Cross-linked crystals of chloroperoxidase

Diverse crystalline protein scaffolds through metal-dependent polymorphism

As protein crystals are increasingly finding diverse applications as scaffolds, controlled crystal polymorphism presents a facile strategy to form crystalline assemblies with controllable porosity with minimal to no protein engineering. Polymorphs of consensus tetratricopeptide repeat proteins with varying porosity were obtained through co-crystallization with metal salts, exploiting the innate metal ion geometric requirements. A single structurally exposed negative amino acid cluster was responsible for metal coordination, despite the abundance of negatively charged residues. Density functional theory calculations showed that while most of the crystals were the most thermodynamically stable assemblies, some were kinetically trapped states. Thus, crystalline porosity diversity is achieved and controlled with metal coordination, opening a new scope in the application of proteins as biocompatible protein-metal-organic frameworks (POFs). In addition, metal-dependent polymorphic crystals allow direct comparison of metal coordination preferences.

Performance of cross-linked enzyme crystals of engineered halohydrin dehalogenase HheG in different chemical reactor systems

Halohydrin dehalogenase HheG is an industrially interesting biocatalyst for the preparation of different β-substituted alcohols starting from bulky internal epoxides. We previously demonstrated that the immobilization of different HheG variants in the form of cross-linked enzyme crystals (CLECs) yielded stable and reusable enzyme immobilizes with increased resistance regarding temperature, pH, and the presence of organic solvents. Now, to further establish their preparative applicability, HheG D114C CLECs cross-linked with bis-maleimidoethane have been successfully produced on a larger scale using a stirred crystallization approach, and their application in different chemical reactor types (stirred tank reactor, fluidized bed reactor, and packed bed reactor) was systematically studied and compared for the ring opening of cyclohexene oxide with azide. This revealed the highest obtained space-time yield of 23.9 kgproduct  gCLEC -1  h-1  Lreactor volume -1 along with the highest achieved product enantiomeric excess [64%] for application in a packed-bed reactor. Additionally, lyophilization of those CLECs yielded a storage-stable HheG preparation that still retained 67% of initial activity (after lyophilization) after 6 months of storage at room temperature.

Crystal Contact Engineering for Enhanced Cross-Linking Efficiency of HheG Crystals

The generation of cross-linked enzyme crystals is a very attractive method for immobilization of enzymes displaying high crystalizability. However, the commonly used cross-linker glutaraldehyde is not always compatible with enzyme activity. Therefore, we previously reported the engineering of halohydrin dehalogenase HheG from Ilumatobacter coccineus to enable thiol-specific cross-linking during CLEC generation by insertion of cysteine residues in the crystal contact. To broaden the applicability of this approach, herein crystal contact engineering of HheG has been performed to incorporate additional lysine residues as defined cross-linking sites for CLEC generation. Using the primary amine-specific cross-linker dithiobis(succinimidyl propionate) (DSP), CLECs of HheG variant V46K were obtained that displayed a high gain in thermal stability compared to wild-type HheG, while using only a low cross-linker concentration. Moreover, respective V46K CLECs exhibited a 10 K higher reaction temperature optimum as well as significantly improved activity and stability at acidic pH and in the presence of organic co-solvents. Overall, our study demonstrates that lysine-specific cross-linkers can also be used as an alternative to glutaraldehyde for stable CLEC generation of halohydrin dehalogenases, and that cross-linking efficiency is significantly improved upon crystal contact engineering.

Immobilization of a Bienzymatic System via Crosslinking to a Metal‐Organic Framework

