Aromatic hydroxylation catalyzed by toluene 4-monooxygenase in organic solvent/aqueous buffer mixtures

Toluene 4-monooxygenase is a four-protein component diiron enzyme complex. The enzyme catalyzes the hydroxylation of toluene to give p-cresol with approximately 96% regioselectivity. The performance of the enzyme in two-phase reaction systems consisting of toluene, hexane, or perfluorohexane and an aqueous buffer was tested. In each of the cosolvent systems, containing up to 93% (v/v) of solvent, the enzyme was active and exhibited regioselectivity indistinguishable from the aqueous reaction. Using the perfluorohexane/buffer system, a number of polycyclic aromatic hydrocarbons were oxidized that were not readily oxidized in aqueous buffer. An instability of the hydroxylase component and a substantial uncoupling of NADH utilization and product formation were observed in reactions that were continued for longer than approximately 3 min. More stable enzyme complexes will be needed for broad applicability of this hydroxylating system in nonaqueous media.

Industrial biocatalysis today and tomorrow

The use of biocatalysis for industrial synthetic chemistry is on the verge of significant growth. Biocatalytic processes can now be carried out in organic solvents as well as aqueous environments, so that apolar organic compounds as well as water-soluble compounds can be modified selectively and efficiently with enzymes and biocatalytically active cells. As the use of biocatalysis for industrial chemical synthesis becomes easier, several chemical companies have begun to increase significantly the number and sophistication of the biocatalytic processes used in their synthesis operations.

Separation of alpha-acid glycoprotein glycoforms using affinity-based reversed micellar extraction and separation

A new method for the preparation of the glycoforms of bovine alpha(1)-acid glycoprotein (AGP) is described relying on affinity-reversed micellar extraction and separation (ARMES). This method has proven effective in separating structurally similar glycoproteins and separating glycoproteins from nonglycosylated proteins from natural sources. In this method, individual glycoforms complex with the lectin, concanavalin A (ConA) are extracted into an organic-phase reversed micellar solution formed by Aerosol OT (AOT). The purity of three AGP glycoforms isolated was assessed by hydroxyapatite high-performance liquid chromatography (HPLC), gel-permeation chromatography and SDS-PAGE. The glycan structure of the pure glycoforms was analyzed. Oligosaccharide mapping using capillary electrophoresis (CE) and PAGE showed the glycans obtained from each glycoform to be distinctly different. ARMES can be used for the semi-preparative scale resolution of the glycoforms of bovine AGP or other therapeutic glycoproteins.

Enzyme-catalyzed synthesis of sugar-containing monomers and linear polymers

Commercially available proteases and lipases were screened for their ability to acylate regioselectively sucrose and trehalose with divinyladipic acid ester. Opticlean M375 (subtilisin from Bacillus licheniformis) was observed to form sucrose 1'-O-adipate and trehalose 6-O-adipate in anhydrous pyridine. Novozym-435 (lipase B from Candida antarctica) catalyzed the synthesis of sucrose 6, 6'-O-divinyladipate and trehalose 6, 6'-O-divinyladipate in acetone. These diesters were then employed as monomers in polycondensation reactions with various diols (aliphatic and aromatic) catalyzed by Novozym-435 in organic solvents to yield linear polyesters with M(w)'s up to 22,000 Da. Spectroscopic analysis confirmed that only the vinyl end groups of sugar esters reacted in the enzymatic polymerization with the diol, and not the internal sugar-adipate linkages. The two-step enzymatic strategy to yield sugar-based polyesters, which is the first report of its kind, results in higher molecular weights and faster reaction times than one-step enzymatic polyester synthesis.

