Enzyme stabilization—recent experimental progress

Rahman RNZR, Leow TC, Ali MSM, Basri M, Salleh AB. Improvement of thermal stability via outer-loop ion pair interaction of mutated T1 lipase from Geobacillus zalihae strain T1

Mutant D311E and K344R were constructed using site-directed mutagenesis to introduce an additional ion pair at the inter-loop and the intra-loop, respectively, to determine the effect of ion pairs on the stability of T1 lipase isolated from Geobacillus zalihae. A series of purification steps was applied, and the pure lipases of T1, D311E and K344R were obtained. The wild-type and mutant lipases were analyzed using circular dichroism. The T_m for T1 lipase, D311E lipase and K344R lipase were approximately 68.52 °C, 70.59 °C and 68.54 °C, respectively. Mutation at D311 increases the stability of T1 lipase and exhibited higher T_m as compared to the wild-type and K344R. Based on the above, D311E lipase was chosen for further study. D311E lipase was successfully crystallized using the sitting drop vapor diffusion method. The crystal was diffracted at 2.1 Å using an in-house X-ray beam and belonged to the monoclinic space group C2 with the unit cell parameters a = 117.32 Å, b = 81.16 Å and c = 100.14 Å. Structural analysis showed the existence of an additional ion pair around E311 in the structure of D311E. The additional ion pair in D311E may regulate the stability of this mutant lipase at high temperatures as predicted in silico and spectroscopically.

Silver nanoparticle (AgNPs) doped gum acacia-gelatin-silica nanohybrid: an effective support for diastase immobilization

An effective carrier matrix for diastase alpha amylase immobilization has been fabricated by gum acacia-gelatin dual templated polymerization of tetramethoxysilane. Silver nanoparticle (AgNp) doping to this hybrid could significantly enhance the shelf life of the impregnated enzyme while retaining its full bio-catalytic activity. The doped nanohybrid has been characterized as a thermally stable porous material which also showed multipeak photoluminescence under UV excitation. The immobilized diastase alpha amylase has been used to optimize the conditions for soluble starch hydrolysis in comparison to the free enzyme. The optimum pH for both immobilized and free enzyme hydrolysis was found to be same (pH=5), indicating that the immobilization made no major change in enzyme conformation. The immobilized enzyme showed good performance in wide temperature range (from 303 to 323 K), 323 K being the optimum value. The kinetic parameters for the immobilized, (K(m)=10.30 mg/mL, V(max)=4.36 μmol mL(-1)min(-1)) and free enzyme (K(m)=8.85 mg/mL, V(max)=2.81 μmol mL(-1)min(-1)) indicated that the immobilization improved the overall stability and catalytic property of the enzyme. The immobilized enzyme remained usable for repeated cycles and did not lose its activity even after 30 days storage at 40°C, while identically synthesized and stored silver undoped hybrid lost its ~31% activity in 48 h. Present study revealed the hybrids to be potentially useful for biomedical and optical applications.

Changes in morphology and activity of transglutaminase following cross-linking and immobilization on a polypropylene microporous membrane

Transglutaminase (TGase) was cross-linked with glutaraldehyde, and cross-linked crystalline transglutaminase was immobilized on a polypropylene microporous membrane by UV-induced grafting. Immobilized enzyme activity were calculated to be 0.128 U/cm² polypropylene microporous membrane. The microstructure and enzyme characteristics of free, cross-linked and immobilized transglutaminase were compared. The optimum temperature of free transglutaminase was determined to be approximately 40 °C, while cross-linking and immobilization resulted in an increase to approximately 45 °C and 50 °C. At 60 °C, immobilized, cross-linked and free transglutaminase retained 91.7 ± 1.20%, 63.2 ± 1.05% and 37.9 ± 0.98% maximum activity, respectively. The optimum pH was unaffected by the state of transglutaminase. However, the thermal and pH stabilities of cross-linked and immobilized transglutaminase were shown to increase.

A simple colorimetric enzymatic-assay for okadaic acid detection based on the immobilization of protein phosphatase 2A in sol-gel

Okadaic acid (OA), a lipophilic toxin, is produced by Dinophysis and Prorocentrum, and causes diarrheic shellfish poisoning to humans. The mechanism of OA action is based on the reversible inhibition of protein phosphatase type 2A (PP2A) by the toxin. Therefore, this inhibition could be used to develop assay for OA detection. In this work, a colorimetric test based on the PP2A inhibition was developed for OA detection. PP2A from GTP and Millipore was immobilized on silica sol-gel, and the detection was performed. A limit of detection of 0.29 and 1.14 μg/L was respectively observed for enzyme from GTP and Millipore. The immobilization technique provided a tool to preserve the enzymatic activity, which is very unstable in solution. The PP2A immobilized sol-gel exhibited a storage stability of near 5 months, when microtiter plate with enzyme-immobilized polymer was kept at -18C°. The combination of the simplicity of the colorimetric method, along with long storage stability achieved by sol-gel immobilization, demonstrated the potentiality of this technique to be used for commercial purpose.