A leading biotechnological advancement in the field of biocatalysis is the immobilization of enzymes on solid supports to create more stable and recyclable systems. Metal-organic frameworks (MOFs) are porous materials that have been explored as solid supports for enzyme immobilization. Composed of organic linkers and inorganic nodes, MOFs feature empty void space with large surface areas and have the ability to be modified post-synthesis. Our target enzyme system for immobilization is glucose oxidase (GOx) and chloroperoxidase (CPO). Glucose oxidase catalyzes the oxidation of glucose and is used for many applications in biosensing, biofuel cells, and food production. Chloroperoxidase is a fungal heme enzyme that catalyzes peroxide-dependent halogenation, oxidation, and hydroxylation. These two enzymes work sequentially in this enzyme system by GOx producing peroxide, which activates CPO that reacts with a suitable substrate. This study focuses on using a zirconium-based MOF, UiO-66-NH2, to immobilize the enzyme system via crosslinking with the MOF’s amine group on the surface of the MOF. This study investigates two different crosslinkers: disuccinimidyl glutarate (DSG) and 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC)/N-hydroxysuccinidimide (NHS), providing stable crosslinking of the MOF to the enzymes. The two crosslinkers are used to covalently bond CPO and GOx onto UiO-66-NH2, and a comparison of the recyclability and enzymatic activity of the single immobilization of CPO and the doubly immobilized CPO and GOx is discussed through assays and characterization analyses. The DSG-crosslinked composites displayed enhanced activity relative to the free enzyme, and all crosslinked enzyme/MOF composites demonstrated recyclability, with at least 30% of the activity being retained after four catalytic cycles. The results of this report will aid researchers in utilizing CPO as a biocatalyst that is more active and has greater recyclability.

Creation of Cross-Linked Crystals With Intermolecular Disulfide Bonds Connecting Symmetry-Related Molecules Allows Retention of Tertiary Structure in Different Solvent Conditions

Protein crystals are generally fragile and sensitive to subtle changes such as pH, ionic strength, and/or temperature in their crystallization mother liquor. Here, using T4 phage lysozyme as a model protein, the three-dimensional rigidification of protein crystals was conducted by introducing disulfide cross-links between neighboring molecules in the crystal. The effect of cross-linking on the stability of the crystals was evaluated by microscopic observation and X-ray diffraction. When soaking the obtained cross-linked crystals into a precipitant-free solution, the crystals held their shape without dissolution and diffracted to approximately 1.1 Å resolution, comparable to that of the non-cross-linked crystals. Such cross-linked crystals maintained their diffraction even when immersed in other solutions with pH values from 4 to 10, indicating that the disulfide cross-linking made the packing contacts enforced and resulted in some mechanical strength in response to changes in the preservation conditions. Furthermore, the cross-linked crystals gained stability to permit soaking into solutions containing high concentrations of organic solvents. The results suggest the possibility of obtaining protein crystals for effective drug screening by introducing appropriate cross-linked disulfide bonds.

Magnetic thermosensitive polymer composite carrier with target spacing for enhancing immobilized enzyme performance

A novel magnetic thermosensitive polymer composite carrier with target spacing was developed. In this strategy, thermosensitive polymer grafted on magnetic Fe₃O₄ for enhancing immobilized penicillin G acylase (PGA) performance and introduce immobilized target spacing into magnetic carriers for the first time. Fe₃O₄ nanoparticles were synthesized by a reverse microemulsion method. The modifier used was the silane coupling agent γ-methylacryloxypropyl trimethoxysilane (KH570) and then reacting with a reversible-adaptive fragmentation chain transfer (RAFT) reagent, 2-cyano-2-propyldodecyl trithiocarbonate (CPDTC). The thermo-sensitive nanoparticle-composite carrier of Fe₃O₄-grafted-poly N, N-diethyl acrylamide-block-poly β-Hydroxyethyl methacrylate-block-random copolymer of glycidyl methacrylate and methyl methacrylate (Fe₃O₄-g-PDEA-b-PHEMA-b-P(MMA-co-GMA)) were synthesized by RAFT polymerization technique that used N, N-diethyl acrylamide (DEA), β-Hydroxyethyl methacrylate (HEMA), Glycidyl methacrylate (GMA) and Methyl methacrylate (MMA) as monomer, then which were employed as functional carriers for the immobilization of PGA. Within the carrier, the epoxy group of GMA segment was a target immobilization site for PGA and the introduction of MMA reflected the target space of immobilized PGA to improve catalytic activity and catalytic activity recovery rate of the immobilized PGA. Characterizations demonstrated that the triblock copolymers grafted Fe₃O₄ nanoparticles were successfully fabricated by the structure design. Besides, under these circumstances the enzyme activity (EA), enzyme loading capacity (ELC) and catalytic activity recovery ration (CAR) reached 31235 U/g, 128.39 mg/g and 93.32 %, respectively. The catalytic activity of immobilized PGA maintained 87.4 % of initial value and the recovery ratio (R) of immobilized PGA reached 96.22 % after recycling 12 times. Furthermore, the immobilized PGA exhibited advantages of low temperature homogeneous catalysis and magnetic separation, which indicated broad application prospects in the biocatalysts' field.

