Laccases structure

Claus, H. (2004). Laccases structure, reactions and distribution, Micron. 35, 93-96. 

Claus, H. 2004. Laccases Structure, reactions, distribution. Micron, 35 93-96. 

Removal of the type 2 copper according to a reported procedure (20) leads to significant decreases of the absorbance at 330 and 750 nm. These decreases indicate that type 2 copper contributes to the absorbance in these regions. For fungal and tree laccase, structures based on tetragonal six, five, or square-planar four coordination, as found in several low-molecular-weight copper complexes, were proposed (37). 

In-situ STM of laccase has so far been attempted using only basal-plane pyrolytic graphite surfaces [54]. No adsotption could be detected. This is perhaps not surprising as laccase voltammetry requires edge-plane graphite and/or pre-adsorption of promoters. Low-resolution ex-situ micron-scale laccase structures coidd be recorded during evaporation of laccase solution where individual molecular-size structures, possibly dislodged by mechanical tip contact, could also be observed. 

Figure 9 demonstrates that [G2]-PEG5k[G2] notably facilitates the oxidation of BP in comparison to the linear analogues. The result is not surprising in view of the reported superior PAH binding capacity of the nanoporous hydrophobic domains formed by this linear-dendritic copolymer in water (8). It is assumed in analogy with the micelles that the dendritic fragments anchored at the carbohydrate surfaces are able to passively bind and transport both NHN and BP in close proximity to the active center in the laccase structure. Thus the linear-dendritic laccase complexes detailed here may be used to facilitate the transformation of those substrates that have unfavorable size and solubility, issues udiich were raised by D Acunzo et al. (22) for laccase-catalyzed oxidations. 

Figure 17.3 Anatomy of a redox enzyme representation of the X-ray crystallographic structure of Trametes versicolor laccase III (PDB file IKYA) [Bertrand et al., 2002]. The protein is represented in green lines and the Cu atoms are shown as gold spheres. Sugar moieties attached to the surface of the protein are shown in red. A molecule of 2,5-xyhdine that co-crystallized with the protein (shown in stick form in elemental colors) is thought to occupy the broad-specificity hydrophobic binding pocket where organic substrates ate oxidized by the enzyme. Electrons from substrate oxidation are passed to the mononuclear blue Cu center and then to the trinuclear Cu active site where O2 is reduced to H2O. (See color insert.)...

Figure 17.6 Redox hydrogel approach to immobilizing multiple layers of a redox enzyme on an electrode, (a) Structure of the polymer, (b) Voltammograms for electrocatalytic O2 reduction by a carbon fiber electrode modified with laccase in the redox hydrogel shown in (a) (long tether) or a version with no spacer atoms in the tether between the backbone and the Os center (short tether). Reprinted with permission fi om Soukharev et al., 2004. Copyright (2004) American Chemical Society.

Bertrand T, Jolivalt C, Briozzo P, Caminade E, Joly N, Madzak C, Mougin C. 2002. Crystal structure of a four-copper laccase complexed with an arylamine Insights into substrate recognition and correlation with kinetics. Biochemistry 41 7325-7333. 

Hakulinen N, Kiiskinen LL, Kruus K, Saloheimo M, Paananen A, Koivula A, Rouvinen J. 2002. Crystal structure of a laccase from Melanocarpus albomyces with an intact trinuclear copper site. Nature Struct Biol 9 601-605. 

Piontek K, Antorini M, Choinowski T. 2002. Crystal structure of a laccase from the fungus Trametes versicolor at 1.90 A resolution containing a full complement of coppers. J Biol Chem277 37663-37669. 

Reported redox potentials of laccases are lower than those of non-phenolic compounds, and therefore these enzymes cannot oxidize such substances [7]. However, it has been shown that in the presence of small molecules capable to act as electron transfer mediators, laccases are also able to oxidize non-phenolic structures [68, 69]. As part of their metabolism, WRF can produce several metabolites that play this role of laccase mediators. They include compounds such as /V-hvdi oxvacetan i I ide (NHA), /V-(4-cyanophenyl)acetohydroxamic acid (NCPA), 3-hydroxyanthranilate, syringaldehyde, 2,2 -azino-bis(3-ethylben-zothiazoline-6-sulfonic acid) (ABTS), 2,6-dimethoxyphenol (DMP), violuric acid, 1-hydroxybenzotriazole (HBT), 2,2,6,6-tetramethylpipperidin-iV-oxide radical and acetovanillone, and by expanding the range of compounds that can be oxidized, their presence enhances the degradation of pollutants [3]. 

The oxidized mediator form, produced in the course of the enzymatic reaction, can nonenzymatically oxidize compounds (including nonphenolic lignin structures) with ionization potentials exceeding the potentials of laccases (Morozova and others 2007). 

