced, which indicates that these ligands afford protection of the essential sulfhydryl. Further, in the presence of ligands, the inactivation stoichiometry is 0.5, consistent with half-of-the-site reactivity of the essential sulfhydryl. Thus, at a low ratio of N-ethylmaleimide to enzyme, nucleotide or substrate affords essentially complete protection of the nonessential sulfhydryl from derivatization. The binding characteristics of NADH to both the native and N-ethylmaleimide-derivatized enzyme have been compared by fluorescence spectroscopy. Quenching of intrinsic tryptophan fluorescence of the protein shows that the enzyme, derivatized with N-ethylmaleimide either in the absence or in the presence of NAD+, binds NADH but with a reduced Kd (approximately 50 microM as compared with approximately 20 microM for native enzyme). However, a critical change has occurred in that resonance energy transfer from protein to bound NADH, observed in the native enzyme, is abolished in the N-ethylmaleimide-derivatized enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)     

An electron cytochemical study of 6-phosphogluconate dehydrogenase activity in infected erythrocytes during malaria

The distribution of 6-phosphogluconate dehydrogenase (6-PGD) was examined by an electron microscopic technique in erythrocytes of rodents infected with $\text{Plasmodium berghei}$, chickens with $\text{P. gallinaceum}$ and rhesus monkeys with $\text{P. knowlesi}$. Unlike glucose-6-phosphate dehydrogenase, which is restricted to the host erythrocyte, 6-PGD was found to be present in the parasite as well as the host erythrocyte in all infections studied. The implications of these findings are discussed in relation to the metabolism of malaria parasites.     

The foetal development of lactate dehydrogenase isoenzymes, glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase from human striated muscle

1. Human foetal skeletal muscles involved in support and in periodic contractility were studied for their content of total extractable lactate dehydrogenase, glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase activities as well as for the relative distribution of lactate dehydrogenase isoenzymes. 2. During foetal development a linear steady increase in total lactate dehydrogenase activity as well as a linear decrease in the H/M sub-unit ratio of the isoenzymes was found. 3. No significant changes were found in the activities of the enzymes of the hexose monophosphate shunt (C-6 oxidation). 4. The changes found suggest a steady increased synthesis of lactate dehydrogenase M-sub-units in human skeletal muscles during foetal development. 5. The weekly changes in the total lactate dehydrogenase activity and in lactate dehydrogenase isoenzymes are lower in muscles involved in support than in those involved in periodic contractility. 6. These findings, together with the literature available, are consistent with the morphological fact that foetal development of skeletal muscles mostly concerns the white muscle fibres and not the red muscle fibres.     

The role of 6-phosphogluconate dehydrogenase in Rhizobium.

A nicotinamide adenine dinucleotide (NAD) linked 6-phosphogluconate (6-PG)dehydrogenase has been detected in Rhizobium. The enzyme activity is similar in both slow- and fast-growing rhizobia. The nicotinamide adenine dinucleotide phosphate (NADP) dependent 6-PG dehydrogenase was detected only in the fast growers and was more than twice as active as the NAD-linked enzyme. Partial characterization of the products of 6-PG oxidation in Rhizobium suggests that the NADP-linked enzyme is the decarboxylating enzyme of the pentose phosphate (PP) pathway (EC 1.1.1.44) whereas a phosphorylated six-carbon compound, containing ketonic group(s), is the product of the oxidation catalyzed by the NAD-linked enzyme.     

The 6-phosphogluconate dehydrogenase reaction in Escherichia coli.