Production of Cross-Linked Lipase Crystals at a Preparative Scale

The autoimmobilization of enzymes via cross-linked enzyme crystals (CLECs) has regained interest in recent years, boosted by the extensive knowledge gained in protein crystallization, the decrease of cost and laboriousness of the process, and the development of potential applications. In this work, we present the crystallization and preparative-scale production of reinforced cross-linked lipase crystals (RCLLCs) using a commercial detergent additive as a raw material. Bulk crystallization was carried out in 500 mL of agarose media using the batch technique. Agarose facilitates the homogeneous production of crystals, their cross-linking treatment, and their extraction. RCLLCs were active in an aqueous solution and in hexane, as shown by the hydrolysis of p-nitrophenol butyrate and α-methylbenzyl acetate, respectively. RCLLCs presented both high thermal and robust operational stability, allowing the preparation of a packed-bed chromatographic column to work in a continuous flow. Finally, we determined the three-dimensional (3D) models of this commercial lipase crystallized with and without phosphate at 2.0 and 1.7 Å resolutions, respectively.

Cross-Linked Artificial Enzyme Crystals as Heterogeneous Catalysts for Oxidation Reactions

Designing systems that merge the advantages of heterogeneous catalysis, enzymology, and molecular catalysis represents the next major goal for sustainable chemistry. Cross-linked enzyme crystals display most of these essential assets (well-designed mesoporous support, protein selectivity, and molecular recognition of substrates). Nevertheless, a lack of reaction diversity, particularly in the field of oxidation, remains a constraint for their increased use in the field. Here, thanks to the design of cross-linked artificial nonheme iron oxygenase crystals, we filled this gap by developing biobased heterogeneous catalysts capable of oxidizing carbon-carbon double bonds. First, reductive O₂ activation induces selective oxidative cleavage, revealing the indestructible character of the solid catalyst (at least 30 000 turnover numbers without any loss of activity). Second, the use of 2-electron oxidants allows selective and high-efficiency hydroxychlorination with thousands of turnover numbers. This new technology by far outperforms catalysis using the inorganic complexes alone, or even the artificial enzymes in solution. The combination of easy catalyst synthesis, the improvement of "omic" technologies, and automation of protein crystallization makes this strategy a real opportunity for the future of (bio)catalysis.

Improved Biodegradation of Synthetic Azo Dye by Anionic Cross-Linking of Chloroperoxidase on ZnO/SiO2 Nanocomposite Support

A novel ZnO nanowire/macroporous SiO₂ composite was used as a support to immobilize chloroperoxidase (CPO) by in situ cross-linking method. An anionic bi-epoxy compound was synthesized and used as a long-chained anionic cross-linker, and it was adsorbed on the surface of ZnO nanowires through static interaction before reaction with CPO, creating a new approach to change the structure, property, and catalytic performance of the produced cross-linking enzyme aggregates (CLEAs) of CPO. The immobilized CPO showed high activity in the decolorization of three azo dyes. The effect of various conditions such as the loading amount of CPO, solution pH, temperature, and dye concentration was optimized on the decolorization. Under optimized conditions, the decolorization percentage of Acid Blue 113, Direct Black 38, and Acid Black 10 BX reached as high as 95.4, 92.3, and 89.1%, respectively. The immobilized CPO exhibited much better thermostability and resistance to pH inactivation than free CPO. The storage stability and reusability were greatly improved through the immobilization. It was found from the decolorization of Acid Blue 113 that 83.6% of initial activity retained after incubation at 4 °C for 60 days and that 80.9% of decolorization efficiency retained after 12 cycles of reuses.

Stabilization of porcine pancreatic elastase crystals by glutaraldehyde cross-linking

Elastase is a serine protease from the chymotrypsin family of enzymes with the ability to degrade elastin, an important component of connective tissues. Excessive elastin proteolysis leads to a number of pathological diseases. Porcine pancreatic elastase (PPE) is often used for drug development as a model for human leukocyte elastase (HLE), with which it shares high sequence identity. Crystals of PPE were grown overnight using sodium sulfate and sodium acetate at acidic pH. Cross-linking the crystals with glutaraldehyde was needed to resist the soaking procedure with a diethyl N-(methyl)pyridinyl-substituted oxo-β-lactam inhibitor. Crystals of PPE bound to the inhibitor belonged to the orthorhombic space group P2₁2₁2₁, with unit-cell parameters a = 51.0, b = 58.3, c = 74.9 Å, and diffracted to 1.8 Å resolution using an in-house X-ray source.

