Chloroperoxidase production by Caldariomyces fumago biofilms

Chloroperoxidase (CPO) is a versatile enzyme, which is secreted by the marine fungus Caldariomyces fumago (Leptoxyphium fumago). However, the application of the enzyme is hampered by its high price, which is due to the costly, labor‐intensive purification process. One challenge of the downstream process is the removal of a coproduced black pigment that forms a complex with the active enzyme. While strain development can be considered as an option to reduce the synthesis of the interfering pigment, the metabolism of the microorganism can be altered alternatively by using the biofilm growth mode of the fungus. The aim of this study was to reduce pigment formation during CPO synthesis. We investigated for the first time CPO production during C. fumago biofilm growth initiated through the presence of different microstructured stainless steel surfaces (material number: 1.4571; AISI 316Ti). CPO production by C. fumago was similar when grown as a biofilm or in suspension, whereas pigment formation was drastically reduced by cells grown on moderately structured surfaces (R_a = 0.13 ± 0.02 μm). The possibilities of biofilm growth for changing cell properties and for continuous fermentation are discussed.     

Immobilization of chloroperoxidase on silica-based materials for 4,6-dimethyl dibenzothiophene oxidation

Silica-based materials have been used as effective supports for the immobilization of enzymes. Moreover, the understanding on the oxidation of sulfur compounds by immobilized chloroperoxidase represents a step further in the development of a biocatalytic desulfurization process of fossil fuels. Here, chloroperoxidase from Caldariomyces fumago was immobilized on amorphous and structured silica-based materials either physically or covalently using an organosilane derivative for the oxidation of a recalcitrant organosulfur compound currently found in gas oil and diesel, such as 4,6-dimethyldibenzothiophene (4,6-DMDBT). Such materials were characterized by FTIR, N₂-adsorption, XRD, SEM and TEM. We have found that the chemical differences on the silanol/siloxane groups of SG/67 and SBA15 mesoporous materials deeply modify the enzymatic load, activity, thermal stability and reusability. The physical immobilization of CPO was characterized by a high adsorption capacity (q_m) and affinity constants (K_m) when compared to the covalent approach, but it resulted more sensitive to temperature than free, the silanized and covalently immobilized enzyme. The thermal residual activity as well as reusability of CPO were first improved by silanization, then by covalent immobilization in a support with a large pore size and high silanol/siloxane ratio.     

A colorimetric method for detection and quantification of chlorinating activity of hemeperoxidases

A method of detecting and assaying the halogenating activity of hemeperoxidases using the colored substrate, thionin, is reported here. In the presence of suitable amounts of peroxide and chloride, chloroperoxidase chlorinates thionin and bleaches the intense color of this substrate. The kinetics was quite similar to that of the established monochlorodimedone assay. Therefore, the thionin assay can be taken as an index of chlorinating activity. This reaction affords an escape from background problems and allows easy processing of large volumes of samples.     

Low temperature photo-oxidation of chloroperoxidase Compound II

Oxidation of the heme-thiolate enzyme chloroperoxidase (CPO) from Caldariomyces fumago with peroxynitrite (PN) gave the Compound II intermediate, which was photo-oxidized with 365 nm light to give a reactive oxidizing species. Cryo-solvents at pH ≈ 6 were employed, and reactions were conducted at temperatures as low as − 50 °C. The activity of CPO as evaluated by the chlorodimedone assay was unaltered by treatment with PN or by production of the oxidizing transient and subsequent reaction with styrene. EPR spectra at 77 K gave the amount of ferric protein at each stage in the reaction sequence. The PN oxidation step gave a 6:1 mixture of Compound II and ferric CPO, the photolysis step gave an approximate 1:1 mixture of active oxidant and ferric CPO, and the final mixture after reaction with excess styrene contained ferric CPO in 80% yield. In single turnover reactions at − 50 °C, styrene was oxidized to styrene oxide in high yield. Kinetic studies of styrene oxidation at − 50 °C displayed saturation kinetics with an equilibrium constant for formation of the complex of K_bind = 3.8 × 10⁴ M^− 1 and an oxidation rate constant of k_ox = 0.30 s^− 1. UV-Visible spectra of mixtures formed in the photo-oxidation sequence at ca. − 50 °C did not contain the signature Q-band absorbance at 690 nm ascribed to CPO Compound I prepared by chemical oxidation of the enzyme, indicating that different species were formed in the chemical oxidation and the photo-oxidation sequence.     

First detection of a chloroperoxidase in bryophytes

Chlorinated cyclic bisbibenzyls of the isoplagiochin type are the first verified halometabolites from bryophytes. They could be obtained by in vitro chlorination of isoplagiochin C with chloroperoxidase from Caldariomyces fumago. Furthermore, an enzyme of this type was detected for the first time in bryophytes namely in the liverwort Bazzania trilobata using the monochlorodimedon assay.     

