Elucidating solvent effects on lipase-catalyzed peroxyacid synthesis through activity-based kinetics and molecular dynamics

Peroxyacid synthesis is the first step in Prilezhaev epoxidation, which is an industrial method to form epoxides. Motivated by the development of a kinetic model as a tool for solvent selection, the effect of solvent type and acid chain length on the lipase-catalyzed peroxyacid synthesis was studied. A thermodynamic activity-based ping-pong kinetic expression was successfully applied to predict the effect of the reagent loadings in hexane. The activity-based reaction quotients provided a prediction of solvent-independent equilibrium constants. However, this strategy did not achieve satisfactory estimations of initial rates in solvents of higher polarity. The lack of compliance with some assumptions of this methodology could be confirmed through molecular dynamics calculations i.e. independent solvation energies and lack of solvent interaction with the active site. A novel approach is proposed combining the activity-based kinetic expression and the free binding energy of the solvent with the active site to predict kinetics upon solvent change. Di-isopropyl ether generated a strong interaction with the enzyme's active site, which was detrimental to kinetics. On the other hand, toluene or limonene gave moderate interaction with the active site rendering improved catalytic yield compared with less polar solvents, a finding sharpened when peroctanoic acid was produced.

Enzymatic Performance of Aspergillus oryzae α-Amylase in the Presence of Organic Solvents: Activity, Stability, and Bioinformatic Studies

Enzymatic reactions can be modulated by the incorporation of organic solvents, leading to alterations in enzyme stability, activity, and reaction rates. These solvents create a favorable microenvironment that enables hydrophobic reactions, facilities enzyme-substrate complex formation, and reduces undesirable water-dependent side reactions. However, it is crucial to understand the impact of organic solvents on enzymatic activity, as they can also induce enzyme inactivation. In this study, the enzymatic performance of Aspergillus oryzae α-amylase (Taka-amylase) in various organic solvents both experimentally and computationally was investigated. The results demonstrated that ethanol and ether sustain Taka-amylase activity up to 20% to 25% of the organic solvents, with ether providing twice the stability of ethanol. Molecular dynamics simulations further revealed that Taka-amylase has a more stable structure in ether and ethanol relative to other organic solvents. In addition, the analysis showed that the loop located near the active site in the AB-domain is a vulnerable site for enzyme destabilization when exposed to organic solvents. The ability of Taka-amylase to preserve the secondary loop structure in ether and ethanol contributed to the enzyme's activity. In addition, the solvent accessibility surface area of Taka-amylase is distributed throughout all enzyme structures, thereby contributing to the instability of Taka-amylase in the presence of most organic solvents.

Leveraging liquid-liquid phase separation and volume modulation to regulate the enzymatic activity of formate dehydrogenase

Engineering of reaction media is an exciting alternative for modulating kinetic properties of biocatalytic reactions. We addressed the combined effect of an aqueous two-phase system (ATPS) and high hydrostatic pressure on the kinetics of the Candida boidinii formate dehydrogenase-catalyzed oxidation of formate to CO2. Pressurization was found to lead to an increase of the binding affinity (decrease of KM, respectively) and a decrease of the turnover number, kcat. The experimental approach was supported using thermodynamic modeling with the electrolyte Perturbed-Chain Statistical Associating Fluid Theory (ePC-SAFT) equation of state to predict the liquid-liquid phase separation and the molecular crowding effect of the ATPS on the kinetic properties. The ePC-SAFT was able to quantitatively predict the KM-values of the substrate in both phases at 1 bar as well as up to a pressure of 1000 bar. The framework presented enables significant advances in bioprocess engineering, including the design of processes with significantly fewer experiments and trial-and-error approaches.

Efficient enzymatic synthesis of L-ascorbyl palmitate using Candida antarctica lipase B-embedded metal-organic framework

The Candida antarctica lipase B (CALB) was embedded in the metal-organic framework, zeolitic imidazolate framework-8 (ZIF-8), and applied in the enzymatic synthesis of L-ascorbic acid palmitate (ASP) for the first time. The obtained CALB@ZIF-8 achieved the enzyme loading of 80 mg g^-1 with 11.3 U g^-1 (dry weight) unit activity, 59.8% activity recovery, and 92.7% immobilization yield. Under the optimal condition, ASP was synthesized with over 75.9% conversion of L-ascorbic acid in a 10-batch reaction. Continuous synthesis of ASP was subsequently performed in a packed bed bioreactor with an outstanding average space-time yield of 58.1 g L^-1  h^-1 , which was higher than ever reported continuous ASP biosynthesis reactions.

Thermodynamics and foundations of mass-action kinetics

A critical overview is given of phenomenological thermodynamic approaches to reaction rate equations of the type based on the law of mass-action. The review covers treatments based on classical equilibrium and irreversible (linear) thermodynamics, extended irreversible, rational and continuum thermodynamics. Special attention is devoted to affinity, the applications of activities in chemical kinetics and the importance of chemical potential. The review shows that chemical kinetics survives as the touchstone of these various thermody-namic theories. The traditional mass-action law is neither demonstrated nor proved and very often is only introduced post hoc into the framework of a particular thermodynamic theory, except for the case of rational thermodynamics. Most published “thermodynamic'’ kinetic equations are too complicated to find application in practical kinetics and have merely theoretical value. Solely rational thermodynamics can provide, in the specific case of a fluid reacting mixture, tractable rate equations which directly propose a possible reaction mechanism consistent with mass conservation and thermodynamics. It further shows that affinity alone cannot determine the reaction rate and should be supplemented by a quantity provisionally called constitutive affinity. Future research should focus on reaction rates in non-isotropic or non-homogeneous mixtures, the applicability of traditional (equilibrium) expressions relating chemical potential to activity in non-equilibrium states, and on using activities and activity coefficients determined under equilibrium in non-equilibrium states.

Solvent stable microbial lipases: current understanding and biotechnological applications

OBJECTIVE

This review examines on our current understanding of microbial lipase solvent tolerance, with a specific focus on the molecular strategies employed to improve lipase stability in a non-aqueous environment.

RESULTS

It provides an overview of known solvent tolerant lipases and of approaches to improving solvent stability such as; enhancing stabilising interactions, modification of residue flexibility and surface charge alteration. It shows that judicious selection of lipase source supplemented by appropriate enzyme stabilisation, can lead to a wide application spectrum for lipases.

CONCLUSION

Organic solvent stable lipases are, and will continue to be, versatile and adaptable biocatalytic workhorses commonly employed for industrial applications in the food, pharmaceutical and green manufacturing industries.

Enzymatic Production of Biodiesel: Strategies to Overcome Methanol Inactivation

Lipase-catalyzed transesterification of triglycerides and alcohols to obtain biodiesel is an environmentally friendly and sustainable route for fuels production since, besides proceeding in mild reaction conditions, it allows for the use of low-cost feedstocks that contain water and free fatty acids, for example non-edible oils and waste oils. This review article reports recent advances in the field and focus in particular on a major issue in the enzymatic process, the inactivation of most lipases caused by methanol, the preferred acyl acceptor used for alcoholysis. The recent results about immobilization of enzymes on nano-materials and the use of whole-cell biocatalysts, as well as the use of cell-surface display technologies and metabolic engineering strategies for microbial production of biodiesel are described. It is discussed also insight into the effects of methanol on lipases obtained by modeling approaches and report on studies aimed at mining novel alcohol stable enzymes or at improving robustness in existing ones by protein engineering.

Lipases as Biocatalyst for Biodiesel Production

Global shortages of fossil fuels, significant rise in the price of crude oil, and increased environmental concerns have stimulated the rapid growth of biodiesel production. Biodiesel is generally produced through transesterification reaction catalyzed either chemically or enzymatically. Enzymatic transesterification is of interest since it shows advantages over the chemical process and, in addition, is considered a "green" process. This chapter reviews the current status of biodiesel production with a lipase biocatalysis approach, including sources of lipases, kinetics, lipase immobilization techniques, and lipase reaction mechanism for biodiesel production. Factors affecting biodiesel production and the economic feasibility of lipase biodiesel production are also covered.

Binding of Solvent Molecules to a Protein Surface in Binary Mixtures Follows a Competitive Langmuir Model

The binding of solvent molecules to a protein surface was modeled by molecular dynamics simulations of of Candida antarctica (C. antarctica) lipase B in binary mixtures of water, methanol, and toluene. Two models were analyzed: a competitive Langmuir model which assumes identical solvent binding sites with a different affinity toward water (KWat), methanol (KMet), and toluene (KTol) and a competitive Langmuir model with an additional interaction between free water and already bound water (KWatWat). The numbers of protein-bound molecules of both components of a binary mixture were determined for different compositions as a function of their thermodynamic activities in the bulk phase, and the binding constants were simultaneously fitted to the six binding curves (two components of three different mixtures). For both Langmuir models, the values of KWat, KMet, and KTol were highly correlated. The highest binding affinity was found for methanol, which was almost 4-fold higher than the binding affinities of water and toluene (KMet ≫ KWat ≈ KTol). Binding of water was dominated by the water-water interaction (KWatWat). Even for the three protein surface patches of highest water affinity, the binding affinity of methanol was 2-fold higher than water and 8-fold higher than toluene (KMet > KWat > KTol). The Langmuir model provides insights into the protein destabilizing mechanism of methanol which has a high binding affinity toward the protein surface. Thus, destabilizing solvents compete with intraprotein interactions and disrupt the tertiary structure. In contrast, benign solvents such as water or toluene have a low affinity toward the protein surface. Water is a special solvent: only few water molecules bind directly to the protein; most water molecules bind to already bound water molecules thus forming water patches. A quantitative mechanistic model of protein-solvent interactions that includes competition and miscibility of the components contributes a robust basis for solvent and protein engineering.

Medium-engineering: a useful tool for modulating lipase activity and selectivity

The design of a specific reaction medium capable to enhance activity, stability, and productivity of biocatalysts has been a recurring topic of study during the last three decades. The remarkable properties and valuable applications of enzymes, especially lipases, have inspiried different strategies for improving their performance in near-anhydrous media. As lipases are the most frequently used enzymes in organic synthesis, understanding the influence of reaction media on their activity and selectivity is crucial. In this paper, we review the key features of lipases and demonstrate how medium-engineering is a useful tool to modulate the activity and selectivity of lipase-catalyzed reactions.

Molecular mechanism of deactivation of C. antarctica lipase B by methanol

The catalytic activity of Candida antarctica lipase B upon alcoholysis of a constant concentration of 15.2% vinyl acetate (vol/vol) and varying concentrations of methanol (0.7-60%) in toluene was determined experimentally by measuring the initial reaction velocity. The molecular mechanism of the deactivation of the enzyme by methanol was investigated by fitting the experimental data to a kinetic model and by molecular dynamics simulations of C. antarctica lipase B in toluene-methanol-water mixtures. The highest catalytic activity (280 U/mg) was observed at methanol concentrations as low as 0.7% methanol (vol/vol), followed by a sharp decrease at higher methanol concentrations. For methanol concentrations above 10% (vol/vol), catalytic activity was at 30% of the maximum activity. A variation of water activity in the range 0.02-0.09 had only minor effects. These experimental observations are described by a simple kinetic model using three assumptions: (1) a ping-pong bi-bi mechanism of the enzyme, (2) competitive inhibition by the substrate methanol, and (3) by describing enzyme kinetics by the thermodynamic activities of the substrates rather than by their concentrations. Two equilibrium constants of methanol (KM,MeOH=0.05 and Ki,MeOH=0.23) were derived by modeling methanol binding to the substrate binding site of the lipase in molecular dynamics simulations of protein-solvent systems at atomic resolution. Thus, the sharp maximum of catalytic activity of C. antarctica lipase B at 0.7% methanol is a direct consequence of the fact that methanol-toluene mixtures are far from ideal. Understanding the thermodynamics of solvent mixtures is prerequisite to a quantitative model of enzymatic activity in organic solvents.

Kinetic investigation of a solvent-free, chemoenzymatic reaction sequence towards enantioselective synthesis of a β-amino acid ester

A solvent-free, chemoenzymatic reaction sequence for the enantioselective synthesis of β-amino acid esters has been kinetically and thermodynamically characterized. The coupled sequence comprises a thermal aza-Michael addition of cheap starting materials and a lipase catalyzed aminolysis for the kinetic resolution of the racemic ester. Excellent ee values of >99% were obtained for the β-amino acid ester at 60% conversion. Kinetic constants for the aza-Michael addition were obtained by straightforward numerical integration of second-order rate equations and nonlinear fitting of the progress curves. A different strategy had to be devised for the biocatalytic reaction. Initially, a simplified Michaelis-Menten model including product inhibition was developed for the reaction running in THF as an organic solvent. Activity based parameters were used instead of concentrations in order to facilitate the transfer of the kinetic model to the solvent-free system. Observed solvent effects not accounted for by the use of thermodynamic activities were incorporated into the kinetic model. Enzyme deactivation was observed to depend on the ratio of the applied substrates and also included in the kinetic model. The developed simple model is in very good agreement with the experimental data and allows the simulation and optimization of the solvent-free process.

Lipases as biocatalyst for biodiesel production

The global shortages of fossil fuels, significant increase in the price of crude oil, and increased environmental concerns have stimulated the rapid growth in biodiesel production. Biodiesel is generally produced through transesterification reaction catalyzed either chemically or enzymatically. Enzymatic transesterification draws high attention because that process shows certain advantages over the chemical catalysis of transesterification and it is "greener." This paper reviews the current status of biodiesel production with lipase-biocatalysis approach, including sources of lipases, kinetics, and reaction mechanism of biodiesel production using lipases, and lipase immobilization techniques. Factors affecting biodiesel production and economic feasibility of biodiesel production using lipases are also covered.

Thermodynamical methods for the optimization of lipase-catalyzed reactions

A basic insight on different thermodynamical strategies reported for the optimization of lipase-catalyzed reactions is presented. The significance of selecting the appropriate reaction media in order to enhance selectivity and operational stability of enzymes is discussed. From this analysis, the importance of developing thermodynamic strategies for controlling both the reaction kinetics and equilibrium is emphasized. A theoretical model (Conductor-like Screening Model for Realistic Solvation) for calculating thermodynamic properties in fluid phases is proposed as a powerful tool for predicting equilibrium and kinetic behavior in biocatalytic processes.

Continuous lipase-catalyzed production of esters from crude high-oleic sunflower oil

The objective of this work was to develop an economically relevant enzymatic process of butyl ester production using crude high-oleic sunflower oil. Novozym 435, a non-regiospecific biocatalyst, provided the best compromise between activity and butyl-ester yield. The inhibition caused by the presence of phopholipids in crude oil was eliminated by using tert-butanol. It demonstrates the key role of the medium polarity in order to insure the stability of a process. Initial substrate concentrations and their molar ratio were optimized in a continuous packed-bed reactor to maximize product yield and productivity. The best compromise was obtained for an initial oil concentration of 500 mM and a molar ratio of 5. It enabled a high productivity of 13.8 tons year(-1)kg Novozym 435(-1) with a butyl-ester purity of 96.5% to be obtained. Experiments with the continuous reactor were performed over 50 days without any loss of enzyme activity.

Detection of ligand- and solvent-induced shape alterations of cell-growth-regulatory human lectin galectin-1 in solution by small angle neutron and x-ray scattering

The bioactivity of galectin-1 in cell growth regulation and adhesion prompted us to answer the questions whether ligand presence and a shift to an aprotic solvent typical for bioaffinity chromatography might alter the shape of the homodimeric human lectin in solution. We used small angle neutron and synchrotron x-ray scattering studies for this purpose. Upon ligand accommodation, the radius of gyration of human galectin-1 decreased from 19.1 +/- 0.1 A in the absence of ligand to 18.2 +/- 0.1 A. In the aprotic solvent dimethyl sulfoxide, which did not impair binding capacity, galectin-1 formed dimers of a dimer, yielding tetramers with a cylindrical shape. Intriguingly, no dissociation into subunits occurred. In parallel, NMR monitoring was performed. The spectral resolution was in accord with these data. In contrast to the properties of the human protein, a nonhomologous agglutinin from mistletoe sharing galactose specificity was subject to a reduction in the radius of gyration from approximately 62 A in water to 48.7 A in dimethyl sulfoxide. Evidently, the solvent caused opposite responses in the two tested galactoside-binding lectins with different folding patterns. We have hereby proven that ligand presence and an aprotic solvent significantly affect the shape of galectin-1 in solution.