Chemoenzymatic construction of a four-component Ugi combinatorial library

The chemoenzymatic preparation of a nine-member Ugi condensation library is described. The carboxylic acid and amine precursors are based on 3-hydroxybutyrate and 4-amino-1-butanol, respectively, and have been acylated selectively using a variety of acyl donors catalyzed by porcine pancreatic lipase. The enzyme is selective for the hydroxyl functionalities on both precursors, thereby yielding 3-acyl-butyric acid and 4-amino-1-acyl compounds. These enzymatically generated derivatives were then subjected to a four-component Ugi condensation reaction in the presence of acetaldehyde and methyl isocyanoacetate. Isolated yields of the alpha-(acylamino)amide Ugi products ranged from 72-95%. The inherent chemoselectivity of enzymatic catalysis may play an increasingly important role in expanding the structural diversity that can be achieved by chemical multicomponent condensation reactions.

Investigating the effects of polymer chemistry on activity of biocatalytic plastic materials

The affects of polymer chemistry on the organic solvent activity of alpha-chymotrypsin-containing biocatalytic plastic materials are investigated in this study. To incorporate alpha-chymotrypsin into the polymer, the enzyme is first acryloylated, then solubilized into organic solvents via hydrophobic ion paring with surfactant molecules. Once in the organic solvent, a vinyl monomer and crosslinker are added and copolymerized with the enzyme. Due to the intimate contact between the enzyme and the resulting polymer network, the polymer chemistry plays an important role in the activity of these biocatalytic materials. The chemical composition of the monomer/polymer has the greatest effect on catalytic activity. The activity spans a range of 100-fold and appears to correlate with the hydrophilicity of the monomer, with the lowest activity exhibited for poly(methyl methacrylate) and the highest for poly(2-hydroxyethyl methacrylate). The effect of the chemical structure of the monomer/polymer appears to be an intrinsic kinetic effect, whereas other polymer chemistry conditions investigated, including crosslinker concentration and length and ratio of solvent:monomer during synthesis, appear to effect the rate of substrate diffusion, thereby affecting observed enzyme activity. Changes in the conditions of polymer synthesis can cause as much as a 20-fold change in activity for a given polymeric material. This is most likely due to an increase in the porosity of the materials, and thus a relaxation of diffusional limitations.

Enzymatically and combinatorially generated array-based polyphenol metal ion sensor

Phenolic polymers were synthesized via soybean hull peroxidase catalysis and used as metal-based sensor components in a polymer array. A sensor array for Fe(3+), Cu(2+), Co(2+), and Ni(2+) has been developed consisting of 15 phenolic homopolymers and copolymers generated from five phenolic monomers by peroxidase-catalyzed oxidative polymerization. Sensing was based on the change of intrinsic polyphenol fluorescence upon addition of a metal ion or a metal ion mixture to an aqueous suspension of a polyphenol. Importantly, the fluorescence response of copolymers differed, in some cases dramatically, from the constituent homopolymers and was dependent upon the relative ratio of monomers that comprise the polymer. This finding suggests that an extremely broad range of sensor arrays can be generated from a limited number of phenolic monomers. Using a statistical analysis, histograms constructed for the four different metal ions yielded unique fingerprints of the array response and can be used to identify specific metal ions.

Intrinsic effects of solvent polarity on enzymic activation energies

The effect of organic solvents on subtilisin Carlsberg catalysis has been investigated with the aid of a thermodynamic analysis. Saturation solubility experiments were performed to provide a quantitative measure of substrate desolvation from the reaction medium. This enabled calculation of the intrinsic enzymic activation energy and resulted in a linear free energy relationship with respect to solvent polarity. The results indicate that the intrinsic activation energy of subtilisin catalysis is lowest in polar organic solvents, which may be due to transition state stabilization of the enzyme's polar transition state for transesterification.