Thurston CF. 1994. The structure and function of fungal laccases. Microbiology 140(1) 19—26. 

Both purified laccase as well as the crude enzyme from the WRF Cerrena unicolor were used to convert the dyes in aqueous solution. Biotransformation of the dyes was followed spectrophotometrically and confirmed by high performance liquid chromatography. The results indicate that the decolorization mechanism follows MichaeliseMenten kinetic and that the initial rate of decolorization depends both on the structure of the dye and on the concentration of the dye. Surprisingly, one recalcitrant azo dye (AR 27) was decolorized merely by purified laccase in the absence of any redox mediator [46],... 

Kandelbauer A, Erlacher A, Cavaco-Paulo A, Guebitz GM (2004) Laccase catalyzed decolorization of the synthetic dye Diamond Black PV 200 and some structurally related derivatives. Biocatal Biotransformation 22 331-339... 

Various spectroscopic methods have been used to probe the nature of the copper centers in the members of the blue copper oxidase family of proteins (e.g. see ref. 13). Prior to the X-ray determination of the structure of ascorbate oxidase in 1989, similarities in the EPR and UV-vis absorption spectra for the blue multi-copper oxidases including laccase and ceruloplasmin had been observed [14] and a number of general conclusions made for the copper centers in ceruloplasmin as shown in Table 1 [13,15]. It was known that six copper atoms were nondialyzable and not available to chelation directly by dithiocarbamate and these coppers were assumed to be tightly bound and/or buried in the protein. Two of the coppers have absorbance maxima around 610 nm and these were interpreted as blue type I coppers with cysteine and histidine ligands, and responsible for the pronounced color of the protein. However, they are not equivalent and one of them, thought to be involved in enzymatic activity, is reduced and reoxidized at a faster rate than the second (e.g. see ref. 16). There was general concurrence that there are two type HI... 

Type II copper enzymes generally have more positive reduction potentials, weaker electronic absorption signals, and larger EPR hyperfine coupling constants. They adopt trigonal, square-planar, five-coordinate, or tetragonally distorted octahedral geometries. Usually, type II copper enzymes are involved in catalytic oxidations of substrate molecules and may be found in combination with both Type I and Type III copper centers. Laccase and ascorbate oxidase are typical examples. Information on these enzymes is found in Tables 5.1, 5.2, and 5.3. Superoxide dismutase, discussed in more detail below, contains a lone Type II copper center in each of two subunits of its quaternary structure. 

Table 5.2 contains data about selected copper enzymes from the references noted. It should be understood that enzymes from different sources—that is, azurin from Alcaligenes denitrificans versus Pseudomonas aeruginosa, fungal versus tree laccase, or arthropodan versus molluscan hemocyanin—will differ from each other to various degrees. Azurins have similar tertiary structures—in contrast to arthropodan and molluscan hemocyanins, whose tertiary and quaternary structures show large deviations. Most copper enzymes contain one type of copper center, but laccase, ascorbate oxidase, and ceruloplasmin contain Type I, Type II, and Type III centers. For a more complete and specific listing of copper enzyme properties, see, for instance, the review article by Solomon et al.4... 

This discussion of copper-containing enzymes has focused on structure and function information for Type I blue copper proteins azurin and plastocyanin, Type III hemocyanin, and Type II superoxide dismutase s structure and mechanism of activity. Information on spectral properties for some metalloproteins and their model compounds has been included in Tables 5.2, 5.3, and 5.7. One model system for Type I copper proteins39 and one for Type II centers40 have been discussed. Many others can be found in the literature. A more complete discussion, including mechanistic detail, about hemocyanin and tyrosinase model systems has been included. Models for the blue copper oxidases laccase and ascorbate oxidases have not been discussed. Students are referred to the references listed in the reference section for discussion of some other model systems. Many more are to be found in literature searches.50... 

Copper oxidases are widely distributed in nature, and enzymes from plants, microbes, and mammals have been characterized (104,105). The blue copper oxidases, which include laccases, ascorbate oxidases, and ceruloplasmin, are of particular interest in alkaloid transformations. The principle differences in specificity of these copper oxidases are due to the protein structures as well as to the distribution and environment of copper(II) ions within the enzymes (106). While an in vivo role in metabolism of alkaloids has not been established for these enzymes, copper oxidases have been used in vitro for various alkaloid transformations. 

Figure 14.8 The overall structure of the CotA laccase from Bacillus subtilis showing the entrance and exit channels for dioxygen and water, above and below the trinuclear cluster, respectively. (From Bento et al., 2006. With kind permission of Springer Science and Business Media.)...

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