Cross-linked protein crystals by glutaraldehyde and their applications

The mechanism of cross-linked protein crystals using glutaraldehyde, and their properties and applications are discussed in detail.

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.

Formation of cross-linked chloroperoxidase aggregates in the pores of mesocellular foams: characterization by SANS and catalytic properties

No escape: The formation of cross-linked chloroperoxidase aggregates (CPO-CLEAs) in the pores of mesocellular foam materials results in active biocatalysts that are more resistant to leaching than the conventional catalyst prepared by physisorption of chloroperoxidase. Small-angle neutron scattering (SANS) experiments clearly confirm that the CPO-CLEAs are located in the pores of the mesocellular foams.

Techno-economic analysis of biocatalytic processes for production of alkene epoxides

A techno-economic analysis of two different bioprocesses was conducted, one for the conversion of propylene to propylene oxide (PO) and other for conversion of styrene to styrene epoxide (SO). The first process was a lipase-mediated chemo-enzymatic reaction, whereas the second one was a one-step enzymatic process using chloroperoxidase. The PO produced through the chemo-enzymatic process is a racemic product, whereas the latter process (based on chloroperoxidase) produces an enantio-pure product. The former process thus falls under the category of high-volume commodity chemical (PO); whereas the latter is a low-volume, high-value product (SO).A simulation of the process was conducted using the bioprocess engineering software SuperPro Designer v6.0 (Intelligen, Inc., Scotch Plains, NJ) to determine the economic feasibility of the process. The purpose of the exercise was to compare biocatalytic processes with existing chemical processes for production of alkene expoxides. The results show that further improvements are needed in improving biocatalyst stability to make these bioprocesses competitive with chemical processes.

Encapsulation of crosslinked subtilisin microcrystals in hydrogel beads for controlled release applications

Enzymes are less stable in harsh conditions and hence to overcome this nature, several methodologies are being developed. It was found that crosslinked enzyme crystals are the most promising strategy for the stabilization of the enzymes [Emilia Abraham, T., Jegan Roy, J., Bindhu, L.V., Jayakumar, K.K., 2004. Crosslinked enzyme crystals of glucoamylase as a potent catalyst for biotransformations. Carbohydr. Res. 339, 1099-1104; Navia, M., St. Clair, N., 1997. Crosslinked enzyme crystals. Biosens. Bioelectron. 12, 7]. A cost effective methodology of crystallization of protease (Bacillus subtilis) with ammonium sulphate (65%, w/v) and then crosslinking the crystals with glutaraldehyde (4%, v/v) in isopropanol for 20min gave a stable and active enzyme. SEM studies showed that the protease is in small cubic shaped crystals of 1-2 microm size. Crosslinked enzyme crystal (CLEC) of protease has good stability in polar and nonpolar organic solvents, such as hexane, toluene, benzene and carbon tetrachloride and it had high thermal stability up to 60 degrees C and hence can be used as a catalyst for the biotransformation of compounds which are not soluble in aqueous medium. The CLECs were entrapped in the alginate:guar gum (3:1) composite beads which were resistant to low pH conditions in the stomach and hence was found to be useful for the oral drug delivery. This method can be used to deliver the protein and peptide drugs which require high concentrations at the delivery stage, and which normally degrades in the stomach before reaching the jejunum. Application of these pH-sensitive beads for the controlled release of subtilisin in in vitro was studied and found to be a viable strategy.