Increased thermostability of microbial transglutaminase by combination of several hot spots evolved by random and saturation mutagenesis

The thermostability of microbial transglutaminase (MTG) of Streptomyces mobaraensis was further improved by saturation mutagenesis and DNA-shuffling. High-throughput screening was used to identify clones with increased thermostability at 55°C. Saturation mutagenesis was performed at seven "hot spots", previously evolved by random mutagenesis. Mutations at four positions (2, 23, 269, and 294) led to higher thermostability. The variants with single amino acid exchanges comprising the highest thermostabilities were combined by DNA-shuffling. A library of 1,500 clones was screened and variants showing the highest ratio of activities after incubation for 30 min at 55°C relative to a control at 37°C were selected. 116 mutants of this library showed an increased thermostability and 2 clones per deep well plate were sequenced (35 clones). 13 clones showed only the desired sites without additional point mutations and eight variants were purified and characterized. The most thermostable mutant (triple mutant S23V-Y24N-K294L) exhibited a 12-fold higher half-life at 60°C and a 10-fold higher half-life at 50°C compared to the unmodified recombinant wild-type enzyme. From the characterization of different triple mutants differing only in one amino acid residue, it can be concluded that position 294 is especially important for thermostabilization. The simultaneous exchange of amino acids at sites 23, 24, 269 and 289 resulted in a MTG-variant with nearly twofold higher specific activity and a temperature optimum of 55°C. A triple mutant with amino acid substitutions at sites 2, 289 and 294 exhibits a temperature optimum of 60°C, which is 10°C higher than that of the wild-type enzyme.

Enhancing the functional properties of thermophilic enzymes by chemical modification and immobilization

The immobilization of proteins (mostly typically enzymes) onto solid supports is mature technology and has been used successfully to enhance biocatalytic processes in a wide range of industrial applications. However, continued developments in immobilization technology have led to more sophisticated and specialized applications of the process. A combination of targeted chemistries, for both the support and the protein, sometimes in combination with additional chemical and/or genetic engineering, has led to the development of methods for the modification of protein functional properties, for enhancing protein stability and for the recovery of specific proteins from complex mixtures. In particular, the development of effective methods for immobilizing large multi-subunit proteins with multiple covalent linkages (multi-point immobilization) has been effective in stabilizing proteins where subunit dissociation is the initial step in enzyme inactivation. In some instances, multiple benefits are achievable in a single process. Here we comprehensively review the literature pertaining to immobilization and chemical modification of different enzyme classes from thermophiles, with emphasis on the chemistries involved and their implications for modification of the enzyme functional properties. We also highlight the potential for synergies in the combined use of immobilization and other chemical modifications.

Bromo-oxidation reaction in enzyme-entrapped alginate hollow microfibers

In this article, the authors present the fabrication of an enzyme-entrapped alginate hollow fiber using a microfluidic device. Further use of enzyme-entrapped alginate hollow fibers as a biocatalytic microchemical reactor for chemical synthesis is also deliberated in this article. To ensure that there is no enzyme leaching from the fiber, fiber surfaces were coated with chitosan. To confine the mobility of reactants and products within the porous hollow fibers the entire fibers were embedded into a transparent polydimethylsiloxane (PDMS) matrix which also works as a support matrix. A vanadium-containing bromoperoxidase enzyme isolated from Corallina confusa was used as a model enzyme to demonstrate the use of these alginate hollow-fiber reactors in bromo-oxidation of phenol red to bromophenol blue at different dye flow rates. Stability of the entrapped enzyme at different temperatures and the effect of the chitosan coating on the reaction conversion were also studied. It was observed that molecules as big as 27 kDa can be retained in the matrix after coating with chitosan while molecules with molecular-weight of around 378 Da can still diffuse in and out of the matrix. The kinetic conversion rate in this microfluidic bioreactor was more than 41-fold faster when compared with the standard test-tube procedure.

