Enantioselectivity ofCandida RugosaLipase Toward Carboxylic Acids: A Predictive Rule from Substrate Mapping and X-Ray Crystallography

Microwave-induced biocatalytic reactions toward medicinally important compounds

Microwaves in the presence of enzymes are used to execute a number of reactions for the preparation of biologically active compounds. The success of microwave-induced enzymatic reactions depends on frequencies, field strength, waveform, duration, and modulation of the exposure. Enzymes under microwave irradiation become activated and this activation is sufficient to investigate simple to complex reactions that were not reported under these reaction conditions before. Enzymatic catalysis together with microwave technology and solvent-free chemical reaction is a nature-friendly procedure. The most interesting reactions that are performed by enzymes in the microwave are documented here with reference to examples that are related to medicinally active molecules.

Computer-aided understanding and engineering of enzymatic selectivity

Enzymes offering chemo-, regio-, and stereoselectivity enable the asymmetric synthesis of high-value chiral molecules. Unfortunately, the drawback that naturally occurring enzymes are often inefficient or have undesired selectivity toward non-native substrates hinders the broadening of biocatalytic applications. To match the demands of specific selectivity in asymmetric synthesis, biochemists have implemented various computer-aided strategies in understanding and engineering enzymatic selectivity, diversifying the available repository of artificial enzymes. Here, given that the entire asymmetric catalytic cycle, involving precise interactions within the active pocket and substrate transport in the enzyme channel, could affect the enzymatic efficiency and selectivity, we presented a comprehensive overview of the computer-aided workflow for enzymatic selectivity. This review includes a mechanistic understanding of enzymatic selectivity based on quantum mechanical calculations, rational design of enzymatic selectivity guided by enzyme-substrate interactions, and enzymatic selectivity regulation via enzyme channel engineering. Finally, we discussed the computational paradigm for designing enzyme selectivity in silico to facilitate the advancement of asymmetric biosynthesis.

Characterised Flavin-Dependent Two-Component Monooxygenases from the CAM Plasmid of Pseudomonas putida ATCC 17453 (NCIMB 10007): ketolactonases by Another Name

The CAM plasmid-coded isoenzymic diketocamphane monooxygenases induced in Pseudomonas putida ATCC 17453 (NCIMB 10007) by growth of the bacterium on the bicyclic monoterpene (rac)-camphor are notable both for their interesting history, and their strategic importance in chemoenzymatic syntheses. Originally named 'ketolactonase-an enzyme system for cyclic lactonization' because of its characterised mode of action, (+)-camphor-induced 2,5-diketocamphane 1,2-monooxygenase was the first example of a Baeyer-Villiger monooxygenase activity to be confirmed in vitro. Both this enzyme and the enantiocomplementary (-)-camphor-induced 3,6-diketocamphane 1,6-monooxygenase were mistakenly classified and studied as coenzyme-containing flavoproteins for nearly 40 years before being correctly recognised and reinvestigated as FMN-dependent two-component monooxygenases. As has subsequently become evident, both the nature and number of flavin reductases able to supply the requisite reduced flavin co-substrate for the monooxygenases changes progressively throughout the different phases of camphor-dependent growth. Highly purified preparations of the enantiocomplementary monooxygenases have been exploited successfully for undertaking both nucleophilic and electrophilic biooxidations generating various enantiopure lactones and sulfoxides of value as chiral synthons and auxiliaries, respectively. In this review the chequered history, current functional understanding, and scope and value as biocatalysts of the diketocamphane monooxygenases are discussed.

Lipases in polymer chemistry

Lipases are highly active in the polymerization of a range of monomers. Both ring-opening polymerization of cyclic monomers such as lactones and carbonates as well as polycondensation reactions have been investigated in great detail. Moreover, in combination with other (chemical) polymerization techniques, lipase-catalyzed polymerization has been employed to synthesize a variety of polymer materials. Major advantages of enzymatic catalysts are the often-observed excellent regio-, chemo- and enantioselectivity that allows for the direct preparation of functional materials. In particular, the application of techniques such as Dynamic Kinetic Resolution (DKR) in the lipase-catalyzed polymerization of racemic monomers is a new development in enzymatic polymerization. This paper reviews selected examples of the application of lipases in polymer chemistry covering the synthesis of linear polymers, chemoenzymatic polymerization and applications of enantioselective techniques for the synthesis and modification of polymers.

