Competitive inhibition of phosphoglucose isomerase of apple leaves by sorbitol 6-phosphate

Apple leaf cytosolic phosphoglucose isomerase (PGI, EC 5.3.1.9) was purified to an apparent homogeneity with a specific activity of 2456units/mg protein, and chloroplastic PGI was partially purified to a specific activity of 72units/mg protein to characterize their biochemical properties. These two isoforms showed differential responses to heat treatment; incubation at 50 degrees C for 10min resulted in a complete loss of the chloroplastic PGI activity, whereas the cytosolic PGI only lost 50% of its activity. Apple cytosolic PGI is a dimeric enzyme with a molecular mass of 66kDa for each monomer. The activity of both isoforms was strongly inhibited by erythrose 4-phosphate (E4P) with a K(i) of 1.2 and 3.0muM for the cytosolic PGI and chloroplastic PGI, respectively. Sorbitol 6-phosphate (Sor6P), an intermediate in sorbitol biosynthesis, was found to be a competitive inhibitor for both cytosolic and chloroplastic PGIs with a K(i) of 61 and 40muM, respectively. PGIs from both spinach and tomato leaves were also inhibited by Sor6P in a similar manner. The possible physiological significance of this finding is discussed.     

Competitive inhibition of Trypanosoma brucei phosphoglucose isomerase by D-arabinose-5-phosphate derivatives

We report four new strong high energy intermediate analog competitive inhibitors of fructose-6-phosphate isomerization catalyzed by purified Trypanosoma brucei phosphoglucose isomerase: D-arabinonhydroxamic acid-5-phosphate, D-arabinonate-5-phosphate, D-arabinonamide-5-phosphate and D-arabinonhydrazide-5-phosphate. For comparison, the inhibitory properties of the corresponding non-phosphorylated analogues D-arabinonhydroxamic acid, D-arabinonate, D-arabinonamide and D-arabinonhydrazide were also evaluated. D-Arabinonhydroxamic acid-5-phosphate appears as the most potent competitive inhibitor ever evaluated on a phosphoglucose isomerase with an inhibition constant value of 50 nM and a Michaelis constant over inhibition constant ratio of about 2000. Our results show that anionic high energy intermediate analogues, and more particularly D-arabinonhydroxamic acid-5-phosphate, display a weak but significant specificity for Trypanosoma brucei phosphoglucose isomerase versus yeast phosphoglucose isomerase, while neutral high energy intermediate analogues are not selective at all. This would indicate the presence of more positively charged residues in the active site for Trypanosoma brucei phosphoglucose isomerase as compared to that of yeast phosphoglucose isomerase.     

Comparative inactivation and inhibition of the anomerase and isomerase activities of phosphoglucose isomerase

Summary Several metabolic compounds have been found to be competitive inhibitors of the anomerase activity of phosphoglucose isomerase (EC 5.3.1.9).K_i values for erythrose 4-phosphate, 6-phosphogluconate, and fructose 1,6-bisphosphate for the anomerase reaction are 0.32 μM, 21 μM, and 84 μM respectively at 0° and pH 8.2. A significant difference between the fructose, 1,6-bisphosphate inhibition constants for both activities was found (k_i(isomerase)=800 μM and K_i(anomerase)=84 μM). Also the K_m values for both activities were found to be significantly different (K_m(isomerase)=140 μM and K_m(anomerase)=3.6 μM). Attempts to independently alter the anomerase to isomerase activity ratio through protein modification yielded mixed results. While several modifying reagents destroyed the catalytic activities at identical rates, inactivation by iodoacetamide or pyridoxal 5′ phosphate sensitized photo-oxidation displayed differential initial effects on the two activities with the anomerase activity being the less affected. These data support the theory that an imidazole residue is catalytically important for isomerization, but less so for anomerization.     