Preparation and isolation of neoglycoconjugates using biotin-streptavidin complexes

Glycoproteins commercially available in multi-gram quantities, were used to prepare milligram amounts of neoglycoproteins. The glycoproteins bromelain and bovine gamma-globulin were proteolyzed to obtain glycopeptides or converted to a mixture of glycans through hydrazinolysis. The glycan mixture was structurally simplified by carbohydrate remodeling using exoglycosidases. Glycopeptides were biotinylated using N-hydroxysuccinimide activated-long chain biotin while glycoprotein-derived glycans were first reductively aminated with ammonium bicarbonate and then biotinylated. The resulting biotinylated carbohydrates were structurally characterized and then bound to streptavidin to afford neoglycoproteins. The peptidoglycan component of raw, unbleached heparin (an intermediate in the manufacture of heparin) was similarly biotinylated and bound to streptavidin to obtain milligram amounts of a heparin neoproteoglycan. The neoglycoconjugates prepared contain well defined glycan chains at specific locations on the streptavidin core and should be useful for the study of protein-carbohydrate interactions and affinity separations.

Optimizing the salt-induced activation of enzymes in organic solvents: effects of lyophilization time and water content

The addition of simple inorganic salts to aqueous enzyme solutions prior to lyophilization results in a dramatic activation of the dried powder in organic media relative to enzyme with no added salt. Activation of both the serine protease subtilisin Carlsberg and lipase from Mucor javanicus resulting from lyophilization in the presence of KCl was highly sensitive to the lyophilization time and water content of the sample. Specifically, for a preparation containing 98% (w/w) KCl, 1% (w/w) phosphate buffer, and 1% (w/w) enzyme, varying the lyophilization time showed a direct correlation between water content and activity up to an optimum, beyond which the activity decreased with increasing lyophilization time. The catalytic efficiency in hexane varied as much as 13-fold for subtilisin Carlsberg and 11-fold for lipase depending on the lyophilization time. This dependence was apparently a consequence of including the salt, as a similar result was not observed for the enzyme freeze-dried without KCl. In the case of subtilisin Carlsberg, the salt-induced optimum value of kcat/Km for transesterification in hexane was over 20,000-fold higher than that for salt-free enzyme, a substantial improvement over the previously reported enhancement of 3750-fold (Khmelnitsky, 1994). As was found previously for pure enzyme, the salt-activated enzyme exhibited greatest activity when lyophilized from a solution of pH equal to the pH for optimal activity in water. The active-site content of the lyophilized enzyme samples also depended upon lyophilization time and inclusion of salt, with opposite trends in this dependence observed for the solvents hexane and tetrahydrofuran. Finally, substrate selectivity experiments suggested that mechanism(s) other than selective partitioning of substrate into the enzyme-salt matrix are responsible for salt-induced activation of enzymes in organic solvents.

Testing for diffusion limitations in salt-activated enzyme catalysts operating in organic solvents

The dramatic activation of serine proteases in nonaqueous media resulting from lyophilization in the presence of KCl is shown to be unrelated to relaxation of potential substrate diffusional limitations. Specifically, lyophilizing subtilisin Carlsberg in the presence of KCl and phosphate buffer in different proportions, ranging from 99% (w/w) enzyme to 1% (w/w) enzyme in the final lyophilized solids, resulted in biocatalyst preparations that were not influenced by substrate diffusion. This result was made evident through use of a classical analysis whereby initial catalytic rates, normalized per weight of total enzyme in the catalyst material, were measured as a function of active enzyme for biocatalyst preparations containing different ratios of active to inactive enzyme. The active enzyme content of a given biocatalyst preparation was controlled by mixing native subtilisin with subtilisin preinactivated with PMSF, a serine protease inhibitor, and lyophilizing the enzyme mixture in the presence of different fractions of KCl and phosphate buffer. Plots of initial reaction rates as a function of percent active subtilisin in the biocatalyst were linear for all biocatalyst preparations. Thus, enzyme activation (reported elsewhere to be as high as 3750-fold in hexane for the transesterification of N-Ac-L-Phe-OEt with n-PrOH) is a manifestation of intrinsic enzyme activation and not relaxation of diffusional limitations resulting from diluted enzyme preparations. Similar activation is reported herein for thermolysin, a nonserine protease, thereby demonstrating that enzyme activation due to lyophilization in the presence of KCl may be a general phenomenon for proteolytic enzymes.