Intrinsic effects of solvent polarity on enzymic activation energies

Solvent tolerant enzymes in extremophiles: Adaptations and applications

Non-aqueous enzymology has always drawn attention due to the wide range of unique possibilities in biocatalysis. In general, the enzymes do not or insignificantly catalyze substrate in the presence of solvents. This is due to the interfering interactions of the solvents between enzyme and water molecules at the interface. Therefore, information about solvent-stable enzymes is scarce. Yet, solvent-stable enzymes prove quite valuable in the present day biotechnology. The enzymatic hydrolysis of the substrates in solvents synthesizes commercially valuable products, such as peptides, esters, and other transesterification products. Extremophiles, the most valuable yet not extensively explored candidates, can be an excellent source to investigate this avenue. Due to inherent structural attributes, many extremozymes can catalyze and maintain stability in organic solvents. In the present review, we aim to consolidate information about the solvent-stable enzymes from various extremophilic microorganisms. Further, it would be interesting to learn about the mechanism adapted by these microorganisms to sustain solvent stress. Various approaches to protein engineering are used to enhance catalytic flexibility and stability and broaden biocatalysis's prospects under non-aqueous conditions. It also describes strategies to achieve optimal immobilization with minimum inhibition of the catalysis. The proposed review would significantly aid our understanding of non-aqueous enzymology.

Solvent-free esterifications mediated by immobilized lipases: a review from thermodynamic and kinetic perspectives

Esters are a highly relevant class of compounds in the industrial context, and biocatalysis applied to ester syntheses is already a reality for some chemical companies.

What do we learn from enzyme behaviors in organic solvents? - Structural functionalization of ionic liquids for enzyme activation and stabilization

Enzyme activity in nonaqueous media (e.g. conventional organic solvents) is typically lower than in water by several orders of magnitude. There is a rising interest of developing new nonaqueous solvent systems that are more "water-like" and more biocompatible. Therefore, we need to learn from the current state of nonaqueous biocatalysis to overcome its bottleneck and provide guidance for new solvent design. This review firstly focuses on the discussion of how organic solvent properties (such as polarity and hydrophobicity) influence the enzyme activity and stability, and how these properties impact the enzyme's conformation and dynamics. While hydrophobic organic solvents usually lead to the maintenance of enzyme activity, solvents carrying functional groups like hydroxys and ethers (including crown ethers and cyclodextrins) can lead to enzyme activation. Ionic liquids (ILs) are designable solvents that can conveniently incorporate these functional groups. Therefore, we systematically survey these ether- and/or hydroxy-functionalized ILs, and find most of them are highly compatible with enzymes leading to high activity and stability. In particular, ILs carrying both ether and tert-alcohol groups are among the most enzyme-activating solvents. Future direction is to learn from enzyme behaviors in both water and nonaqueous media to design biocompatible "water-like" solvents.

Cochineal Waxy Residues as Source of Policosanol: Chemical Hydrolysis and Enzymatic Transesterification

The aim of this study was to obtain and characterise the long-chain alcohols present in policosanol derived from waste from the production of carminic acid, a natural colouring agent widely used in the food industry. The effectiveness of different methods designed for extraction of policosanol from waxy waste was investigated and its content and composition was determined. Triacontanol was the main component in policosanol produced by chemical processes, and it yields up to 13% by alkaline hydrolysis in water and chloroform extraction. Regarding enzymatic transesterification, policosanol was obtained using lipase Candida antarctica recombinant in Aspergillus niger (CAL-Bn) in a reaction medium with toluene. To improve the reaction, different acyl receptors, propanol, butanol, and isopropanol, were tested and molecular sieves were employed to maintain an anhydrous reaction medium. In this case, the policosanol was made up of other long-chain alcohols, but triacontanol was obtained in yields of up to 19% using isopropanol as an acyl receptor. Triacontanol has a great commercial value due to its effect as a promoter of plant growth, and these results contribute to the use and application of this agroindustrial waste in obtaining value-added products.