Effect of 6-Phosphogluconate on Phosphoglucose Isomerase in Rat Brain In Vitro and In Vivo

The activity of phosphoglucose isomerase, its kinetic properties, and the effect of 6-phosphogluconate on its activity in the forward (glucose 6-phosphate----fructose 6-phosphate) and the reverse (fructose 6-phosphate----glucose 6-phosphate) reactions were determined in adult rat brain in vitro. The activity of phosphoglucose isomerase (in nmol/min/mg of whole brain protein) was 1,865 +/- 20 in the forward reaction and 1,756 +/- 32 in the reverse reaction at pH 7.5. It was 1,992 +/- 28 and 2,620 +/- 46, respectively, at pH 8.5. The apparent Km and Vmax of phosphoglucose isomerase were 0.593 +/- 0.031 mM and 2,291 +/- 61 nmol/min/mg of protein, respectively, for glucose 6-phosphate and 0.095 +/- 0.013 mM and 2,035 +/- 98 nmol/min/mg of protein, respectively, for fructose 6-phosphate. The activity of phosphoglucose isomerase was inhibited intensely and competitively by 6-phosphogluconate, with an apparent Ki of 0.048 +/- 0.005 mM for glucose 6-phosphate and 0.042 +/- 0.004 mM for fructose 6-phosphate as the substrate. With glucose 6-phosphate as the substrate, at concentrations from 0.05 to 0.5 mM, the activity of the enzyme was inhibited completely in the presence of 0.5-2.0 mM 6-phosphogluconate. With 0.05-0.2 mM fructose 6-phosphate as the substrate, it was inhibited greater than or equal to 85% at the same concentrations of the inhibitor. No significant changes were observed in the values of Km, Vmax, and Ki for phosphoglucose isomerase in the brain of 6-aminonicotinamide-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Studies on 3-deoxy-D-arabinoheptulosonate-7-phosphate synthetase(phe)from Escherichia coli K12. 2. Kinetic properties.

1. Co2+ is not a cofactor for 3-deoxy-D-arabinoheptulosonate-7-phosphate synthetase(phe). 2. The following analogues of phosphoenolpyruvate were tested as inhibitors of 3-deoxy-D-arabinoheptolosonate-7-phosphate synthetase(phe): pyruvate, lactate, glycerate, 2-phosphoglycerate, 2,3-bisphosphoglycerate, 3-methylphosphoenolpyruvate, 3-ethylphosphoenolpyruvate and 3,3-demethylphosphoenolpyruvate. The rusults obtained indicate that the binding of phosphoenolpyruvate to the enzyme requires a phosphoryl group on the C-2 position of the substrate and one free hydrogen atom at the C-3 position. 3. The dead-end inhibition pattern observed with the substrate analogue 2-phosphoglycerate when either phosphoenolpyruvate or erythrose 4-phosphate was the variable substrate is inconsistent with a ping-pong mechanism and indicates that the reaction mechanism for this enzyme must be sequential. The following kinetic constants were determined:Km for phosphoenolpyruvate, 0.08 +/- 0.04 mM; Km for erythrose 4-phosphate, 0.9 +/- 0.3 mM; K is for competitive inhibition by 2-phosphoglycerate with respect to phosphoenolpyruvate, 1.0 +/- 0.1 mM. 4. The enzyme was observed to have a bell-shaped pH PROFILE WITH A PH OPTIMUM OF 7.0. The effects of pH ON V and V/(Km for phosphoenolpyruvate) indicated that an ionizing group of pKa 8.0-8.1 is involved in the catalytic activity of the enzyme. The pKa of this group is unaffected by the binding of phosphoenolpyruvate.     

The crystal structure of phosphoglucose isomerase/autocrine motility factor/neuroleukin complexed with its carbohydrate phosphate inhibitors suggests its substrate/receptor recognition

Phosphoglucose isomerase catalyzes the reversible isomerization of glucose 6-phosphate to fructose 6-phosphate. In addition, phosphoglucose isomerase has been shown to have functions equivalent to neuroleukin, autocrine motility factor, and maturation factor. Here we present the crystal structures of phosphoglucose isomerase complexed with 5-phospho-D-arabinonate and N-bromoacetylethanolamine phosphate at 2.5- and 2.3-A resolution, respectively. The inhibitors bind to a region within the domains' interface and interact with a histidine residue (His(306)) from the other subunit. We also demonstrated that the inhibitors not only affect the enzymatic activity of phosphoglucose isomerase, but can also inhibit the autocrine motility factor-induced cell motility of CT-26 mouse colon tumor cells. These results indicate that the substrate and the receptor binding sites of phosphoglucose isomerase and autocrine motility factor are located within close proximity to each other. Based on these two complex structures, together with biological and biochemical results, we propose a possible isomerization mechanism for phosphoglucose isomerase.     

