Characterization of a laccase from a wood-feeding termite, Coptotermes formosanus

Coptotermes formosanus Shiraki is a wood-feeding termite which secretes a series of lignolytic and cellulolytic enzymes for woody biomass degradation. However, the lignin modification mechanism in the termite is largely elusive, and the characteristics of most lignolytic enzymes in termites remain unknown. In this study, a laccase gene lac1 from C. formosanus was heterogeneously expressed in insect Sf9 cells. The purified Lac1 showed strong activities toward hydroquinone (305 mU/mg) and 2,6-dimethoxyphenol (2.9 mU/mg) with low K_m values, but not veratryl alcohol or 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid). Lac1 could function well from pH 4.5 to 7.5, and its activity was significantly inhibited by H₂ O₂ at above 4.85 mmol/L (P < 0.01). In addition, the lac1 gene was found to be mainly expressed in the salivary glands and foregut of C. formosanus, and seldom in the midgut or hindgut. These findings suggested that Lac1 is a phenol-oxidizing laccase like RflacA and RflacB from termite Reticulitermes flavipes, except that Lac1 was found to be more efficient in phenol oxidation, and it did not require H₂ O₂ for its function. It is suspected that this kind of termite laccase might only be able to directly oxidize low redox-potential substrates, and the high redox-potential groups in lignin might be oxidized by other enzymes in the termite or by using the Fenton reaction.