Developments and trends in enzyme catalysis in nonconventional media

Recent Advances in Applications of Oxidases and Peroxidases Polymer-Based Enzyme Biocatalysts in Sensing and Wastewater Treatment: A Review

Oxidase and peroxidase enzymes have attracted attention in various biotechnological industries due to their ease of synthesis, wide range of applications, and operation under mild conditions. Their applicability, however, is limited by their poor stability in harsher conditions and their non-reusability. As a result, several approaches such as enzyme engineering, medium engineering, and enzyme immobilization have been used to improve the enzyme properties. Several materials have been used as supports for these enzymes to increase their stability and reusability. This review focusses on the immobilization of oxidase and peroxidase enzymes on metal and metal oxide nanoparticle-polymer composite supports and the different methods used to achieve the immobilization. The application of the enzyme-metal/metal oxide-polymer biocatalysts in biosensing of hydrogen peroxide, glucose, pesticides, and herbicides as well as blood components such as cholesterol, urea, dopamine, and xanthine have been extensively reviewed. The application of the biocatalysts in wastewater treatment through degradation of dyes, pesticides, and other organic compounds has also been discussed.

Examining polymer-protein biophysical interactions with small-angle x-ray scattering and quartz crystal microbalance with dissipation

Polymer-protein hybrids can be deployed to improve protein solubility and stability in denaturing environments. While previous work used robotics and active machine learning to inform new designs, further biophysical information is required to ascertain structure-function behavior. Here, we show the value of tandem small-angle x-ray scattering (SAXS) and quartz crystal microbalance with dissipation (QCMD) experiments to reveal detailed polymer-protein interactions with horseradish peroxidase (HRP) as a test case. Of particular interest was the process of polymer-protein complex formation under thermal stress whereby SAXS monitors formation in solution while QCMD follows these dynamics at an interface. The radius of gyration (Rg ) of the protein as measured by SAXS does not change significantly in the presence of polymer under denaturing conditions, but thickness and dissipation changes were observed in QCMD data. SAXS data with and without thermal stress were utilized to create bead models of the potential complexes and denatured enzyme, and each model fit provided insight into the degree of interactions. Additionally, QCMD data demonstrated that HRP deforms by spreading upon surface adsorption at low concentration as shown by longer adsorption times and smaller frequency shifts. In contrast, thermally stressed and highly inactive HRP had faster adsorption kinetics. The combination of SAXS and QCMD serves as a framework for biophysical characterization of interactions between proteins and polymers which could be useful in designing polymer-protein hybrids.

Biotechnological potential of psychrophilic microorganisms as the source of cold-active enzymes in food processing applications

Microorganisms striving in extreme environments and exhibiting optimal growth and reproduction at low temperatures, otherwise known as psychrophilic microorganisms, are potential sources of cold-active enzymes. Owing to higher stability and cold activity, these enzymes are gaining enormous attention in numerous industrial bioprocesses. Applications of several cold-active enzymes have been established in the food industry, e.g., β-galactosidase, pectinase, proteases, amylases, xylanases, pullulanases, lipases, and β-mannanases. The enzyme engineering approaches and the accumulating knowledge of protein structure and function have made it possible to improve the catalytic properties of interest and express the candidate enzyme in a heterologous host for a higher level of enzyme production. This review compiles the relevant and recent information on the potential uses of different cold-active enzymes in the food industry.

Improving the catalytic efficiency and substrate affinity of a novel esterase from marine Klebsiella aerogenes by random and site-directed mutation

A novel esterase (EstKa) from marine Klebsiella aerogenes was characterized with hydrolytic activity against p-nitrophenyl caprylate (pNPC, C₈) under optimum conditions (50 °C and pH 8.5). After two rounds of mutagenesis, two highly potential mutants (I6E9 and L7B11) were obtained with prominent activity, substrate affinity and thermostability. I6E9 (L90Q/P96T) and L7B11 (A37S/Q100L/S133G/R138C/Q156R) were 1.56- and 1.65-fold higher than EstKa in relative catalytic efficiency. The influence of each amino acid on enzyme activity was explored by site-directed mutation. The mutants Pro96Thr and Gln156Arg showed 1.29- and 1.48-fold increase in catalytic efficiency (Kcat/Km) and 54.4 and 36.2% decrease in substrate affinity (Km), respectively. The compound mutant Pro96Thr/Gln156Arg exhibited 68.9% decrease in Km and 1.41-fold increase in Kcat/Km relative to EstKa. Homology model structure analysis revealed that the replacement of Gln by hydrophilic Arg on the esterase surface improved the microenvironment stability and the activity. The replacement of Pro by Thr enabled the esterase enzyme to retain 90% relative activity after 3 h incubation at 45 °C. Structural analysis confirmed that the formation of a hydrogen bond leads to a notable increase of catalytic efficiency under high temperature conditions.