High-level expression and molecular characterization of a recombinant prolidase from Escherichia coli NovaBlue

Long-term use of organophosphorus (OP) compounds has become an increasing global problem and a major threat to sustainability and human health. Prolidase is a proline-specific metallopeptidase that can offer an efficient option for the degradation of OP compounds. In this study, a full-length gene from Escherichia coli NovaBlue encoding a prolidase (EcPepQ) was amplified and cloned into the commercially-available vector pQE-30 to yield pQE-EcPepQ. The overexpressed enzyme was purified from the cell-free extract of isopropyl thio-β-D-galactoside IPTG-induced E. coli M15 (pQE-EcPepQ) cells by nickel-chelate chromatography. The molecular mass of EcPepQ was determined to be about 57 kDa by 12% sodium dodecyl sulfate–polyacrylamide gel electrophoresis and the result of size-exclusion chromatography demonstrated that the enzyme was mainly present in 25 mM Tris–HCl buffer (pH 8.0) as a dimeric form. The optimal conditions for EcPepQ activity were 60 °C, pH 8.0, and 0.1 mM Mn²⁺ ion. Kinetic analysis with Ala-Pro as the substrate showed that the K_m and k_cat values of EcPepQ were 8.8 mM and 926.5 ± 2.0 s⁻¹, respectively. The thermal unfolding of EcPepQ followed a two-state process with one well-defined unfolding transition of 64.2 °C. Analysis of guanidine hydrochloride (GdnHCl)-induced denaturation by tryptophan emission fluorescence spectroscopy revealed that the enzyme had a [GdnHCl]_0.5,N-U value of 1.98 M. The purified enzyme also exhibited some degree of tolerance to various water/organic co-solvents. Isopropanol and tetrahydrofuran were very detrimental to the enzymatic activity of EcPepQ; however, other more hydrophilic co-solvents, such as formamide, methanol, and ethylene glycol, were better tolerated. Eventually, the non-negative influence of some co-solvents on both catalytic activity and structural stability of EcPepQ allows to adjust the reaction conditions more suitable for EcPepQ-catalyzed bioprocess.

Differences in the Abilities to Mechanically Eliminate Activation Energies for Unimolecular and Bimolecular Reactions

Mechanochemistry, i.e. the application of forces, F, at the molecular level, has attracted significant interest as a means of controlling chemical reactions. The present study uses quantum chemical calculations to explore the abilities to mechanically eliminate activation energies, ΔE^‡, for unimolecular and bimolecular reactions. The results demonstrate that ΔE^‡ can be eliminated for unimolecular reactions by applying sufficiently large F along directions that move the reactant and/or transition state (TS) structures parallel to the zero-F reaction coordinate, S₀. In contrast, eliminating ΔE^‡ for bimolecular reactions requires the reactant to undergo a force-induced shift parallel to S₀ irrespective of changes in the TS. Meeting this requirement depends upon the coupling between F and S₀ in the reactant. The insights regarding the differences in eliminating ΔE^‡ for unimolecular and bimolecular reactions, and the requirements for eliminating ΔE^‡, may be useful in practical efforts to control reactions mechanochemically.

Solvent effects in catalysis: implementation for modelling of kinetics

A mathematical framework is developed for analysis of solvent dependent reaction rates and selectivity in the case of complex catalytic reactions by incorporating solvent permittivity into the rate expressions.

Organic co-solvents affect activity, stability and enantioselectivity of haloalkane dehalogenases

Haloalkane dehalogenases are microbial enzymes with a wide range of biotechnological applications, including biocatalysis. The use of organic co-solvents to solubilize their hydrophobic substrates is often necessary. In order to choose the most compatible co-solvent, the effects of 14 co-solvents on activity, stability and enantioselectivity of three model enzymes, DbjA, DhaA, and LinB, were evaluated. All co-solvents caused at high concentration loss of activity and conformational changes. The highest inactivation was induced by tetrahydrofuran, while more hydrophilic co-solvents, such as ethylene glycol and dimethyl sulfoxide, were better tolerated. The effects of co-solvents at low concentration were different for each enzyme-solvent pair. An increase in DbjA activity was induced by the majority of organic co-solvents tested, while activities of DhaA and LinB decreased at comparable concentrations of the same co-solvent. Moreover, a high increase of DbjA enantioselectivity was observed. Ethylene glycol and 1,4-dioxane were shown to have the most positive impact on the enantioselectivity. The favorable influence of these co-solvents on both activity and enantioselectivity makes DbjA suitable for biocatalytic applications. This study represents the first investigation of the effects of organic co-solvents on the biocatalytic performance of haloalkane dehalogenases and will pave the way for their broader use in industrial processes.