This study is an attempt to relate in vivo use of the 6-phosphogluconate dehydrogenase reaction in Escherichia coli with the characteristics of the enzyme determined in vitro. 1) The enzyme was obtained pure by affinity chromatography and kinetically characterized; as already known, ATP and fructose-1,6-P2 were inhibitors. 2) A series of isogenic strains were made in which in vivo use of thereaction might differ, e.g. a wild type strain versus a mutant lacking 6-phosphogluconate dehydrase, as grown on gluconate; a phosphoglucose isomerase mutant grown on glucose or glycerol. 3) The in vivo rate of use of the 6-phosphogluconate dehydrogenase reaction was determined from measurements of growth rate and yield and from the specific activity of alanine after growth in 1-14C-labeled substrates. 4) The intracellular concentrations of 6-phosphogluconate, NADP+, fructose-1,6-P2, and ATP were measured for the strains in growth on several carbon sources. 5) The metabolite concentrations were used for assay of the enzyme in vitro. The results allow one to calculate how fast the reaction would function in vivo if ATP and fructose-1,6-P2 were its important effectors and if the in vitro assay conditions apply in vivo. The predicted in vivo rates ranged down to as low as one-tenth of the actual rates, and, accordingly, one cannot yet draw firm conclusions about how the reaction is actually controlled in vivo.     

Kinetic studies of 6-phosphogluconate dehydrogenase from sheep liver

The steady-state kinetics of the oxidative decarboxylation of 6-phosphogluconate catalysed by 6-phosphogluconate dehydrogenase from sheep liver in triethanolamine and phosphate buffers (pH 7.0) have been reinvestigated. In triethanolamine buffer the enzyme is inhibited by high NADP+ concentrations in the presence of low fixed concentrations of 6-phosphogluconate. Data are consistent with an asymmetric sequential mechanism in which NADP+ and 6-phosphogluconate bind randomly and product release is ordered. The pathway through the enzyme--6-phosphogluconate complex appears to be preferred in triethanolamine buffer. Pre-steady-state studies of the oxidative decarboxylation reaction at pH 6.0-8.0 show that hydride transfer is greater than 900 s-1. After the fast formation of NADPH in amounts equivalent to about half of the enzyme-active-centre concentration, the rate of NADPH formation is equal to the steady-state rate. Two possible interpretations are considered. Rapid fluorescence measurements of the displacement of NADPH from its complex with the enzyme at pH 6.0 and 7.0 indicate that the dissociation of NADPH is fast (greater than 800 s-1) and cannot be the rate-limiting step in oxidative decarboxylation. Coenzyme binding studies at equilibrium have been extended to include the determination of the dissociation constants for the binary complexes of enzyme with NADPH and NADP+ at pH 6.0-8.0 and the dissociation constant for NADPH in the ternary enzyme--6-phosphogluconate--NADPH complex in triethanolamine buffer, pH 7.0.     

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.     

Identification of a cDNA encoding 6-phosphogluconate dehydrogenase from a human heart cDNA library

A full-length cDNA clone for human 6-phosphogluconate dehydrogenase (PGD) was isolated from a human adult heart cDNA library. The clone encoded an open reading frame of 483 amino acids. When the amino acid sequences of human PGD and sheep PGD were aligned, 94.2% identity between these two proteins was found. Its calculated molecular weight is 53,149 daltons. The predicted isoelectric point is 6.85. When the secondary structure of human PGD was examined by the PROSIS software, 36% α-helix and 9% β-sheet were found.     

Effectors of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase of rat liver.

Summary The lower Vmax of 6PGDH with respect to G6PDH and its higher sensitivity to inhibition by NADPH, suggest the existence of an imbalance between the two dehydrogenases of the pentose phosphate pathway in rat liver. Possible modulators of these activities, particularly in relation with the inhibition by NADPH in physiological conditions, have been investigated. The results suggest that in both cases the inhibition by NADPH is strictly isosteric and that the relative affinities for the reduced and oxidized forms of the pyridine nucleotide are unaffected by glutathion, the intermediates of the pentose phosphate shunt or some divalent ions.Abbreviations G6PDH glucose-6-phosphate dehydrogenase (EC 1.1.1.49) - 6PGDH 6-phosphogluconate dehydrogenase (EC 1.1.1.44)On leave from the Instituto de Bioquímica, Facultad de Ciencias, Universidad Austral de Chile, Casilla 567, Valdivia, Chile.     

