Functional substitution of domain 3 (T1 copper center) of a novel laccase with Cu ions

Abstract

A bacterial laccase having potential to work in industry without mediator is of special interest. In this work, gene (1.83kb) encoding a novel laccase from Rheinheimera sp., having potential to deink waste paper without mediator, was cloned, over-expressed and the induced 69kDa protein (RhLacc) was purified and characterized. rhlacc gene was mutated by error prone PCR and mutants were sequenced. One mutant showed protein truncation resulting in the absence of domain 3 that contains T1 copper center. It is known that redox potential of T1 is the key parameter for substrate oxidation. Overexpression of this mutant gene showed induced 41.1kDa protein (∆RhLacc) that exhibited laccase activity but in the presence of added copper, compared to RhLacc which showed activity without added copper ions. Optimum temperature for both was 55°C. However, optimum pH varied with substrates. Kinetic studies showed ∆RhLacc had lower affinity for substrates except for guaiacol and reduced kcat in comparison to RhLacc. Both were able to deink old newspaper and degrade indigo carmine without mediator. The study suggests that the novel property to deink waste paper without mediator may not depend on the redox potential of T1 but other mechanisms using domains 1 and 2 may be involved.