Laccase function

The EPR-nondetectable ions in laccase function as a cooperative 2-e" unit (5). With cytochrome oxidase redox titrations based on the heme absorption bands (2) indicate the presence of a high and a low potential site (380 and 220 mV, respectively). On the other hand, the quasi equilibrium established in the rapid initial transfer of electrons from reduced cytochrome c to the primary electron acceptor in the oxidase, cytochrome a, indicates a potential of 285 mV for this site (18). 

Laccases function over a wide range of potentials -1-500 mV (versus a normal hydrogen electrode) is characteristic of a iow-potential laccase and -1-800 mV is typical for a high-potential laccase . Laccase from Trametes versicolor belongs to the latter class. 

Plou, F.)., and Alcalde, M. (2008) Altering the laccase functionality by in vivo assembly of mutant libraries with different mutational spectra. Proteins, 71, 250-260. 

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

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... 

The produchon of oxidase enzymes in native strains is often not constitutive, since these enzymes are only required under specific conditions, such as nutrient stress. This imposes limitations on their application in continuous bioprocesses and one way of dealing with this issue is to develop improved expression systems. Laccase from Trametes sp. C30, which in its native strain is inducible and only weakly expressed, has been cloned and expressed in yeast to provide a functional recombinant enzyme with the same high activity and low redox potential as the native enzyme [68]. 

The multi-copper oxidases include laccase, ceruloplasmin, and ascorbate oxidase. Laccase can be found in tree sap and in fungi ascorbate oxidase, in cucumber and related plants and ceruloplasmin, in vertebrate blood serum. Laccases catalyze oxidation of phenolic compounds to radicals with a concomitant 4e reduction of O2 to water, and it is thought that this process may be important in the breakdown of lignin. Ceruloplasmin, whose real biological function is either quite varied or unknown, also catalyzes oxidation of a variety of substrates, again via a 4e reduction of O2 to water. Ferroxidase activity has been demonstrated for it, as has SOD activity. Ascorbate oxidase catalyzes the oxidation of ascorbate, again via a 4e reduction of O2 to water. Excellent reviews of these three systems can be found in Volume 111 of Copper Proteins and Copper Enzymes (Lontie, 1984). 

Laccase, 36 318, 329, 40 122 see also Blue copper oxidases amino-acid sequences, 40 141 anaerobic reduction, 40 158-160 biological function, 40 124 electrochemistry, 36 360 fungal, 40 145-152 evolution, 40 153-154 inhibition, 40 162 kinetic properties, 40 157-162 molecular and spectroscopic properties, 40 125-126... 

The other small blue proteins are only poorly characterized at present It is assumed their function is that of electron transfer. Rusticyanin from Thiobacillus ferrooxidans is thought to be the initial electron acceptor from iron(II) in the respiratory chain at pH 2. Rusticyanin contains 159 residues, with one cysteine, three methionine and five histidine residues. The protein is unusually stable at low pH, in accord with its presence in an acidophilic organism. The midpoint potential of rusticyanin is high (+680 mV), and is second in magnitude only to that of Polyporus laccase. 

As noted above, the type 3 site has similarities to the cyt a%- -CuB pair in cytochrome oxidase. Both pairs function as 02-reducing centres and are ESR-silent due to antiferromagnetic interactions in the oxidized and fully reduced enzymes. ESR signals have now been detected from the type 3 Cu11 in laccase intermediates in which the second Cu has been reduced to Cu1.960,961 Similarly, the ESR-silent Cu in cytochrome oxidase has been detected in an intermediate species. The ESR signals for the type 3 Cu in laccase and the Cu 1 in cytochrome oxidase are very similar. 

Pal et al. (1994) compared the catalysis of oxidative coupling reactions of various phenolic compounds by the enzymes, laccase and tyrosinase, and mineral catalyst, birnessite. Birnessite acts as a heterogeneous catalyst whereas laccase and tyrosinase function as homogeneous catalysts. Laccase and tyrosinase continue to oxidize catechol after repeated additions of the chemical, while birnessite lost its oxidizing activity after the first addition of catechol (Figure 2.20). In the case of birnessite,... 

Copper proteins are involved in a variety of biological functions, including electron transport, copper storage and many oxidase activities. A variety of reviews on this topic are available (Sykes, 1985 Chapman, 1991). Several copper proteins are easily identified by their beautiful blue colour and have been labelled blue copper proteins. The blue copper proteins can be divided into two classes, the oxidases (laccase, ascorbate oxidase, ceruloplasmin) and the electron carriers (plastocyanin, stellacyanin, umecyanin, etc.). 

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