Reaction Engineering Aspects of α-l,4-D-Glucan Phosphorylase Catalysis

Some important process properties of α-l,4-D-ghican phosphorylases isolated from the bacteriumCorynebacterium callunae and potato tubers (Solatium tuberosum) were compared. Apart from minor differences in their stability and specificity (represented by the maximum degree of maltodextrin conversion) and a 10-fold higher affinity of the plant phosphorylase for maltodextrin (K _M of 1.3 g/L at 300 mM of orthophosphate), the performances of both enzymes in a continuous ultrafiltration membrane reactor were almost identical. Product synthesis was carried out over a time course of 300–400 h in the presence or absence of auxiliary pullulanase (increasing the accessibility of the glucan substrate for phosphorolytic attack up to 15–20%). The effect of varied dilution rate and reaction temperature on the resulting productivities was quantitated, and a maximum operational temperature of 40°C was identified.     

Purification and characterization of a laccase from the white-rot fungus Trametes multicolor

The wood-degrading fungus Trametes multicolor secretes several laccase isoforms when grown on a simple medium containing copper in the millimolar range for stimulating laccase synthesis. The main isoenzyme laccase II was purified to apparent homogeneity from the culture supernatant by using anion-exchange chromatography and gel filtration. Laccase II is a monomeric glycoprotein with a molecular mass of 63 kDa as determined by sodium dodecylsulfate polyacrylamide gel electrophoresis, contains 18% glycosylation, and has a pI of 3.0. It oxidizes a variety of phenolic substrates as well as ferrocyanide and iodide. The pH optimum depends on the substrate employed and shows a bell-shaped pH activity profile with an optimum of 4.0 to 5.0 for the phenolic substrates, while the nonphenolic substrates ferrocyanide and 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulfonate) show a monotonic pH profile with a rate decreasing with increasing pH.     

Targeting the Substrate Binding Site of E. coli Nitrile Reductase QueF by Modeling,Substrate and Enzyme Engineering

Nitrile reductase QueF catalyzes the reduction of 2‐amino‐5‐cyanopyrrolo[2,3‐d]pyrimidin‐4‐one (preQ₀) to 2‐amino‐5‐aminomethylpyrrolo[2,3‐d]pyrimidin‐4‐one (preQ₁) in the biosynthetic pathway of the hypermodified nucleoside queuosine. It is the only enzyme known to catalyze a reduction of a nitrile to its corresponding primary amine and could therefore expand the toolbox of biocatalytic reactions of nitriles. To evaluate this new oxidoreductase for application in biocatalytic reactions, investigation of its substrate scope is prerequisite. We report here an investigation of the active site binding properties and the substrate scope of nitrile reductase QueF from Escherichia coli. Screenings with simple nitrile structures revealed high substrate specificity. Consequently, binding interactions of the substrate to the active site were identified based on a new homology model of E. coli QueF and modeled complex structures of the natural and non‐natural substrates. Various structural analogues of the natural substrate preQ₀ were synthesized and screened with wild‐type QueF from E. coli and several active site mutants. Two amino acid residues Cys190 and Asp197 were shown to play an essential role in the catalytic mechanism. Three non‐natural substrates were identified and compared to the natural substrate regarding their specific activities by using wild‐type and mutant nitrile reductase.     

Production of Hemicellulose- and Cellulose-Degrading enzymes by various strains ofSclerotium Rolfsii

A number of wild-type isolates ofSclerotium rolfsii were screened for their capacity to produce lignocellulolytic enzymes when grown on a cellulose-based medium.S. rolfsii proved to be an efficient producer of hemicellulolytic enzymes under the conditions selected for this screening, although there was a great variability in enzyme activities formed by the different isolates. In addition to xylanase and mannanase, which were produced in remarkably high levels, a number of accessory enzymes, which are important for the complete degradation of substituted hemicelluloses and include a-arabinosidase, acetyl esterase, and a-galactosidase, are formed byS. rolfsii. Efficient production of xylanase and mannanase was achieved when cellulose-based media were used for growth. Under these conditions, enhanced levels of endoglucanase were formed as well. Formation of xylanase and mannanase could be more specifically induced when using xylan or mannan as growth substrates, although the enzyme activities thus obtained were significantly lower compared to cultivations on cellulose as main inducing substrate.

     

Enzymatic Synthesis of α-Glucose-1-phosphate: A Study Employing a New α-1,4 Glucan Phosphorylase from Corynebacterium callunae 1

Abstract

In synthetic pathways to complex carbohydrates such as oligosaccharides or nucleotide sugars the activated sugar 1-phosphates serve as important starting molecules. In this study the enzymatic synthesis of α-glucose-1-phosphate (Glc-1-P) has been investigated using a new bacterial α-glucan phosphorylase from Corynebacterium callunae. The major factors governing the rate of reaction and the attainable degree of substrate conversion have been identified and, accordingly, for optimizing the yield and limiting reaction time for the enzymatic process several points must be considered: (i) the pH-dependent equilibrium of reaction, (ii) product inhibition of the phosphorylase and (iii) enzymatic cleavage of α-1,6 glycosidic linkages present in α-1,4-glucans such as starch or maltodextrins by pullulanases to improve their phosphorolytic conversion. Results obtained in continuous experiments with the phosphorylase retained in an ultrafiltration membrane reactor confirmed the complete operational stability of the enzyme for several days at 30 °C. Since no more than approximately 18 % of the inorganic phosphate can be converted into Glc-1-P an efficient procedure for phosphate and product recovery will be particularly important.     

Hydrolysis of lactose by β-glycosidase CelB from hyperthermophilic archaeon Pyrococcus furiosus

Applied Biochemistry and Biotechnology - Recombinant β-glycosidase CelB from the hyperthermophilic archaeon Pyrococcus furiosus was produced through expression of the plasmid-encoded gene in...     

Identification of Candida tenuis xylose reductase as highly selective biocatalyst for the synthesis of aromatic alpha-hydroxy esters and improvement of its efficiency by protein engineering

Wild-type Candida tenuis xylose reductase and two Trp-23 mutants thereof catalyze NADH-dependent reduction of a homologous series of aromatic alpha-keto esters with absolute pseudo re-face stereoselectivity and broad tolerance for the substituent on the aromatic ring, producing the corresponding R-alcohols in high yield.