X-ray crystal structures of active site mutants of the vanadium-containing chloroperoxidase from the fungus Curvularia inaequalis

The X-ray structures of the chloroperoxidase from Curvularia inaequalis, heterologously expressed in Saccharomyces cerevisiae, have been determined both in its apo and in its holo forms at 1.66 and 2.11?Å resolution, respectively. The crystal structures reveal that the overall structure of this enzyme remains nearly unaltered, particularly at the metal binding site. At the active site of the apo-chloroperoxidase structure a clearly defined sulfate ion was found, partially stabilised through electrostatic interactions and hydrogen bonds with positively charged residues involved in the interactions with the vanadate in the native protein. The vanadate binding pocket seems to form a very rigid frame stabilising oxyanion binding. The rigidity of this active site matrix is the result of a large number of hydrogen bonding interactions involving side chains and the main chain of residues lining the active site. The structures of single site mutants to alanine of the catalytic residue His404 and the vanadium protein ligand His496 have also been analysed. Additionally we determined the structural effects of mutations to alanine of residue Arg360, directly involved in the compensation of the negative charge of the vanadate group, and of residue Asp292 involved in forming a salt bridge with Arg490 which also interacts with the vanadate. The enzymatic chlorinating activity is drastically reduced to approximately 1% in mutants D292A, H404A and H496A. The structures of the mutants confirm the view of the active site of this chloroperoxidase as a rigid matrix providing an oxyanion binding site. No large changes are observed at the active site for any of the analysed mutants. The empty space left by replacement of large side chains by alanines is usually occupied by a new solvent molecule which partially replaces the hydrogen bonding interactions to the vanadate. The new solvent molecules additionally replace part of the interactions the mutated side chains were making to other residues lining the active site frame. When this is not possible, another side chain in the proximity of the mutated residue moves in order to satisfy the hydrogen bonding potential of the residues located at the active site frame.     

Cross-linked crystals of chloroperoxidase

Chloroperoxidase from Caldariomyces fumago was crystallized. The crystals were modified with several cross-linkers, but only glurataldehyde was able to produce catalytically active and insoluble crystals. Unlike other immobilized chloroperoxidase preparations, these catalytic crystals are more thermostable than the unmodified soluble enzyme. The enhanced stability is probably due to the structure conservation in the crystalline matrix. In addition, non-cross-linked chloroperoxidase crystals retained more activity than the soluble enzyme after incubation in an organic solvent with low water content. Although the cross-linked crystals were catalytically active, they showed lower specific activity than the soluble enzyme. This low activity may be due to non-specific reactions between the cross-linker and essential residues for catalysis. Alternative cross-linking strategies are discussed.     

Improvement of chloroperoxidase stability by covalent immobilization on chitosan membranes

Chloroperoxidase (CPO) from Caldariomyces fumago was optimally covalently immobilized on chitosan membranes pretreated with 0.8 M glutaraldehyde at pH 3.5 to give 3.18 mg CPO g⁻¹ support. Using monochlorodimedone (MCD) as assay substrate, the immobilized-CPO retained 40% activity at 50°C after 40 min whereas free CPO retained only 0.02%. The residual activity for immobilized-CPO was 99 and 58% compared with 68 and 43% for free CPO in the presence of 1.5 M urea and 300 μM H₂O₂, respectively, after 20 h.     

BioNano engineered hybrids for hypochlorous acid generation

Enzyme-based systems represent a user- and environmentally-friendly alternative to current corrosive and/or toxic decontamination technologies used for microbial decontamination. Herein an easily deployable enzyme-nanosupport hybrid system was developed for in situ generation of hypochlorous acid (HOCl), a strong decontaminant. The user-controlled strategy allowed co-immobilization of two different enzymes at a nanosupport interface and decontaminant generation through a chain reaction. For this, glucose oxidase was used as the working enzyme and co-immobilized onto multi-walled carbon nanotubes along with chloroperoxidase. Our hypothesis was that hydrogen peroxide produced at the nanosupport interface through the glucose oxidase enzymatic reaction can further be used as substrate by the co-immobilized CPO to convert (Cl^?) into HOCl. The chemistry of the immobilization method, as well as the enzyme loading, activity, kinetics and enzyme stability at the nanointerface were evaluated. The multi-enzyme system was found to be able to initiate and propagate the chain reaction resulting in decontaminant production. The strong capability of HOCl generation can be viewed as an important first step toward creating self-sustainable microbial decontamination coatings to be used against various pathogens such as bacteria and spores.     

Monoterpenes as novel substrates for oxidation and halo-hydroxylation with chloroperoxidase from Caldariomyces fumago

Chloroperoxidase (CPO) from Caldariomyces fumago was analysed for its ability to oxidize ten different monoterpenes with hydrogen peroxide as oxidant. In the absence of halide ions geraniol and, to a lesser extent, citronellol and nerol were converted into the corresponding aldehydes, whereas terpene hydrocarbons did not serve as substrates under these conditions. In the presence of chloride, bromide and iodide ions, every terpene tested was converted into one or more products. (1S)-(+)-3-carene was chosen as a model substrate for the CPO-catalysed conversion of terpenes in the presence of sodium halides. With chloride, bromide and iodide, the reaction products were the respective (1S,3R,4R,6R)-4-halo-3,7,7-trimethyl-bicyclo[4.1.0]-heptane-3-ols, as identified by ¹H and ¹³C nuclear magnetic resonance. These product formations turned out to be strictly regio- and stereoselective and proceeded very rapidly and almost quantitatively. Initial specific activities of halohydrin formation increased from 4.22 U mg⁻¹ with chloride to 12.22 U mg⁻¹ with bromide and 37.11 U mg⁻¹ with iodide as the respective halide ion. These results represent the first examples of the application of CPO as a highly efficient biocatalyst for monoterpene functionalization. This is a promising strategy for ‘green’ terpene chemistry overcoming drawbacks usually associated with cofactor-dependent oxygenases, whole-cell biocatalysts and conventional chemical methods used for terpene conversions.