Introduction to the field of enzyme immobilization and stabilization

Enzyme stabilization is important for any biomedical or industrial application of enzymes. In many applications, the goal is to provide extended active lifetime at normal environmental conditions with traditional substrates at low concentrations in buffered solutions. However, as enzymes are used for more and more applications, there is a desire to use them in extreme environmental conditions (i.e., high temperatures), in high substrate concentration, and in nontraditional solvent systems. This chapter introduces the topic of enzyme stabilization and the methods used for enzyme stabilization including enzyme immobilization.

Engineering protein stability

This article defines protein stability, emphasizes its importance and surveys some notable recent publications (2004-2008) in the field of protein stability/stabilization. Knowledge of the factors stabilizing proteins has emerged from denaturation studies and from study of thermophilic (and other extremophilic) proteins. One can enhance stability by protein engineering strategies, the judicious use of solutes and additives, immobilization, and chemical modification in solution. General protocols are set out on how to measure the kinetic thermal stability of a given protein and how to undertake chemical modification of a protein in solution.

Conjugation of laccase from the white rot fungus Trametes versicolor to chitosan and its utilization for the elimination of triclosan

A commercial laccase from Trametes versicolor was conjugated with biopolymer chitosan using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) as the cross-linking agent. Laccase-chitosan conjugation strategies were tested using different molar ratios of glucosamine monomer/protein with different molar excess ratios of EDC relative to laccase. Immobilization techniques were developed to improve the stability against thermal and chemical denaturation, storage and reusability of this biocatalyst. The conjugation resulted in a solid biocatalyst with an apparent laccase activity of ±626 U/g, 12 and 60 folds higher in the conjugation efficiency of biocatalyst relative to the immobilized and free laccase activity respectively when compared with zero EDC/laccase ratio used in conjugation solution. The conjugated laccases formed successfully eliminated the emerging pollutant triclosan (TCS) from aqueous solutions, having a higher potential to transform TCS than free laccase. UPLC-QTOF results indicate the formation of TCS oligomers. Furthermore, they are the first evidence of direct dechlorination of TCS mediated by the oxidative action of laccases.

Stabilization of dextransucrase from Leuconostoc mesenteroides NRRL B-640

Stabilization of dextransucrase from Leuconostoc mesenteroides NRRL B-640 with various stabilizers at different temperatures was studied. Dextransucrase was stable at lower temperatures (10-30°C) and lost the activity at above 30°C. The salts such as CaCl(2), CoCl(2) and MgCl(2) enhanced the dextransucrase activity. A 22% higher dextransucrase activity was obtained by 4 mM CoCl(2). The dextransucrase activity was lost by 50% at 1 mM EDTA. Urea denatured the enzyme and caused 45%, 90% and 98% loss of activity in 30 min when treated with 1 M, 3 M, and 5 M urea concentrations, respectively. Amongst the stabilizers Tween 80, glycerol, PEG-8000, dextran (500 kDa) and glutaraldehyde, Tween 80 provided the maximum stability at 30°C. In the presence of Tween 80 the enzyme lost only 8% activity at 30°C in 20 h but, it lost 65% of activity with out any stabilizer. The enzyme lost 92% of activity with in 4 days at 30°C and lost only 25% of activity at -20°C after 14 days.

Increasing the stability of an enzyme toward hostile organic solvents by directed evolution based on iterative saturation mutagenesis using the B-FIT method

Mutants of the lipase from Bacillus subtilis, previously engineered for enhanced thermostability using directed evolution based on the B-FIT method, show significantly increased tolerance to hostile organic solvents.

Effect of prolonged exposure to organic solvents on the active site environment of subtilisin Carlsberg

The potential of enzyme catalysis as a tool for organic synthesis is nowadays indisputable, as is the fact that organic solvents affect an enzyme's activity, selectivity and stability. Moreover, it was recently realized that an enzyme's initial activity is substantially decreased after prolonged exposure to organic media, an effect that further hampers their potential as catalysts for organic synthesis. Regrettably, the mechanistic reasons for these effects are still debatable. In the present study we have made an attempt to explain the reasons behind the partial loss of enzyme activity on prolonged exposure to organic solvents. Fluorescence spectroscopic studies of the serine protease subtilisin Carlsberg chemically modified with polyethylene glycol (PEG-SC) and inhibited with a Dancyl fluorophore, and dissolved in two organic solvents (acetonitrile and 1,4-dioxane) indicate that when the enzyme is initially introduced into these solvents, the active site environment is similar to that in water; however prolonged exposure to the organic medium causes this environment to resemble that of the solvent in which the enzyme is dissolved. Furthermore, kinetic studies show a reduction on both V(max) and K(M) as a result of prolonged exposure to the solvents. One interpretation of these results is that during this prolonged exposure to organic solvents the active-site fluorescent label inhibitor adopts a different binding conformation. Extrapolating this to an enzymatic reaction we argue that substrates bind in a less catalytically favorable conformation after the enzyme has been exposed to organic media for several hours.