The kinetics of a purified form of 3-deoxy-D-arabino heptulosonate-7-phosphate synthase (tryptophan) from Neurospora crassa.

1. A method is described for the purification of a form of 3-deoxy-D-arabinoheptulosonate-7-phosphate synthase (tryptophan) that probably differs from that of the native enzyme. 2. The kinetics of the reaction catalysed by 3-deoxy-D-arabinoheptulosonate-7-phosphate synthase (tryptophan) shows that the reaction proceeds via a ping-pong bi-bi mechanism, with activation by phosphoenolpyruvate (P-Prv), the first substrate, and inhibition by erythrose 4-phosphate (Ery-P) the second substrate. At low substrate concentrations, KP-Prv is 0.1 mM and KEry-P is 0.13 mM. 3. The substrates phosphoenolpyruvate and erythrose 4-phosphate and the product inorganic phosphate can protect the purified enzyme against heat denaturation, whereas the inhibitor, tryptophan, has no effect, although it binds to the enzyme in the absence of other ligands. 4. Product inhibition by inorganic phosphate is linear non-competitive with respect to phosphoenolpyruvate (Ki, slope = 22 mM and Ki, intercept = 54 mM) and substrate-linear competitive with respect to erythrose 4-phosphate (Ki, slope = 25 mM). 5. The enzyme has an activity optimum at pH 7.3 and a tryptophan inhibition optimum at pH 6.4, Trp 0.5 is 4 microM. Inhibition by tryptophan is non-competitive with respect to phosphoenolpyrovate and substrate-parabolic competitive with respect to erythrose 4-phosphate. 6. The role of the enzyme in metabolic regulation is discussed.     

Functional multiplicity of phosphoglucose isomerase from Lactobacillus casei

Phosphoglucose isomerase (D-glucose-6-phosphate ketolisomerase, EC 5.3.1.9), purified from Lactobacillus casei, showed multiplicity with respect to electrophoretic mobility, molecular weight, kinetic properties and responses to erythrose 4-phosphate. Among the three forms isolated, one having a dimeric conformation, was specific for glucose 6-phosphate. Erythrose 4-phosphate inhibited this preparation in a sigmoid fashion, while this compound activated the enzyme for isomerization of ribose 5-phosphate. In tetrameric conformation of the similar subunits, the enzyme was more specific for ribose 5-phosphate and the inhibition exerted by erythrose 4-phosphate was hyperbolic. The possible implications of these observations have been discussed.     

Stimulation of mutases and isomerases by vanadium.

The effects of vanadate and vanadate complexes on the rates of exchange of phosphoryl groups in the reactions catalyzed by the enzymes phosphoglucomutase and the coupled system formed by phosphoglycerate mutase and enolase, and the effects of vanadyl complexes on the interconversion of aldehyde and keto groups catalyzed by the enzymes phosphomannose isomerase, phosphoribose isomerase, and phosphoglucose isomerase, were measured using one-dimensional 31P nuclear magnetic resonance spectroscopy. Chemical exchange was investigated by observing the transfer of magnetization achieved by selective irradiation of resonances using the DANTE pulse sequence. The presence of vanadium stimulated the catalytic activity of the enzymes in vitro, with the exception of enolase whose activity was not affected. Addition of vanadate also increased the rate constants of the interconversion of glucose 6-phosphate and fructose 6-phosphate in hemolysates. 51V nuclear magnetic resonance spectroscopy and electron paramagnetic resonance spectroscopy were employed to investigate the interactions between ammonium vanadate and sugar phosphates and the formation of vanadium--sugar phosphate complexes that may be involved in the stimulation of the catalytic activity of the isomerases.     