Multi-catalysis reactions: new prospects and challenges of biotechnology to valorize lignin

Considerable effort has been dedicated to the chemical depolymerization of lignin, a biopolymer constituting a possible renewable source for aromatic value-added chemicals. However, these efforts yielded limited success up until now. Efficient lignin conversion might necessitate novel catalysts enabling new types of reactions. The use of multiple catalysts, including a combination of biocatalysts, might be necessary. New perspectives for the combination of bio- and inorganic catalysts in one-pot reactions are emerging, thanks to green chemistry-driven advances in enzyme engineering and immobilization and new chemical catalyst design. Such combinations could offer several advantages, especially by reducing time and yield losses associated with the isolation and purification of the reaction products, but also represent a big challenge since the optimal reaction conditions of bio- and chemical catalysis reactions are often different. This mini-review gives an overview of bio- and inorganic catalysts having the potential to be used in combination for lignin depolymerization. We also discuss key aspects to consider when combining these catalysts in one-pot reactions.

Thermostable variants constructed via the structure-guided consensus method also show increased stability in salts solutions and homogeneous aqueous-organic media

Enzyme instability is a major factor preventing widespread adoption of enzymes for catalysis. Stability at high temperatures and in the presence of high salt concentrations and organic solvents would allow enzymes to be employed for transformations of compounds not readily soluble in low temperature or in purely aqueous systems. Furthermore, many redox enzymes require costly cofactors for function and consequently a robust cofactor regeneration system. In this work, we demonstrate how thermostable variants developed via an amino acid sequence-based consensus method also showed improved stability in solutions with high concentrations of kosmotropic and chaotropic salts and water-miscible organic solvents. This is invaluable to protein engineers since deactivation in salt solutions and organic solvents is not well understood, rendering a priori design of enzyme stability in these media difficult. Variants of glucose 1-dehydrogenase (GDH) were studied in solutions of different salts along the Hofmeister series and in the presence of varying amounts of miscible organic solvent. Only the most stable variants showed little deactivation dependence on salt-type and salt concentration. Kinetic stability, expressed by the deactivation rate constant k(d,obs), did not always correlate with thermodynamic stability of variants, as measured by melting temperature T(m). However, a strong correlation (R(2) > 0.95) between temperature stability and organic solvent stability was found when plotting T(50)(60) versus C(50)(60) values. All GDH variants retained stability in homogeneous aqueous-organic solvents with >80% v/v of organic solvent.

Water dynamics and salt-activation of enzymes in organic media: mechanistic implications revealed by NMR spectroscopy

Deuterium spin relaxation was used to examine the motion of enzyme-bound water on subtilisin Carlsberg co-lyophilized with inorganic salts for activation in different organic solvents. Spectral editing was used to ensure that the relaxation times were associated with relatively mobile deuterons, which were contributed almost entirely by D(2)O rather than hydrogen-deuteron exchange on the protein. The results indicate that the timescale of motion for residual water molecules on the biocatalyst, (tau(c))(D(2)O), in hexane decreased from 65 ns (salt-free) to 0.58 ns (98% CsF) as (k(cat)/K(M))(app) of the biocatalyst preparation increased from 0.092 s(-1) x M(-1) (salt-free) to 1,140 s(-1) x M(-1) (98% CsF). A similar effect was apparent in acetone; the timescale decreased from 24 ns (salt-free) to 2.87 ns (98% KF), with a corresponding increase in (k(cat)/K(M))(app) of 0.140 s(-1) x M(-1) (salt-free) to 12.8 s(-1) x M(-1) (98% KF). Although a global correlation between water mobility and enzyme activity was not evident, linear correlations between ln[(k(cat)/K(M))(app)] and (tau(c))(D(2)O) were obtained for salt-activated enzyme preparations in both hexane and acetone. Furthermore, a direct correlation was evident between (k(cat)/K(M))(app) and the total amount of mobile water per mass of enzyme. These results suggest that increases in enzyme-bound water mobility mediated by the presence of salt act as a molecular lubricant and enhance enzyme flexibility in a manner functionally similar to temperature. Greater flexibility may permit a larger degree of local transition-state mobility, reflected by a more positive entropy of activation, for the salt-activated enzyme compared with the salt-free enzyme. This increased mobility may contribute to the dramatic increases in biocatalyst activity.

