Laccase reduction reactions

Hexacyanoferrate(III) was added to the samples in order to oxidize reductive interferents such as ascorbic acid, and the hexacyanofer-rate(II) formed was reoxidized by a laccase-catalyzed reaction. This approach appears to be particularly useful for samples with extreme concentrations of oxidizable compounds, such as urine. 

The results obtained make it clear that laccase in the immobilized state is an effective catalyst for the oxygen reduction reaction in neutral solutions. 

The effect of the distance between the active center and the electrode on the reaction rate has been studied using as an example the electrocatalysis of the oxygen reduction reaction by laccase adsorbed on soot. Variation in the distance between the active center and the electroconductive substrate was achieved by inserting an intermediate monolayer of lipid molecules flatly and vertically oriented cholesterol molecules and vertically oriented lecithin molecules (scheme in Figure 36). In this case, the conditions of obtaining compact lipid monolayers were fulfilled. The subsequent setting of laccase did not lead to their desorption. 

The decomposition of pinacyanol chloride dye using octahedral layered mixed-valent manganese oxides has been published [81]. Catalytic reduction reactions using birnessite have been tried for removing pentachlorophenol (PCP) from soil and water (detoxyfication) [82]. Transformation and dechlorination of PCP incubated with peroxidase, laccase, or birnessite is decreased in the presence of humic monomers as cosubstrate. The dehalogenation number for birnessite is 3.3, compared with 3.5 for peroxidase and 1.5 for laccase [82]. 

Laccase has been commonly used in cathodes of en2yme-based biofuel cells. Only recently, it was discovered that bacteria can also catalyze the oxygen reduction reaction in MFCs. Biocathode here refers to bacteria-based cathode. 

Parirni NS, Umasankar Y, Atanassov P, Ramasamy RP. Electrochemical kinetic and mechanistic parameters of laccase-catalyzed oxygen reduction reaction. ACS Catal 2012 2 38-44. 

Gupta G, Rajendran V, Atanassov P. Bioelectrocatalysis of oxygen reduction reaction by laccase on gold electrode. Electroanalysis 2004 16 1182-1185. 

The catalytic cycle of laccase includes several one-electron transfers between a suitable substrate and the copper atoms, with the concomitant reduction of an oxygen molecule to water during the sequential oxidation of four substrate molecules [66]. With this mechanism, laccases generate phenoxy radicals that undergo non-enzymatic reactions [65]. Multiple reactions lead finally to polymerization, alkyl-aryl cleavage, quinone formation, C> -oxidation or demethoxylation of the phenolic reductant [67]. 

The reaction of CO J radicals with human ceruloplasmin produced RSSR radicals. Their first-order decay occurred at the reduction rate of type-1 Cu. The same mechanism was observed with fungal and tree laccase... 

In the discussion of the biochemistry of copper in Section 62.1.8 it was noted that three types of copper exist in copper enzymes. These are type 1 ( blue copper centres) type 2 ( normal copper centres) and type 3 (which occur as coupled pairs). All three classes are present in the blue copper oxidases laccase, ascorbate oxidase and ceruloplasmin. Laccase contains four copper ions per molecule, and the other two contain eight copper ions per molecule. In all cases oxidation of substrate is linked to the four-electron reduction of dioxygen to water. Unlike cytochrome oxidase, these are water-soluble enzymes, and so are convenient systems for studying the problems of multielectron redox reactions. The type 3 pair of copper centres constitutes the 02-reducing sites in these enzymes, and provides a two-electron pathway to peroxide, bypassing the formation of superoxide. Laccase also contains one type 1 and one type 2 centre. While ascorbate oxidase contains eight copper ions per molecule, so far ESR and analysis data have led to the identification of type 1 (two), type 2 (two) and type 3 (four) copper centres. 

Kinetic studies with laccase have shown that the enzyme must be reduced by the organic substrate before reaction with dioxygen occurs. The first electron from the substrate is accepted by the type 1 Cu2+, and the second by the type 2 Cu2+. The electrons from these reduced sites are then transferred to the type 3 copper pair, which then binds dioxygen with reduction to peroxide. It is possible that the type 2 and type 3 centres are in the same cavity, which only becomes accessible to the solvent when the type 1 Cu+ is oxidized. 

---