Stabilization of invertase by molecular engineering

Extracellular invertase (EC 3.2.1.26) of Saccharomyces cerevisiae was stabilized against thermal denaturation by intermolecular and intramolecular crosslinking of the surface nucleophilic functional groups with diisocyanate homobifunctional reagents (O==C==N(CH(2))(n)N==C==O) of various lengths (n = 4, 6, 8). Crosslinking with 1,4-diisocyanatobutane (n = 4) proved most effective in enhancing thermostability. Stability was improved dramatically by crosslinking 0.5 mg/mL of protein with 30 mumol/mL of the reagent. Molecular engineering by crosslinking reduced the first-order thermal denaturation constant at 60 degrees C from 1.567 min(-1) (for the native enzyme) to 0.437 min(-1) (for the stabilized enzyme). Similarly, the best crosslinking treatment increased the activation energy for denaturation from 391 kJ mol(-1) (for the native protein) to 466 kJ mol(-1) (for the stabilized enzyme). Crosslinking was confirmed by sodium dodecyl sulfate polyacrylamide gel electrophoresis.

LysK, the enzyme lysing Staphylococcus aureus cells: specific kinetic features and approaches towards stabilization

LysK, the enzyme lysing cells of Staphylococcus aureus, can be considered as perspective antimicrobial agent. Knowledge of LysK properties and behavior would allow optimizing conditions of its storage as well as formulating strategy towards its stabilization. Reaction of LysK with substrate (suspension of autoclaved Staphylococcus aureus cells) has been found to be adequately described by the two-stage Michaelis-Menten kinetic scheme. Ionization of the enzyme and enzyme-substrate complex is important for revealing catalytic activity, which is controlled by two ionogenic groups with pK 6.0 and 9.6. Ionization energy of the group with pK 6.0 is of 30 kJ/mol, thus, pointing out on His residue; pK 9.6 might be attributed to metal ion or metal-bound water molecule. At temperatures lower than 40 degrees C, LysK stability depends on its concentration, pH and presence of low molecular weight additives. Results of electrophoresis under native and denaturing conditions as well as sedimentation analysis strongly suggest that aggregation is behind LysK inactivation. Decrease in the enzyme concentration, as well as addition of low molecular mass polyols (glycerol, sorbitol, sucrose, trehalose) and Ca(2+) cations resulted in an enhanced (more than 100 times) stability of LysK. Dramatic stability decline observed in a narrow temperature range (40-42 degrees C) was accompanied by changes in LysK secondary structure as confirmed by CD spectroscopy studies. According to computer modeling data, Cys and His residues and metal cation might play a crucial role for LysK catalytic activity. Our data on the enzyme activity in the presence of ethylenediaminetetraacetic acid and different metal cations confirmed the importance of metal cation in LysK catalysis.

Induced allostery in the directed evolution of an enantioselective Baeyer-Villiger monooxygenase

The molecular basis of allosteric effects, known to be caused by an effector docking to an enzyme at a site distal from the binding pocket, has been studied recently by applying directed evolution. Here, we utilize laboratory evolution in a different way, namely to induce allostery by introducing appropriate distal mutations that cause domain movements with concomitant reshaping of the binding pocket in the absence of an effector. To test this concept, the thermostable Baeyer-Villiger monooxygenase, phenylacetone monooxygenase (PAMO), was chosen as the enzyme to be employed in asymmetric Baeyer-Villiger reactions of substrates that are not accepted by the wild type. By using the known X-ray structure of PAMO, a decision was made regarding an appropriate site at which saturation mutagenesis is most likely to generate mutants capable of inducing allostery without any effector compound being present. After screening only 400 transformants, a double mutant was discovered that catalyzes the asymmetric oxidative kinetic resolution of a set of structurally different 2-substituted cyclohexanone derivatives as well as the desymmetrization of three different 4-substituted cyclohexanones, all with high enantioselectivity. Molecular dynamics (MD) simulations and covariance maps unveiled the origin of increased substrate scope as being due to allostery. Large domain movements occur that expose and reshape the binding pocket. This type of focused library production, aimed at inducing significant allosteric effects, is a viable alternative to traditional approaches to "designed" directed evolution that address the binding site directly.