The use of poly(ethylene oxide) for the efficient stabilization of entrapped α-chymotrypsin in silicone elastomers: A chemometric study

The enzyme alpha-chymotrypsin, a model for catalytic proteins, was entrapped in different silicone elastomers that were formed via the condensation-cure room temperature vulcanization (CC-RTV) of silanol terminated poly(dimethylsiloxane) with tetraethyl orthosilicate as a crosslinker, in the presence of different poly(ethylene oxide) oligomers that were functionalized with triethoxysilyl groups. The effects of various chemical factors on both the activity and entrapping efficiency of proteins (leaching) were studied using a 2-level fractional factorial design--a chemometrics approach. The factors studied include the concentration and chain length of poly(ethylene oxide), enzyme content, and crosslinker (TEOS) concentration. The study indicated that poly(ethylene oxide) can stabilize the entrapped alpha-chymotrypsin in silicone rubber: the specific activity can be maximized by incorporating a relatively high content of short chain, functional PEO. Increased enzyme concentration was found to adversely affect the specific activity. The effect of TEOS was found to be insignificant when PEO was present in the elastomer, however, it does affect the activity positively in the case of simple elastomers.

Characteristics of nearly dry enzymes in organic solvents: implications for biocatalysis in the absence of water

We have examined enzymes in nearly anhydrous organic solvents spanning a wide range of dielectric constants using a combination of electron paramagnetic resonance (EPR) spectroscopy, molecular dynamics simulations, high-pressure kinetic studies and the electrostatic model of Kirkwood. This approach enabled us to investigate the relationship between catalytic activity, protein flexibility and solvent polarity for an enzymatic reaction proceeding through a highly polar transition state in the near absence of water. Further insights into water-protein interactions and the involvement of water in enzyme structure and function have been obtained by EPR and multinuclear nuclear magnetic resonance studies of enzymes suspended and immobilized in organic solvents with and without added water. In these systems, correlations were observed between the water content and enzyme activity, flexibility, and active-site polarity, although the structural properties of suspended and immobilized enzymes differed markedly. These results have helped to elucidate the role of water in molecular events at the enzymic active site leading to improved biocatalysis in low-water environments.

Solid-phase peptide synthesis by ion-paired alpha-chymotrypsin in nonaqueous media

Solid-phase synthesis of dipeptides in low-water media was achieved using AOT ion-paired alpha-chymotrypsin solubilized in organic solvents. Multiple solvents and systematic variation of water activity, a(w), were used to examine the rate of coupling between N-alpha-benzyloxycarbonyl-L-phenylalanine methyl ester (Z-Phe-OMe) and leucine as a function of the reaction medium for both solid-phase and solution-phase reactions. In solution, the observed maximum reaction rate in a given solvent generally correlated with measures of hydrophobicity such as the log of the 1-octanol/water partitioning coefficient (log P) and the Hildebrand solubility parameter. The maximum rate for solution-phase synthesis (13 mmol/h g-enzyme) was obtained in a 90/10 (v/v) isooctane/tetrahydrofuran solvent mixture at an a(w) of 0.30. For the synthesis of dipeptides from solid-phase leucine residues, the highest synthetic rates (0.14-1.3 mmol/h g-enzyme) were confined to solvent environments that fell inside abruptly defined regions of solvent parameter space (e.g., log P > 2.3 and normalized electron acceptance index <0.13). The maximum rate for solid-phase synthesis was obtained in a 90/10 (v/v) isooctane/tetrahydrofuran solvent mixture at an a(w) of 0.14. In 90/10 and 70/30 (v/v) isooctane/tetrahydrofuran environments with a(w) set to 0.14, seven different N-protected dipeptides were synthesized on commercially available Tentagel support with yields of 74-98% in 24 h.