Abstract
Laccases or benzenediol oxygen oxidoreductases (EC 1.10.3.2) are polyphenol multicopper oxidases that are known for their structural and functional diversity in various life forms. In the present study, the molecular and physico-chemical properties (redox-potential and secondary structures) of fungal laccase isozymes (FLIs) isolated from a medicinal mushroom Ganoderma lucidum were analyzed and compared with those of the recombinant bacterial laccases (rLac) obtained from different Yersinia enterocolitica strains. It was revealed that the FLIs contained His-Cys-His as the most conserved residue in its domain I Cu site, while the fourth and fifth residues were variable (Ile, Leu, or Phe). Evidently, the cyclic voltammetric measurements of Glac L2 at Type 1 Cu site revealed greater E° for ABTS/ABTS+ (0.312 V) and ABTS+/ABTS2+ (0.773 V) compared to the E° of rLac. Furthermore, circular dichroism-based conformational analysis revealed structural stability of the FLIs at acidic pH (3.0) and low temperature (<30 °C), while the isozymes were destabilized at neutral pH (7.0) and high-temperature conditions (>70 °C). The zymographic studies further confirmed the functional inactivation of FLIs at high temperatures (≥70 °C), predominantly due to domain unfolding. These findings provide novel insight into the evolution of the catalytic efficiency and redox properties of the FLIs, contributing to the existing knowledge regarding stress responses, metabolite production, and the biotechnological utilization of metabolites.
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