Display of bacterial lipase on the Escherichia coli cell surface by using FadL as an anchoring motif and use of the enzyme in enantioselective biocatalysis

We have developed a novel cell surface display system by employing FadL as an anchoring motif, which is an outer membrane protein involved in long-chain fatty acid transport in Escherichia coli. A thermostable Bacillus sp. strain TG43 lipase (44.5 kDa) could be successfully displayed on the cell surface of E. coli in an active form by C-terminal deletion-fusion of lipase at the ninth external loop of FadL. The localization of the truncated FadL-lipase fusion protein on the cell surface was confirmed by confocal microscopy and Western blot analysis. Lipase activity was mainly detected with whole cells, but not with the culture supernatant, suggesting that cell lysis was not a problem. The activity of cell surface-displayed lipase was examined at different temperatures and pHs and was found to be the highest at 50 degrees C and pH 9 to 10. Cell surface-displayed lipase was quite stable, even at 60 and 70 degrees C, and retained over 90% of the full activity after incubation at 50 degrees C for a week. As a potential application, cell surface-displayed lipase was used as a whole-cell catalyst for kinetic resolution of racemic methyl mandelate. In 36 h of reaction, (S)-mandelic acid could be produced with the enantiomeric excess of 99% and the enantiomeric ratio of 292, which are remarkably higher than values obtained with crude lipase or cross-linked lipase crystal. These results suggest that FadL may be a useful anchoring motif for displaying enzymes on the cell surface of E. coli for whole-cell biocatalysis.

Controlling lipase enantioselectivity for organic synthesis

Lipases are used frequently as chiral catalysts in the synthesis of various fine chemicals and intermediates. The increasing need of compounds with high stereochemical purity requires catalysts with an improved and controlled performance. This overview emphasizes some important aspects for the control of lipase enantioselectivity and some examples where the enantioselectivity has been altered or reversed are highlighted. However, in several of these cases the complete explanation for the altered or reversed enantioselectivity remains unclear and needs to be solved. Three different strategies (engineering of the reaction medium, the substrate molecule, and the enzyme) for exploring lipase enantioselectivity at a molecular level are discussed and summarized. These three different approaches represent powerful tools for understanding the molecular basis for lipase enantioselective catalysis and can guide the rational improvement and tailoring of catalyst performance. By combining approaches from chemistry and biology much is learnt about the most important parameters controlling lipase enantioselectivity for organic synthesis.

The enantioselective hydrolysis of 3-hydroxy-5-phenyl-4-pentenoicacidethylester in supercritical carbon dioxide using lipases

A new experimental high-pressure-unit was constructed for the enantioselective enzymatic hydrolysis of 3-hydroxy-5-phenyl-4-pentenoicacidethylester (a precursor for biological interesting substances) in a biphasic buffer/SCCO(2)-system. One objective is to take advantage of the solubility differences of the substrate and the produced acid. Thus the different solubilities of the substrates and the products in the different phases were studied regarding to an overall process integration. One ester enantiomer is preferably hydrolyzed, the other remains in the supercritical phase. And the produced acid enantiomer is concentrated in the buffer phase. The decrease in pressure is followed by an extraction process of the remaining substrate-enantiomer, in consequence it will be possible to combine an enzymatic reaction with a separation step. The catalysis was optimized in regard to enantioselectivity, enantiomeric excess, conversion and reaction time. A high enantioselectivity is achieved for the aromatic substrate using the lipase of Pseudomonas cepacia. The results show that this unconventional reaction system offers tremendous advantages for enzyme process development.

Design and realization of a tailor-made enzyme to modify the molecular recognition of 2-arylpropionic esters by Candida rugosa lipase

Within a research project aimed at probing the substrate specificity and the enantioselectivity of Candida rugosa lipase (CRL), computer modeling studies of the interactions between CRL and methyl (+/-)-2-(3-benzoylphenyl)propionate (Ketoprofen methyl ester) have been carried out in order to identify which amino acids are essential to the enzyme/substrate interaction. Different binding models of the substrate enantiomers to the active site of CRL were investigated by applying a computational protocol based on molecular docking, conformational analysis, and energy minimization procedures. The structural models of the computer generated complexes between CRL and the substrates enabled us to propose that Phe344 and Phe345, in addition to the residues constituting the catalytic triad and the oxyanion hole, are the amino acids mainly involved in the enzyme-ligand interactions. To test the importance of these residues for the enzymatic activity, site-directed mutagenesis of the selected amino acids has been performed, and the mutated enzymes have been evaluated for their conversion and selectivity capabilities toward different substrates. The experimental results obtained in these biotransformation reactions indicate that Phe344 and especially Phe345 influence CRL activity, supporting the findings of our theoretical simulations.

Creation of a synthetically useful lipase with higher than wild-type enantioselectivity and maintained catalytic activity

[reaction: see text] We have found that two Geotrichum candidum lipase isozymes have remarkably different abilities to differentiate between enantiomers of ethyl 2-methyldecanoate. By rational recombination of selected portions of the two isozymes, we have created a novel lipase with an enantioselectivity superior to that of the best wild-type parent isozyme. Site-directed mutagenesis identified two key amino acid residues responsible for the improved enantioselectivity without compromised total activity of the reengineered enzyme.