Phosphofructokinase is responsible for the fructose 2,6-bisphosphate inhibition of hexokinase in tissue extracts

Mammalian and yeast hexokinases were reported to be reversibly inhibited by fructose 2,6-bisphosphate in the presence of cytosolic proteins (H. Niemeyer, C. Cerpa, and E. Rabajille (1987) Arch. Biochem. Biophys. 257, 17-26). Reinvestigation of this finding using a radioassay with [14C]glucose as substrate showed no effect of fructose 2,6-bisphosphate on hexokinase activity of rat liver cytosols. Detailed reexamination of the spectrophotometric assay resulted in the observation that the fructose 2,6-bisphosphate-dependent inhibition was a function of the cytosolic phosphoglucose isomerase and phosphofructokinase activities compared to the amount of glucose-6-phosphate dehydrogenase used as auxiliary enzyme. The diminution or loss of the fructose 2,6-bisphosphate-dependent inhibition produced in aged cytosols was restored by addition of crystalline muscle phosphofructokinase, as well as by decreasing the amount of glucose-6-phosphate dehydrogenase in the assay. When phosphoglucose isomerase, phosphofructokinase, and hexokinase activities were separated by DEAE-chromatography of liver cytosol, no fructose 2,6-bisphosphate-dependent inhibition of hexokinase was found in any single fraction of the chromatogram. However, combination of fractions containing both phosphoglucose isomerase and phosphofructokinase displayed the fructose 2,6-bisphosphate-dependent inhibition on either endogenous hexokinase or added yeast hexokinase. From these results we conclude that the activation of phosphofructokinase elicited by fructose 2,6-bisphosphate is responsible for the hexokinase inhibition observed in the coupled spectrophotometric assay.     

Pathway and regulation of erythritol formation in Leuconostoc oenos.

It was recently observed that Leuconostoc oenos GM, a wine lactic acid bacterium, produced erythritol anaerobically from glucose but not from fructose or ribose and that this production was almost absent in the presence of O2. In this study, the pathway of formation of erythritol from glucose in L. oenos was shown to involve the isomerization of glucose 6-phosphate to fructose 6-phosphate by a phosphoglucose isomerase, the cleavage of fructose 6-phosphate by a phosphoketolase, the reduction of erythrose 4-phosphate by an erythritol 4-phosphate dehydrogenase and, finally, the hydrolysis of erythritol 4-phosphate to erythritol by a phosphatase. Fructose 6-phosphate phosphoketolase was copurified with xylulose 5-phosphate phosphoketolase, and the activity of the latter was competitively inhibited by fructose 6-phosphate, with a Ki of 26 mM, corresponding to the Km of fructose 6-phosphate phosphoketolase (22 mM). These results suggest that the two phosphoketolase activities are borne by a single enzyme. Extracts of L. oenos were also found to contain NAD(P)H oxidase, which must be largely responsible for the reoxidation of NADPH and NADH in cells incubated in the presence of O2. In cells incubated with glucose, the concentrations of glucose 6-phosphate and of fructose 6-phosphate were higher in the absence of O2 than in its presence, explaining the stimulation by anaerobiosis of erythritol production. The increase in the hexose 6-phosphate concentration is presumably the result of a functional inhibition of glucose 6-phosphate dehydrogenase because of a reduction in the availability of NADP.     

Novel pronounced reversible inhibition of phosphofructokinase, glucose 6-phosphate dehydrogenase, and phosphoglucose isomerase by hexacyanoferrate (II).

At pH 6.8, pig kidney phosphofructokinase (PFK) is inhibited 90% by 1 mm hexacyanoferrate(II), in a reaction mixture containing 0.2 mm fructose 6-phosphate (F-6-P) and 1 mm ATP. Glucose 6-phosphate dehydrogenase and phosphoglucose isomerase are inhibited 70% by 5 mm hexacyanoferrate(II), at a 0.2 mm concentration of their respective substrates. Unlike all previously reported inhibitions of glycolytic enzymes by hexacyanoferrate, this inhibition seems not to involve an oxidation of enzyme, substrate, or enzyme-substrate complex. It appears to be due to reversible binding of the hexacyanoferrate at, or near, the hexose phosphate binding site of each enzyme. These inhibition studies were carried out in 50 mm 2-mercaptoethanol, and spectral studies showed that these conditions ensured that all the hexacyanoferrate was in the reduced (II) state. The inhibition of PFK was competitive with respect to the substrate F-6-P. Some reaction between hexacyanoferrate(II) and the substrate could not be definitely ruled out, but such reactions cannot be the major basis for the inhibitions observed. Increasing the magnesium concentration did not overcome the PFK inhibition. For all three enzymes, addition of a high concentration of hexose phosphate substrate to an assay mixture containing highly inhibited enzyme resulted in removal of the inhibition. The inhibition was instantaneous, and there was no increase in inhibition with time of incubation with hexacyanoferrate(II). These results may provide an approach to active-site labeling of these three enzymes at their hexose phosphate binding sites. These results should also be of interest to other workers, especially those involved in oxidative phosphorylation studies, who use ferro- and ferricyanide as research tools. The effects from such experiments may, in some cases, be due to binding of these compounds at, or near, hexose phosphate binding sites in the system.     