Purification and properties of 6-phosphogluconate dehydrogenase from sheep liver

A method is described for the isolation of 6-phosphogluconate dehydrogenase from sheep liver. The product appears to be homogeneous in polyacrylamide-gel electrophoresis and in sedimentation-velocity and sedimentation-equilibrium studies in the ultracentrifuge. The molecular weight is estimated as 129000 from equilibrium sedimentation.     

Kinetic characteristics of 6-phosphogluconate dehydrogenase from bull adrenal cortex

The hyperbolic dependence of the initial rate of 6-phosphogluconate dehydrogenase-catalyzed reaction on 6-phosphogluconate and NADP concentrations has been established. The Lineweaver-Burk plots of V0 against concentration of one substrate with constant unsaturating concentrations of another substrate cross left from the ordinate axis. The Km value for 6-phosphogluconate is equal ot 0.035 mM, for NADP--0.018 mM. It has been shown that NADPH inhibits 6-phosphogluconate dehydrogenase by the competitive type with respect to NADP and by the noncompetitive one with respect to 6-phosphogluconate. Ribulose-5-phosphate inhibits the reaction by the mixed type with respect to NADP and by the noncompetitive type with respect to 6-phosphogluconate. Kinetic data are in agreement with the consecutive mechanism of the reaction: the first substrate is NADP, the last product--NADPH. The Arrhenius plot for the reaction shows a break at 27 degrees C.     

Importance in catalysis of the 6-phosphate-binding site of 6-phosphogluconate in sheep liver 6-phosphogluconate dehydrogenase

The 6-phosphate of 6-phosphogluconate (6PG) is proposed to anchor the sugar phosphate in the active site and aid in orientating the substrate for catalysis. In order to test this hypothesis, alanine mutagenesis was used to probe the contribution of residues in the vicinity of the 6-phosphate to binding of 6PG and catalysis. The crystal structure of sheep liver 6-phosphogluconate dehydrogenase shows that Tyr-191, Lys-260, Thr-262, Arg-287, and Arg-446 contribute a mixture of ionic and hydrogen bonding interactions to the 6-phosphate, and these interactions are likely to provide the majority of the binding energy for 6PG. All mutant enzymes, with the exception of T262A, exhibit an increase in K(6PG) that ranges from 5- to 800-fold. There is also a less pronounced increase in K(NADP), ranging from 3- to 15-fold, with the exception of T262A. The R287A and R446A mutant enzymes exhibit a dramatic decrease in V/E(t) (600- and 300-fold, respectively) as well as in V/K(6PG)E(t) (10(5) - and 10(4)-fold), and therefore no further characterization was carried out with these two mutant enzymes. No change in V/E(t) was observed for the Y191A mutant enzyme, whereas 20- and 3-fold decreases were obtained for the K260A and T262A mutant enzymes, respectively, resulting in a decrease in V/K(6PG)E(t) range from 3- to 120-fold. All mutant enzymes also exhibit at least an order of magnitude increase in 13C-isotope effect -1, indicating that the decarboxylation step has become more rate-limiting. Data are consistent with significant roles for Tyr-191, Lys-260, Thr-262, Arg-287, and Arg-446 in providing the binding energy for 6PG. In addition, these residues also likely ensure proper orientation of 6PG for catalysis and aid in inducing the conformation change that precedes, and sets up the active site for, catalysis.     

Substrate-induced intramolecular proton transfer in 6-phosphogluconate dehydrogenase from Candida utilis

Formation of binary complex between 6-phosphogluconate dehydrogenase (6-phospho-D-gluconate:NADP+ 2-oxidoreductase (decarboxylating), EC 1.1.1.44) from Candida utilis and 6-phosphogluconate was investigated by means of ultraviolet difference spectroscopy. The formation of the enzyme-substrate complex induces in the difference spectrum a positive peak the wavelength and extinction coefficient of which agree well with a tyrosine ionization. Titrimetric studies indicate that the formation of the binary complex is not coupled to a proton release from the protein. These data support an intramolecular proton transfer from a tyrosine to other functional group. This proton transfer could be correlated to the conformational change induced by substrate in 6-phosphogluconate dehydrogenase.     

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.