Cloning, heterologous expression, and characterization of a novel thioesterase from natural sample

A novel thioesterse gene was successfully cloned and sequenced directly from natural sample of Domas Hot Spring, West Java, Indonesia. Homological analysis of the sequence showed that the gene appeared high homology to thioesterase genes with the highest to a putative thioesterase gene from uncultured Acidilobus sp. JCHS at 66% identity. However, phylogenetic analysis showed that the protein was separated from the branch with other known thioesterases. The size of the gene is around 500 base pairs, lied into 2 kb DNA fragment from a random PCR amplicon. The gene was overexpressed in Escherichia coli, a dominant band appeared at 17 kDa in SDS-PAGE with expression level at around 32% of total proteins. The activity of the purified protein using acetyl-CoA as substrate showed that the protein exhibited thioesterase activity. Furthermore, the enzyme also showed esterase activity on p-nitrophenyl ester as substrate. Detail characterization of esterolytic activity showed that the enzyme preferred p-nitrophenyl decanoate as substrate. The optimum activity of the enzyme was at 80 °C and pH 8. Activity of the enzyme was maintained after incubation at 80 °C up to 24 h. In addition, the enzyme was favorable on polar organic solvents. All the data obtained suggested that the enzyme is a novel alkaline thermostable thioesterase.

Non-aqueous green solvents improve alpha-amylase induced fiber opening in leather processing

Severe water deficit and highly polluting effluent generation from leather industries have constantly been pressurizing the tanners to adopt cleaner leather processing systems. The present study aims to minimize the use of water by substituting it with non-aqueous green solvents and also to enhance the enzyme action in alpha-amylase based fiber opening process. The activity of alpha-amylase in select non-aqueous green solvents namely, heptane, polyethylene glycol 200 and propylene glycol is considerably higher by 62, 38 and 31% than in water, respectively. Comparable results are obtained for the catalytic efficiency of alpha-amylase and hence it is further validated in collagen fiber opening trials as well. Scanning electron micrographs, histological images and proteoglycan estimation supported the above findings at 1% alpha-amylase dosage. The final quality of the experimental leathers in terms of physical and bulk properties is comparable to that of control leathers. Recycling studies indicate that it is possible to replace water with green solvents for enzymatic fiber opening with the feasibility to recover more than 85% solvent-enzyme mixture and reuse without any additional alpha-amylase usage. Reduction in pollution load coupled with the efficient catalytic action of enzyme in non-aqueous media favors the present protocol for industrial applications.

Metagenomics: Is it a powerful tool to obtain lipases for application in biocatalysis?

In recent years, metagenomic strategies have been widely used to isolate and identify new enzymes from uncultivable components of microbial communities. Among these enzymes, various lipases have been obtained from metagenomic libraries from different environments and characterized. Although many of these lipases have characteristics that could make them interesting for application in biocatalysis, relatively little work has been done to evaluate their potential to catalyze industrially important reactions. In the present article, we highlight the latest research on lipases obtained through metagenomic tools, focusing on studies of activity and stability and investigations of application in biocatalysis. We also discuss the challenges of metagenomic approaches for the bioprospecting of new lipases.

Elastin-like Polypeptide (ELP) Charge Influences Self-Assembly of ELP-mCherry Fusion Proteins

Self-assembly of protein-polymer bioconjugates presents an elegant strategy for controlling nanostructure and orientation of globular proteins in functional materials. Recent work has shown that genetic fusion of globular protein mCherry to an elastin-like polypeptide (ELP) yields similar self-assembly behavior to these protein-polymer bioconjugates. In the context of studying protein-polymer bioconjugate self-assembly, the mutability of the ELP sequence allows several different properties of the ELP block to be tuned orthogonally while maintaining consistent polypeptide backbone chemistry. This work uses this ELP sequence tunability in combination with the precise control offered by genetic engineering of an amino acid sequence to generate a library of four novel ELP sequences that are used to study the combined effect of charge and hydrophobicity on ELP-mCherry fusion protein self-assembly. Concentrated solution self-assembly is studied by small-angle X-ray scattering (SAXS) and depolarized light scattering (DPLS). These experiments show that fusions containing a negatively charged ELP block do not assemble at all, and fusions with a charge balanced ELP block exhibit a weak propensity for assembly. By comparison, the fusion containing an uncharged ELP block starts to order at 40 wt % in solution and at all concentrations measured has sharper, more intense SAXS peaks than other fusion proteins. These experiments show that charge character of the ELP block is a stronger predictor of self-assembly behavior than the hydrophobicity of the ELP block. Dilute solution small-angle neutron scattering (SANS) on the ELPs alone suggests that all ELPs used in this study (including the uncharged ELP) adopt dilute solution conformations similar to those of traditional polymers, including polyampholytes and polyelectrolytes. Finally, dynamic light scattering studies on ELP-mCherry blends shows that there is no significant complexation between the charged ELPs and mCherry. Therefore, it is proposed that the superior self-assembly of fusion proteins containing uncharged ELP block is due to effective repulsions between charged and uncharged blocks due to local charge correlation effects and, in the case of anionic ELPs, repulsion between like charges within the ELP block.