The sugar phosphate specificity of rat hepatic 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase

The sugar phosphate specificity of the active site of 6-phosphofructo-2-kinase and of the inhibitory site of fructose-2,6-bisphosphatase was investigated. The Michaelis constants and relative Vmax values of the sugar phosphates for the 6-phosphofructo-2-kinase were: D-fructose 6-phosphate, Km = 0.035 mM, Vmax = 1; L-sorbose 6-phosphate, Km = 0.175 mM, Vmax = 1.1; D-tagatose 6-phosphate, Km = 15 mM, Vmax = 0.15; and D-psicose 6-phosphate, Km = 7.4 mM, Vmax = 0.42. The enzyme did not catalyze the phosphorylation of 1-O-methyl-D-fructose 6-phosphate, alpha- and beta-methyl-D-fructofuranoside 6-phosphate, 2,5-anhydro-D-mannitol 6-phosphate, D-ribose 5-phosphate, or D-arabinose 5-phosphate. These results indicate that the hydroxyl group at C-3 of the tetrahydrofuran ring must be cis to the beta-anomeric hydroxyl group and that the hydroxyl group at C-4 must be trans. The presence of a hydroxymethyl group at C-2 is required; however, the orientation of the phosphonoxymethyl group at C-5 has little effect on activity. Of all the sugar monophosphates tested, only 2,5-anhydro-D-mannitol 6-phosphate was an effective inhibitor of the kinase with a Ki = 95 microM. The sugar phosphate specificity for the inhibition of the fructose-2,6-bisphosphatase was similar to the substrate specificity for the kinase. The apparent I0.5 values for inhibition were: D-fructose 6-phosphate, 0.01 mM; L-sorbose 6-phosphate, 0.05 mM; D-psicose 6-phosphate, 1 mM; D-tagatose 6-phosphate, greater than 2 mM; 2,5-anhydro-D-mannitol 6-phosphate, 0.5 mM. 1-O-Methyl-D-fructose 6-phosphate, alpha- and beta-methyl-D-fructofuranoside 6-phosphate, and D-arabinose 5-phosphate did not inhibit. Treatment of the enzyme with iodoacetamide decreased sugar phosphate affinity in the kinase reaction but had no effect on the sensitivity of fructose-2,6-bisphosphatase to sugar phosphate inhibition. The results suggest a high degree of homology between two separate sugar phosphate binding sites for the bifunctional enzyme.     

Competitive inhibition of spinach leaf phosphoglucose isomerase isoenzymes by erythrose 4-phosphate

In plant cells, the reversible isomerization between fructose 6-phosphate (Fru6P) and glucose 6-phosphate (Glc6P) is catalyzed by a cytosolic and a chloroplastic isoenzyme of phosphoglucose isomerase (PGI, EC 5.3.1.9). The extractable activities of both PGI isoenzymes are in large excess compared with the flux required for product synthesis, but the measured Glu6P/Fru6P ratio in illuminated chloroplasts and in whole leaves is always displaced from equilibrium. Cytosolic (PGI 2) and stromal (PGI 1) isoenzymes were purified from spinach leaves and used to investigate the possibility of metabolic regulation at this step. Several metabolites were found to inhibit PGI, but within the physiological concentration range, only erythrose 4-phosphate (Ery4P) inhibited significantly. The inhibition was competitive, with K_i values below 10 μM for PGI 2 and 1. The physiological significance of the inhibition of PGI by Ery4P was assessed in isolated intact spinach chloroplasts. We conclude that, in vivo, this inhibition is probably responsible for the observed displacement from equilibrium in the chloroplasts, but limits the carbon flow towards starch synthesis only when Fru6P is low. In contrast, the inhibition by Ery4P is unlikely to play any role in the cytosolic carbon metabolism because both Fru6P concentration and PGI activity, are much higher than in the chloroplast stroma.     

