Glucose oxidase from Aspergillus niger: the mechanism of action with molecular oxygen, quinones, and one-electron acceptors.

Glucose oxidase from the mold Aspergillus niger (EC 1.1.3.4) oxidizes beta-D-glucose with a wide variety of oxidizing substrates. The substrates were divided into three main groups: molecular oxygen, quinones, and one-electron acceptors. The kinetic and chemical mechanism of action for each group of substrates was examined in turn with a wide variety of kinetic methods and by means of molecular modeling of enzyme-substrate complexes. There are two proposed mechanisms for the reductive half-reaction: hydride abstraction and nucleophilic attack followed by deprotonation. The former mechanism appears plausible; here, beta-D-glucose is oxidized to glucono-delta-lactone by a concerted transfer of a proton from its C1-hydroxyl to a basic group on the enzyme (His516) and a direct hydride transfer from its C1 position to the N5 position in FAD. The oxidative half-reaction proceeds via one- or two-electron transfer mechanisms, depending on the type of the oxidizing substrate. The active site of the enzyme contains, in addition to FAD, three amino acid side chains that are intimately involved in catalysis: His516 with a pK(a)=6.9, and Glu412 with pK(a)=3.4 which is hydrogen bonded to His559, with pK(a)>8. The protonation of each of these residues has a strong influence on all rate constants in the catalytic mechanism.

Cultivation of Polyporus squamosus for pectinase production in aqueous two-phase system containing sugar beet extraction waste.

Cultivation of the fungus Polyporus squamosus for pectinase production was studied in a polyethylene glycol/crude dextran aqueous two-phase system, with sugar beet extraction waste as pectin source. Fungal growth was restricted to the bottom phase and the amounts of biomass and exo-pectinase activity produced were superior to in homogeneous cultivation. The partition coefficients of endo-pectinase and exo-pectinase were 4.26 and 2.78, respectively. The top phase yields in the single extraction step were about 90% for both pectinases.

Production of pectinases by Polyporus squamosus in aqueous two-phase system.

The ability of Polyporus squamosus to grow and produce pectinases in an aqueous two-phase medium composed of polyethylene glycol and crude dextran is reported. Fungal growth was restricted to the bottom phase leaving the top phase cell free. Amounts of produced biomass and endo and exo-pectinase activities were superior or equal to those obtained in homogeneous medium. The partition coefficient for the endo-pectinase was 1.52 followed by a top phase yield of 70.86%. Although the phase system composition favours partition of a greater part of exo-pectinase activity to the bottom phase (K(exo) was 0.6 and yield in top phase 48.56%) the partitioned activity in the top phase was equal to that produced in homogeneous cultivation.

Production and some characteristics of beta-glucosidase in Diaporthe (Phomopsis) helianthi.

Diaporthe (Phomopsis) helianthi Munt.-Cvet. et al. is an important phytopathogenic fungus which causes stem canker of sunflower. When grown in submerged cultures in the presence of milled wheat chaff as a carbon source this phytopathogen produced extracellular and intracellular beta-glucosidases. The optimum pH of these enzymes was 4.8. The temperature of crude intracellular beta-glucosidase activity was at 60 degrees C, whereas the optimum activity of crude extracellular beta-glucosidase was observed in a wide range of temperature between 40 and 70 degrees C. Although, extracellular and intracellular beta-glucosidase activities had identical pH and similar temperature optima, the thermal stability of the intracellular enzyme was significantly higher.

Inhibition by isoproterenol of the passive potassium efflux from pigeon erythrocytes.

Pigeon erythrocytes were carefully washed in an isotonic neutral buffer, devoid of potassium, and the rate of passive unidirectional efflux of potassium from the cells into a K+-free medium was measured after 20 min, at 40 degrees C. Isoproterenol inhibits K+-efflux by 35-45%, at a cell concentration of 1%; the isoproterenol effect is mediated by beta-adrenergic receptors. Cyclic AMP mimics the effect of isoproterenol, but at 4-5 orders of magnitude higher concentrations. Cyclic AMP increases 20-fold the phosphorylation of purified cell membranes by [gamma 32P]ATP.

The influence of nicergoline on reaction kinetics of various forms of cyclic AMP phosphodiesterase.

10-Methoxy-1,6-dimethylergoline-8 beta-methanol-5-bromonicotinate (nicergoline) has in vitro a modulatory effect upon reaction kinetics of various forms of cAMP phosphodiesterase (PDE) from beef heart and mice brain. Nicergoline inhibits low Km form of cAMP PDE from hear and brain; concentration of inhibitor required for 50% inhibition is 3-12 mumol/l. It is believe that the observed in vitro effects of nicergoline on reaction kinetics of various forms of cAMP PDE may in vivo help to normalize the level of cAMP, keep its concentration within certain limits, both by opposing large increase and large decrease in its cellular concentration.

The influence of nicergoline on reaction kinetics of synaptosomal adenosine triphosphatases from porcine brain.