Studies on crystallization and cross-linking of lipase for biocatalysis

The development of robust biocatalysts with increased stability and activity is a major challenge to industry. A major breakthrough in this field was the development of cross-linked enzyme crystals with high specificity and stability. A method is described to produce micro crystals of CLEC lipase, which is thermostable and solvent stable. Lipase from Burkholderia cepacia was crystallized using ammonium sulfate and cross-linked with glutaraldehyde to produce catalytically active enzyme. The maximum yield of CLEC was obtained with 70% ammonium sulfate and cross-linked with 5% (v/v) glutaraldehyde. SEM studies showed small hexagonal-shaped crystals of 2-5 microm size. CLEC lipase had improved thermal and reuse stability. It is versatile, having good activity in both polar and nonpolar organic solvents. CLEC lipase was coated using beta cyclodextrin for improving the storage and reuse stability. CLEC was successfully used for esterification of Ibuprofen and synthesis of ethyl butyrate.

New reaction system for hydrocarbon oxidation by chloroperoxidase

A novel reaction system was developed to maximize the catalytic efficiency of chloroperoxidase (CPO, from Caldariomyces fumago) toward the oxidation of hydrocarbons. The reaction system consisted of an organic/aqueous emulsion comprising pure substrate and aqueous buffer supplemented with the surfactant dioctyl sulfosuccinate. The emulsion system attenuated not only the destabilizing effects of the substrate and product on the enzyme by emulsifying the compounds, but also oxidant toxicity (oxidative stress) by increasing substrate availability. As a result, CPO exhibited total turnover numbers (TTNs, defined as the amount of product produced over the catalytic lifetime of the enzyme) of ca. 20,000 mol product/mol enzyme for the oxidation of styrene, toluene, and o-, m-, p-xylenes. The TTNs are over 10-fold higher than those previously reported for the oxidation of benzylic hydrocarbons by CPO. This study represents a significant step toward the development of CPO as a practical catalyst for large-scale organic syntheses.

Novel interface-binding chloroperoxidase for interfacial epoxidation of styrene

A unique interface-binding chloroperoxidase (CPO) was developed and examined for interfacial biocatalysis. Native CPO was conjugated with polystyrene (PS) to form a surfactant-like structure that self assembled at oil-water interfaces. While enantioselectivity of the enzyme was maintained, the interfacial assembly of the enzyme improved its overall catalytic efficiency as compared to that observed with the enzyme contained in the bulk aqueous phase. The PS conjugated CPO (PS-CPO) showed a 2.5-fold enhancement of enzyme productivity versus native CPO under batch reaction conditions for the epoxidation of styrene, whereas a 25-fold improvement was realized in a continuous feeding reaction to reach a productivity of 10 micromol h-1 mg protein-1. The interface-binding enzyme also demonstrated several other advantages such as suppressing unwanted side reactions including the hydrolysis of styrene epoxide products, stabilizing the enzyme by limiting its exposure to both the oxidant H2O2 and epoxide products, and alleviating the deactivating effect of interfacial stress on enzymes by functioning as a surfactant.

Preparation of protein nanocrystals and their characterization by solid state NMR

Preparation of proteins in their crystalline state has been found to be important in producing stable therapeutic protein formulations, cross-linked enzyme crystals for application in industrial processes, generating novel porous media for separations, and of course in structure elucidation. Of these applications only X-ray crystallography requires large crystals, defined here as being crystals 100s of microns or greater in size. Smaller crystals have attractive attributes in many instances, and are just as useful in structure determination by solid state NMR (ssNMR) as are large crystals. In this paper we outline a simple set of procedures for preparing nanocrystalline protein samples for ssNMR or other applications and describe the characterization of their crystallinity by ssNMR and X-ray powder diffraction. The approach is demonstrated in application to five different proteins: ubiquitin, lysozyme, ribonuclease A, streptavidin, and cytochrome c. In all instances the nanocrystals produced are found to be highly crystalline as judged by natural abundance 13C ssNMR and optical and electron microscopy. We show for ubiquitin that nanocrystals prepared by rapid batch crystallization yield equivalent 13C ssNMR spectra to those of larger X-ray diffraction quality crystals. Single crystal and powder X-ray diffraction measurements are made to compare the degree of order present in polycrystalline, nanocrystalline, and lyophilized ubiquitin. Solid state 13C NMR is also used to show that ubiquitin nanocrystals are thermally robust, giving no indication of loss of local order after repeated temperature cycling between liquid nitrogen and room temperature. The methods developed are rapid and should scale well from the tenths of milligram to multi-gram scales, and as such should find wide utility in the preparation of protein nanocrystals for applications in catalysis, separations, and especially in sample preparation for structural studies using ssNMR.