Fluorine-containing analogues of intermediates in the Shikimate pathway.

The phosphoenolpyruvate analogue (Z)-phosphoenol-3-fluoropyruvate is a substrate for phenylalanine-inhibitable 3-deoxy-D-arabino-heptulosonic acid-7-phosphate synthase from Escherichia coli. In the presence of excess erythrose 4-phosphate, apparent KM values of 65 and 38 muM were observed for phosphoenol-3-fluoropyruvate and phosphoenolpyruvate, respectively. Because the apparent Vmax for phosphoenol-3-fluoropyruvate is only 1.17% of that for phosphoenolpyruvate, one can study the former as an inhibitor of 3-deoxy-arabino-heptulosonic acid-7-phosphate synthase. Kinetic experiments showed phosphoenol-3-fluoropyruvate to be competitive with respect to phosphoenolpyruvate. Two distinguishable Ki values of 8 and 48 muM were obtained. The product (3S)-3-deoxy--3-fluoro-arabino-heptulosonic acid 7-phosphate was purified, characterized, and shown to act as a substrate for 5-dehydroquinate synthase. 3-Deoxy-3-fluoro-arabino-heptulosonic acid 7-phosphate, in contrast to 3-deoxy-arabino-heptulosonic acid 7-phosphate reacts slowly or not at all with reagents specific for 2-keto-3-deoxy sugars and is relatively resistant to oxidative cleavage by sodium periodate. The expected product of periodate oxidation, 3-fluoro-3-formylpyruvate, cannot be detected. This observation was clarified by studies with model compounds.     

Crystal structures of mouse autocrine motility factor in complex with carbohydrate phosphate inhibitors provide insight into structure-activity relationship of the inhibitors

Autocrine motility factor (AMF), a tumor-secreted cytokine, stimulates cell migration in vitro and metastasis in vivo. AMF is identical to the extracellular cytokines neuroleukin and maturation factor and, interestingly, to the intracellular enzyme phosphoglucose isomerase. The cytokine activity of AMF is inhibited by carbohydrate phosphate compounds as they compete for AMF binding with the carbohydrate moiety of the AMF receptor (AMFR), which is a glycosylated seven transmembrane helix protein. Here, we report the first comprehensive high-resolution crystal structure analyses of the inhibitor-free form and the eight types of inhibitor (phosphate, erythrose 4-phosphate (E4P), arabinose 5-phosphate (A5P), sorbitol 6-phosphate (S6P), 6-phosphogluconic acid (6PGA), fructose 6-phosphate (F6P), glucose 6-phosphate (G6P), or mannose 6-phosphate (M6P)) complexes of mouse AMF (mAMF). We assayed the inhibitory activities of these inhibitors against the cytokine activity of mAMF. The inhibitory activities of the six-carbon sugars (G6P, F6P, M6P, and 6PGA) were found to be significantly higher than those of the four or five-carbon sugars (E4P or A5P). The inhibitory activities clearly depend on the length of the inhibitor molecules. A structural comparison revealed that a water-mediated hydrogen bond between one end of the inhibitor and a rigid portion of the protein surface in the shorter-chain inhibitor (E4P) complex is replaced by a direct hydrogen bond in the longer-chain inhibitor (6PGA) complex. Thus, to obtain a new compound with higher inhibitory activities against AMF, water molecules at the inhibitor binding site of AMF should be replaced by a functional group of inhibitors in order to introduce direct interactions with the protein surface. The present structure-activity relationship studies will be valuable not only for designing more effective AMF inhibitors but also for studying general protein-inhibitor interactions.     

The synthesis of 3-deoxyheptulosonic acid 7-phosphate.

3-Deoxyheptulosonic acid 7-phosphate was obtained by a one-step chemical synthesis through condensation of oxalacetate with erythrose 4-phosphate. This reaction occurs at measurable rates only in the presence of a metal ion; Co2+ and Ni2+ are the most effective catalysts. The Co2+ catalyzed condensation of oxalacetate and erythrose 4-phosphate proceeds at room temperature and neutral pH. Since erythrose 4-phosphate can be replaced by any free aldehyde tested thus far, this type of a homogeneous catalysis opens new synthetic routes to alpha-keto-gamma-hydroxy-fatty acids and their derivatives.     