Insights into the molecular basis for fatty acyl specificities of lipases from Geotrichum candidum and Candida rugosa

Despite immense progress in our comprehension of lipase structure and function during the past decade, the basis for lipase acyl specificities has remained poorly understood. This review summarizes some recent advances in the understanding at the molecular-level of substrate acyl recognition by two members in a group of large (Mw approximately 60 kDa) microbial lipases. Two aspects of acyl specificity will be focused upon. (i) The unique preference of a fungal Geotrichum candidum lipase for long-chain cis (delta-9) unsaturated fatty acid moieties in the substrate. Mutational analysis of this lipase identified residues essential for its anomalous acyl preference. This information highlighted for the first time parts in the lipase molecule involved in substrate acyl differentiation. These results are discussed in the context of the 3D-structure of a G. candidum lipase isoenzyme and structures of the related Candida rugosa lipase in complex with inhibitors. (ii) The mechanism by which the yeast C. rugosa lipase discriminates between enantiomers of a substrate with a chiral acyl moiety. Molecular modeling in combination with substrate engineering and kinetic analyses, identified two alternative substrate binding models. This allowed for the proposal of a molecular mechanism explaining how long-chain alcohols can act as enantioselective inhibitors of this enzyme. A picture is thus beginning to emerge of the interplay between lipase structure and fatty acyl specificity.

Molecular modeling of the enantioselectivity in lipase-catalyzed transesterification reactions

Two strategies based on the use of subsets for calculating the enantioselectivity in lipase-catalyzed transesterifications using the CHARMM force field were investigated. Molecular dynamics was used in our search for low energy conformations. Molecular mechanics was used for refining these low energy conformations. A tetrahedral intermediate with a rigid central part was used for mimicking the transition state. The energy differences between the transition states of the diastereomeric enzyme-substrate complexes were calculated. The way of defining the subsets was based on two fundamentally different strategies. The first strategy used predefined parts of the enzyme and the substrate as subsets. The second approach formed energy-based subsets, varying in size with the substrates studied. The selection of residues to be included in these energy-based subsets was based on the energy of the interaction between the specific residue or water molecule and the transition state. The reaction studied was the kinetic resolution of secondary alcohols in transesterifications using the Candida antarctica lipase B as chiral biocatalyst. The secondary alcohols used in the study were 2-butanol, 3-methyl-2-butanol, and 3,3-dimethyl-2-butanol.

Analysis of a lipase-catalyzed kinetic resolution by chiral normal-phase liquid chromatography

As part of our investigation of structure-activity relationships in a Candida rugosa lipase-catalyzed ester hydrolysis reaction, methyl 2-(octylsulfinyl)benzoate has been studied, with respect to rate and enantioselectivity behaviour, using two different lipase preparations. A chiral normal-phase liquid chromatography method has been developed for determination of the experimental data, degree of conversion (c) and enantiomeric excess of the substrate (ees) or the product (eep), needed for calculation of the enantioselectivity in a kinetic resolution of this kind. A recently developed new class of network-polymeric chiral stationary phases, giving baseline-resolution with high selectivities for the ester substrates as well as their corresponding carboxylic acid products, permits, without any derivatization, an accurate and direct determination of the rate of the hydrolysis reaction and of the enantioselectivity from one and the same chromatogram.

A structural basis for enantioselective inhibition of Candida rugosa lipase by long-chain aliphatic alcohols

Molecular modeling showed that the enantiomers of heptyl 2-methyldecanoate are productively bound to the active site of Candida rugosa lipase in quite different conformations. The fast-reacting S-enantiomer may well occupy the previously identified acyl-binding tunnel in the active site of the lipase. By contrast, the slow-reacting R-enantiomer must be bound to the active site, leaving the tunnel empty to allow the formation of two catalytically essential hydrogen bonds between His 449 of the catalytic triad and the transition state of the catalyzed reaction. This information enables us to propose a molecular mechanism explaining how long-chain aliphatic alcohols act as enantioselective inhibitors of this lipase in the resolution of 2-methyldecanoic acid. Long-chain aliphatic alcohols may coordinate to the acyl-binding tunnel of the C. rugosa lipase, thereby selectively inhibiting the turnover of the fast-reacting S-enantiomer, thus resulting in a lowered enantioselectivity in the resolution.

Structural determinants defining common stereoselectivity of lipases toward secondary alcohols

In this review we summarize some aspects of the enantiopreference of the lipase from Candida rugosa following structural analysis of complexes of this lipase with two enantiomers of an analog of a tetrahedral intermediate in the hydrolysis of simple esters. The analysis of the molecular basis of the enantiomeric differentiation suggests that these results can be generalized to a large class of lipases and esterases. We also summarize our experiments on identification of the key regions in the lipases from Geotrichum candidum lipase responsible for differentiation between fatty acyl chains.