Structure–function relationship among bacterial, fungal and plant laccases

Abstract

Abstract Laccases (benzenediol: oxygen oxidoreductase; EC 1.10.3.2), a multicopper oxidase enzyme, widely distributed in plants, fungi and bacteria have ability to catalyze oxidation of various phenolic and non-phenolic compounds as well as many environmental pollutants. The diversified functions of laccases, including the antagonistic ones such as their involvement in lignin biosynthesis (in plants) as well as lignin degradation (in fungi and bacteria), make them an interesting enzyme for study from the point of view of their structure, function and application. Important applications of laccases include delignification, pulp bleaching and bioremediation. The ability of laccases to polymerize natural phenols helps to develop new cosmetic pigments, hair dyeing materials, deodorants, toothpastes, mouthwashes and other useful products. Recently, the utility of enzyme has also been shown in the nanoparticle based biosensor technology as well as in medical fields. In the present review, a comparative account of the bacterial, fungal and plant laccases has been presented from these points of views. Laccases are dimeric or tetrameric glycoproteins usually containing four copper atoms per monomer. To perform catalytic function, laccase depends on Cu atoms that are distributed at the three different copper centers. These copper centers in laccases are categorized into three groups: Type-1 or blue copper center, Type-2 or normal copper and Type-3 or coupled binuclear copper centers. The four copper atoms are differing in their characteristic electronic paramagnetic resonance (EPR) signals. The phylogenetic analysis reveals, laccases from these groups (viz. bacteria, fungi and plant) form independent clades, in consistent with that of taxonomical classification. From previous experimental evidence and from in silico studies, it is evident that despite their wide taxonomic distribution and substrate diversity, molecular architecture of laccases is common to multicopper oxidases. Three dimensional structure predictions, at monomeric level, for all laccases (bacterial, fungal and plant) suggest that they are composed of three sequentially arranged cupredoxin-like domains. Multiple alignment of primary sequences of all three modeled laccases shows that, the copper binding motives are highly conserved in all sequences. The similarities are more significant in the N- and C-terminal regions, corresponding to Domains 1 and 3, as the copper interacting motif is present in Domains 1 and 3 not in Domain 2. This structural conservation reflects a common reaction mechanism for the copper oxidation and the O2 reduction in these enzymes. Putative binding pocket analysis depicts, larger binding cavity for bacterial laccase as compared to those for plants and fungi. An in depth analysis of copper binding site, yielded significant differences in conserved residues for laccases of bacteria, fungi and plants which provided the basis for the dual and contrasting functions of laccases.