Enzymatic reactions involving the heteroatoms from organic substrates

Several enzymatic reactions of heteroatom-containing compounds have been explored as unnatural substrates. Considerable advances related to the search for efficient enzymatic systems able to support a broader substrate scope with high catalytic performance are described in the literature. These reports include mainly native and mutated enzymes and whole cells biocatalysis. Herein, we describe the historical background along with the progress of biocatalyzed reactions involving the heteroatom(S, Se, B, P and Si) from hetero-organic substrates.

Unraveling the effects of amino acid substitutions enhancing lipase resistance to an ionic liquid: a molecular dynamics study

The key properties affecting lipase resistance towards an ionic liquid are uncovered through a molecular dynamics study.

An Overview on the Enhancement of Enantioselectivity and Stability of Microbial Epoxide Hydrolases

Epoxide hydrolases (EHs; 3.3.2.x) catalyze the enantioselective ring opening of racemic epoxides to the corresponding enantiopure vicinal diols and remaining equivalent unreacted epoxides. These epoxides and diols are used for the synthesis of chiral drug intermediates. With an upsurge in the methods for identification of novel microbial EHs, a lot of EHs have been discovered and utilized for kinetic resolution of racemic epoxides. However, there is still a constraint on the account of limited EHs being successfully applied on the preparative scale for industrial biotransformations. This limitation has to be overcome before application of identified functional EHs on large scale. Many strategies such as optimizing reaction media, immobilizing EHs and laboratory-scale directed evolution of EHs have been adopted for enhancing the industrial potential of EHs. In this review, these approaches have been highlighted which can serve as a pathway for the enrichment of already identified EHs for their application on an industrial scale in future studies.

Mechanism-based inhibition of an aldolase at high concentrations of its natural substrate acetaldehyde: structural insights and protective strategies

2-Deoxy-d-ribose-5-phosphate aldolase (DERA) is used in organic synthesis for the enantioselective reaction between acetaldehyde and a broad range of other aldehydes as acceptor molecules. Nevertheless, its application is hampered by a poor tolerance towards high concentrations of acetaldehyde, its natural substrate. While numerous studies have been performed searching for new, more acetaldehyde-resistant DERAs, the mechanism underlying this deactivation process has remained elusive. By using NMR spectroscopy on both the protein and the small-molecule scale, we could show that a reaction product binds to the inner part of the enzyme, and that this effect can be partly reversed via heating. The crystal structure of DERA before and after acetaldehyde incubation was determined at high resolution, revealing a covalently bound reaction product bridging the catalytically active lysine (K167) to a nearby cysteine (C47) in the deactivated enzyme. A reaction mechanism is proposed where crotonaldehyde as the aldol product of two acetaldehyde molecules after water elimination forms a Schiff base with the lysine side chain, followed by Michael addition of the cysteine thiol group to the Cβ atom of the inhibitor. In support of this mechanism, direct incubation of DERA with crotonaldehyde results in a more than 100-fold stronger inhibition, compared to acetaldehyde, whereas mutation of C47 gives rise to a fully acetaldehyde-resistant DERA. Thus this variant appears perfectly suited for synthetic applications. A similar diagnostic and preventive strategy should be applicable to other biocatalysts suffering from mechanism-based inhibition by a reactive substrate, a condition that may be more common than currently appreciated in biotechnology.