The crystal structure of mouse phosphoglucose isomerase at 1.6A resolution and its complex with glucose 6-phosphate reveals the catalytic mechanism of sugar ring opening

Phosphoglucose isomerase (PGI) is an enzyme of glycolysis that interconverts glucose 6-phosphate (G6P) and fructose 6-phosphate (F6P) but, outside the cell, is a multifunctional cytokine. High-resolution crystal structures of the enzyme from mouse have been determined in native form and in complex with the inhibitor erythrose 4-phosphate, and with the substrate glucose 6-phosphate. In the substrate-bound structure, the glucose sugar is observed in both straight-chain and ring forms. This structure supports a specific role for Lys518 in enzyme-catalyzed ring opening and we present a "push-pull" mechanism in which His388 breaks the O5-C1 bond by donating a proton to the ring oxygen atom and, simultaneously, Lys518 abstracts a proton from the C1 hydroxyl group. The reverse occurs in ring closure. The transition from ring form to straight-chain substrate is achieved through rotation of the C3-C4 bond, which brings the C1-C2 region into close proximity to Glu357, the base catalyst for the isomerization step. The structure with G6P also explains the specificity of PGI for glucose 6-phosphate over mannose 6-isomerase (M6P). To isomerize M6P to F6P requires a rotation of its C2-C3 bond but in PGI this is sterically blocked by Gln511.     

Enzymatic Studies on the Conversion of Erythritol Fermentation to d-Mannitol Fermentation in n-Alkane-grown Yeast Candida zeylanoides

In the polyol fermentation by Candida zeylanoides KY6166, which occurred preferentially by keeping the pH of medium at acidic side (below 4.0), phosphate ion played a precise role in the conversion of erythritol fermentation to d-mannitol fermentation. Enzymatic studies on the conversion mechanism provided the following evidences.

The enzymes involved in pentosephosphate cycle were considerably depressed in polyol production phase in which intracellular pH ranged from 5.5 to 5.7. Particularly transaldolase responsible for the synthesis of erythrose 4-phosphate and fructose 6-phosphate from glyceraldehyde 3-phosphate plus d-sedoheptulose 7-phosphate was significantly depressed at pH 5.5. Besides, transketolase which participated directly in the formation of erythrose 4-phosphate from fructose 6-phosphate was significantly inhibited by phosphate ion. Glucose 6-phosphate dehydrogenase was slightly inhibited by phosphate ion.

The enzymes involved in pentosephosphate cycle were considerably depressed in polyol production phase in which intracellular pH ranged from 5.5 to 5.7. Particularly transaldolase responsible for the synthesis of erythrose 4-phosphate and fructose 6-phosphate from glyceraldehyde 3-phosphate plus d-sedoheptulose 7-phosphate was significantly depressed at pH 5.5. Besides, transketolase which participated directly in the formation of erythrose 4-phosphate from fructose 6-phosphate was significantly inhibited by phosphate ion. Glucose 6-phosphate dehydrogenase was slightly inhibited by phosphateion. From these results, the alteration from erythritol fermentation to mannitol fermentation by phosphate ion was explained as the result of the change in the level of erythrose 4-phosphate and fructose 6-phosphate which was caused by the inhibition of transketolase.     

The pentose phosphate pathway of glucose metabolism. Measurement of the non-oxidative reactions of the cycle

Methods for the quantitative determination of ribose 5-phosphate isomerase, ribulose 5-phosphate 3-epimerase, transketolase and transaldolase in tissue extracts are described. The determinations depend on the measurement of glyceraldehyde 3-phosphate by using the coupled system triose phosphate isomerase, α-glycero-phosphate dehydrogenase and NADH. By using additional purified enzymes transketolase, ribose 5-phosphate isomerase and ribulose 5-phosphate epimerase conditions could be arranged so that each enzyme in turn was made rate-limiting in the overall system. Transaldolase was measured with fructose 6-phosphate and erythrose 4-phosphate as substrates, and again glyceraldehyde 3-phosphate was measured by using the same coupled system. Measurements of the activities of the non-oxidative reactions of the pentose phosphate pathway were made in a variety of tissues and the values compared with those of the two oxidative steps catalysed by glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase.