Solvent effect on lipase enantioselectivity. Evidence for the presence of two thermodynamic states

Finite-temperature effect in the O-acylation of (R,S)-propranolol catalyzed by Candida antarctica lipase B

CalB is a triacylglycerol hydrolase (E.C.3.1.1.3) used in the O-acylation of the beta-adrenergic blocking agent (R,S)-propranolol. The catalytic mechanism involves two steps: enzyme acylation and enzyme deacylation. The enantioselectivity of the O-acylation of (R,S)-propranolol originates from the second step, where the acyl-enzyme transfers the acyl group to the racemic substrate. This step occurs via an initial Michaelis complex (MCC) and a tetrahedral intermediate (TI-2). To gain more insight into the molecular basis of this reaction, we performed an exhaustive conformational sampling along the reaction coordinate of the enantioselective step of the reaction (MCC→TI-2→EPC) applying a QM/MM MD protocol (SCC-DFTB/CHARMM) in combination with umbrella sampling and the weighted histogram analysis method. To identify finite temperature effects we compare the PMF and the potential energy pathway. It is found that the effect of the finite temperature in this reaction is a destabilization of the tetrahedral intermediate and an increase of the barrier height of its formation. This increase is higher for the S-enantiomer.

Organic Solvent Effects in Biomass Conversion Reactions

Transforming lignocellulosic biomass into fuels and chemicals has been intensely studied in recent years. A large amount of work has been dedicated to finding suitable solvent systems, which can improve the transformation of biomass into value-added chemicals. These efforts have been undertaken based on numerous research results that have shown that organic solvents can improve both conversion and selectivity of biomass to platform molecules. We present an overview of these organic solvent effects, which are harnessed in biomass conversion processes, including conversion of biomass to sugars, conversion of sugars to furanic compounds, and production of lignin monomers. A special emphasis is placed on comparing the solvent effects on conversion and product selectivity in water with those in organic solvents while discussing the origins of the differences that arise. We have categorized results as benefiting from two major types of effects: solvent effects on solubility of biomass components including cellulose and lignin and solvent effects on chemical thermodynamics including those affecting reactants, intermediates, products, and/or catalysts. Finally, the challenges of using organic solvents in industrial processes are discussed from the perspective of solvent cost, solvent stability, and solvent safety. We suggest that a holistic view of solvent effects, the mechanistic elucidation of these effects, and the careful consideration of the challenges associated with solvent use could assist researchers in choosing and designing improved solvent systems for targeted biomass conversion processes.

Combination of oxyanion Gln114 mutation and medium engineering to influence the enantioselectivity of thermophilic lipase from Geobacillus zalihae

The substitution of the oxyanion Q114 with Met and Leu was carried out to investigate the role of Q114 in imparting enantioselectivity on T1 lipase. The mutation improved enantioselectivity in Q114M over the wild-type, while enantioselectivity in Q114L was reduced. The enantioselectivity of the thermophilic lipases, T1, Q114L and Q114M correlated better with log p as compared to the dielectric constant and dipole moment of the solvents. Enzyme activity was good in solvents with log p < 3.5, with the exception of hexane which deviated substantially. Isooctane was found to be the best solvent for the esterification of (R,S)-ibuprofen with oleyl alcohol for lipases Q114M and Q114L, to afford E values of 53.7 and 12.2, respectively. Selectivity of T1 was highest in tetradecane with E value 49.2. Solvents with low log p reduced overall lipase activity and dimethyl sulfoxide (DMSO) completely inhibited the lipases. Ester conversions, however, were still low. Molecular sieves employed as desiccant were found to adversely affect catalysis in the lipase variants, particularly in Q114M. The higher desiccant loading also increased viscosity in the reaction and further reduced the efficiency of the lipase-catalyzed esterifications.

Modelling substrate specificity and enantioselectivity for lipases and esterases by substrate-imprinted docking

Background

Previously, ways to adapt docking programs that were developed for modelling inhibitor-receptor interaction have been explored. Two main issues were discussed. First, when trying to model catalysis a reaction intermediate of the substrate is expected to provide more valid information than the ground state of the substrate. Second, the incorporation of protein flexibility is essential for reliable predictions.

Results

Here we present a predictive and robust method to model substrate specificity and enantioselectivity of lipases and esterases that uses reaction intermediates and incorporates protein flexibility. Substrate-imprinted docking starts with covalent docking of reaction intermediates, followed by geometry optimisation of the resulting enzyme-substrate complex. After a second round of docking the same substrate into the geometry-optimised structures, productive poses are identified by geometric filter criteria and ranked by their docking scores. Substrate-imprinted docking was applied in order to model (i) enantioselectivity of Candida antarctica lipase B and a W104A mutant, (ii) enantioselectivity and substrate specificity of Candida rugosa lipase and Burkholderia cepacia lipase, and (iii) substrate specificity of an acetyl- and a butyrylcholine esterase toward the substrates acetyl- and butyrylcholine.

Conclusion

The experimentally observed differences in selectivity and specificity of the enzymes were reproduced with an accuracy of 81%. The method was robust toward small differences in initial structures (different crystallisation conditions or a co-crystallised ligand), although large displacements of catalytic residues often resulted in substrate poses that did not pass the geometric filter criteria.

Optimization of enantioselective resolution of racemic ibuprofen by native lipase from Aspergillus niger

Resolution of (R,S)-ibuprofen (2-(4-isobutylphenyl)propionic acid) enantiomers by esterification reaction with 1-propanol in different organic solvents was studied using native Aspergillus niger lipase. The main variables controlling the process (enzyme concentration and 1-propanol:ibuprofen molar ratio) have been optimized using response surface methodology based on a five-level, two-variable central composite rotatable design, in which the selected objective function was enantioselectivity. This enzyme preparation showed preferentially catalyzes the esterification of R(-)-ibuprofen, and under optimum conditions (7% w/v of enzyme and molar ratio of 2.41:1) the enantiomeric excess of active S(+)-ibuprofen and total conversion values were 79.1 and 48.0%, respectively, and the E-value was 32, after 168 h of reaction in isooctane.

Size as a parameter for solvent effects on Candida antarctica lipase B enantioselectivity

Changes in solvent type were shown to yield significant improvement of enzyme enantioselectivity. The resolution of 3-methyl-2-butanol catalyzed by Candida antarctica lipase B, CALB, was studied in eight liquid organic solvents and supercritical carbon dioxide, SCCO(2). Studies of the temperature dependence of the enantiomeric ratio allowed determination of the enthalpic (Delta(R-S)Delta H(++)) as well as the entropic (Delta(R-S)Delta S(++)) contribution to the overall enantioselectivity (Delta(R-S)Delta G(++)= -RTlnE). A correlation of the enantiomeric ratio, E, to the van der Waals volume of the solvent molecules was observed and suggested as one of the parameters that govern solvent effects on enzyme catalysis. An enthalpy-entropy compensation relationship was indicated between the studied liquid solvents. The enzymatic mechanism must be of a somewhat different nature in SCCO(2), as this reaction in this medium did not follow the enthalpy-entropy compensation relation.