10-Methoxy-1,6-dimethylergoline-8beta-methanol-5-bromonicotinate (nicergoline) has in vitro a modulatory effect upon reaction kinetics of two synaptosomal adenosine triphosphatases from porcine brain: Na,K-dependent ATPase and Ca,Mg-dependent ATPase. Nicergoline is a non-competitive inhibitor of synaptosomal Ca,Mg-dependent ATPase in vitro; thus, if its in vivo concentration reaches a sufficiently high level, the rate of exchange of Ca and Mg ions between the intra- and the extracellular spaces of neurons slows down. In vivo effect of nicergoline upon reaction kinetics of synaptosomal Na,K-dependent ATPase is complex; below 80 micromol/l ATP it is an activator, and above 80 micromol/l ATP it is an inhibitor or enzyme. It decreases Km for ATP from 310 micromol/l in the absence, to 70 micromol/l in the presence of 0.3 mmol/l drug. It is believed that, in vivo, nicergoline helps to level or normalize the rate of physiological processes energetically coupled to synaptosomal Na,K-dependent ATPase, by moderately accelerating the slow and strongly inhibiting the fast reaction rates by ATP hydrolysis.

Influence of temperature on hemolysis of erythrocytes by digitonin.

The kinetics of hemolysis of pig erythrocytes by digitonin was continuously monitored by a potassium selective electrode. The following minimal mechanism of hemolysis was postulated, based upon kinetic measurements: 2 D + E (1)in equilibrium E x D2 (2)leads to (E x D2) (3)leads to (E x D2) where D denotes a digitonin molecule and E a specific digitonin binding-site on the membrane. The first step (1) represents a rapid reversible combining of digitonin with specific binding-sites on the membrane. The second step (2) is prelytic, related to the time required for bound digitonin molecules to alter the membrane structure so much that hemolysis may take place; this step has a transition temperature at 26 degrees C, probably related to the "melting" of specific membrane structures at that temperature. The third step (3) is hemolytic, and comprises the changes within the digitonin-altered membrane during hemolysis; it is stongly influenced by temperature. Lowering of temperature slows down the rate of hemolysis and increases the quantity of digitonin required to obtain a fixed extent of hemolysis. It appears that two molecules of digitonin combine with a single binding-site on the outer face of the membrane in a digitonin-membrane complex.

External yeast beta-fructosidase. The role of tryptophyl residues in catalysis.

1. In native invertase at pH 4.6, 23 out of a total of 34 tryptophyl residues are "exposed" to oxidation with N-bromosuccinimide, the other residues being apparently shielded from the oxidant within the molecule. 2. Oxidation of 5-6 tryptophyl residues/molecule with N-bromosuccinimide is proportional to the complete inactivation of the enzyme, and appears to be specific for indole chromophore only. The ligand binding and fluorescence measurements indicate that the oxidation of native enzyme, up to 50% inhibition, apparently does not change the conformation and topography of enzymes surface. 3. Invertase is inhibited by diazonium-1-H-tetrazole. Since tyrosine residues can be excluded by nitration studies as catalytically unimportant, it appears that a mocification of a single histidyl residue/molecule with diazonium-1-H-tetrazole is sufficient to abolish the enzymic activity. However, the absence of inhibition with diethyl pyrocarbonate indicates that the inhibition with diazonium-1-H-tetrazole may be mediated through steric hindrance or other indirect effects. 4. The absence of inhibition with 2,4-dinitrophenylhydrazine, trinitro benzenesulfonic acid and 5,5'-dithiobis-(2-nitrobenzoate) indicates that the carbonyl groups of the carbohydrate moiety, free amino and -SH groups are not essential for activity.

Evidence for a histidine and a cysteine residue in the substrate-binding site of yeast alcohol dehydrogenase.

1. Yeast alcohol dehydrogenase (EC 1.1.1.1) is inhibited by stoicheiometric concentrations of diethyl pyrocarbonate. The inhibition is due to the acylation of a single histidine residue/monomer (mol.wt. 36000). 2. Alcohol dehydrogenase is also inhibited by stoicheiometric amounts of 5,5'-dithiobis-(2-nitrobenzoate), owing to the modification of a single cysteine residue/monomer. 3. Native alcohol dehydrogenase binds two molecules of reduced coenzyme/molecule of enzyme (mol.wt. 144000). 4. Modification of a single histidine residue/monomer by treatment with diethyl pyrocarbonate prevents the binding of acetamide in the ternary complex, enzyme-NADH-acetamede, but does not prevent the binding of NADH to the enzyme. 5. Modification of a single cysteine residue/monomer does not prevent the binding of acetamide to the ternary complex. After the modification of two thiol groups/monomer by treatment with 5,5'-dithiobis-(2-nitrobenzoate), the capacity of enzyme to bind coenzyme in the ternary complex was virtually abolished. 6. From the results presented in this paper we conclude that at least one histidine and one cysteine residue are closely associated in the substrate-binding site of alcohol dehydrogenase.