Engineered catalytic biofilms: Site-specific enzyme immobilization onto E. coli curli nanofibers

Biocatalytic transformations generally rely on purified enzymes or whole cells to perform complex transformations that are used on industrial scale for chemical, drug, and biofuel synthesis, pesticide decontamination, and water purification. However, both of these systems have inherent disadvantages related to the costs associated with enzyme purification, the long-term stability of immobilized enzymes, catalyst recovery, and compatibility with harsh reaction conditions. We developed a novel strategy for producing rationally designed biocatalytic surfaces based on Biofilm Integrated Nanofiber Display (BIND), which exploits the curli system of E. coli to create a functional nanofiber network capable of covalent immobilization of enzymes. This approach is attractive because it is scalable, represents a modular strategy for site-specific enzyme immobilization, and has the potential to stabilize enzymes under denaturing environmental conditions. We site-specifically immobilized a recombinant α-amylase, fused to the SpyCatcher attachment domain, onto E. coli curli fibers displaying complementary SpyTag capture domains. We characterized the effectiveness of this immobilization technique on the biofilms and tested the stability of immobilized α-amylase in unfavorable conditions. This enzyme-modified biofilm maintained its activity when exposed to a wide range of pH and organic solvent conditions. In contrast to other biofilm-based catalysts, which rely on high cellular metabolism, the modified curli-based biofilm remained active even after cell death due to organic solvent exposure. This work lays the foundation for a new and versatile method of using the extracellular polymeric matrix of E. coli for creating novel biocatalytic surfaces.

Kinetics of hydrogen peroxide decomposition by catalase: hydroxylic solvent effects

The effect of water-alcohol (methanol, ethanol, propan-1-ol, propan-2-ol, ethane-1,2-diol and propane-1,2,3-triol) binary mixtures on the kinetics of hydrogen peroxide decomposition in the presence of bovine liver catalase is investigated. In all solvents, the activity of catalase is smaller than in water. The results are discussed on the basis of a simple kinetic model. The kinetic constants for product formation through enzyme-substrate complex decomposition and for inactivation of catalase are estimated. The organic solvents are characterized by several physical properties: dielectric constant (D), hydrophobicity (log P), concentration of hydroxyl groups ([OH]), polarizability (α), Kamlet-Taft parameter (β) and Kosower parameter (Z). The relationships between the initial rate, kinetic constants and medium properties are analyzed by linear and multiple linear regression.

Lipase-coated K2SO4 micro-crystals: preparation, characterization, and application in biodiesel production using various oil feedstocks

This study investigated the preparation and characteristics of protein-coated microcrystals (PCMCs) from Pseudomonas cepacia lipase (PS) and K(2)SO(4), and their application in biodiesel synthesis, via single factorial experiments and response surface methodology (RSM), the optimized PCMC-PS exhibited high activity and stability; the optimal temperature was 60°C (which gave 99.83% conversion), although fairly high activity was exhibited after incubation at different temperatures (25-70°C). The organic solvents stability of the PCMC-PS was improved, and it significantly reduced ethanol toxicity. Circular dichroism (CD) analysis revealed the correlation between the conformation and the enzyme activity. The morphology of the PCMC-PS was also confirmed via scanning electron microscopy (SEM). When catalyzed by PCMC-PS, above 83% biodiesel yield was obtained for most of the seven oils tested. The PCMC-PS (washed with hexane) activity remained relatively stable after eight batch reactions, with only a 15.73% reduction in the conversion (from 99.02% to 83.29%).

Enzyme stabilization and immobilization by sol-gel entrapment

While biocatalysts show tremendous potential for the industrial production of fine chemicals, their integration into large-scale processes has been slow. One of the main reasons for slow acceptance in industry is the inherent instability of the enzymes. Recent developments in bioengineering have shed some light on methods of improving enzyme stability. One method that has been used for many decades, successfully to varying degrees, has been the immobilization of enzymes. To this regards, silica gels have attracted much attention because of the ease of surface functionalization, high surface areas, mechanical and thermal stability, and resistance to both chemical and biological attack. We have previously shown the immobilization of invertase on silica gels with high immobilized activity and significantly improved stability. Here, we provide greater details on the methods for effecting the immobilization.

Molten globule-triggered inactivation of a thermostable and solvent stable lipase in hydrophilic solvents

The use of lipase in hydrophilic solvent is usually hampered by inactivation. The solvent stability of a recombinant solvent stable lipase isolated from thermostable Bacillus sp. strain 42 (Lip 42), in DMSO and methanol were studied at different solvent-water compositions. The enzymatic activities were retained in up to 45% v/v solvent compositions. The near-UV CD spectra indicated that tertiary structures were perturbed at 60% v/v and above. Far-UV CD in methanol indicated the secondary structure in Lip 42 was retained throughout all solvent compositions. Fluorescence studies indicated formations of molten globules in solvent compositions of 60% v/v and above. The enzyme was able to retain its secondary structures in the presence of methanol; however, there was a general reduction in beta-sheet and an increase in alpha-helix contents. The H-bonding arrangements triggered in methanol and DMSO, respectively, caused different forms of tertiary structure perturbations on Lip 42, despite both showing partial denaturation with molten globule formations.