Structural and redox properties of the small laccase Ssl1 from Streptomyces sviceus

Laccases (EC 1.10.3.2) are members of the multicopper oxidase family. They oxidize diverse electron-rich substrates through electron abstraction by the type 1 copper ion in the enzyme active site. Abstracted electrons are transferred to the trinuclear copper cluster, where molecular oxygen serves as final acceptor and is reduced to water. Laccase activity is assumed to depend on the redox potential of its type 1 copper ion. Whereas numerous studies have been undertaken to elucidate the determinants of the redox potential of type 1 copper ions in one-domain cupredoxins and in three-domain laccases, such experimental investigations are lacking for recently described, small, two-domain laccases. In this work, the crystal structure of the small laccase Ssl1 from Streptomyces sviceus was solved, and the positions that might influence the redox potential of Ssl1 were depicted. On the basis of this knowledge, several Ssl1 variants were constructed with an increase in redox potential of 16-81 mV, from 375 mV to 391-456 mV. Mutation of residues in close proximity to the type 1 copper center resulted in a predicted increase in the redox potential of the copper center; however, there was a reduced specific activity for the oxidation of 2,6-dimethoxyphenol, which has a relatively low redox potential. Mutations more distant to the type 1 copper also led to an increased redox potential of the copper center, and resulted in variants able to oxidize the high redox potential substrates 1,2-dihydroxyanthraquinone-3-sulfonic acid (Alizarin Red S) and indigo carmine more efficiently than wild-type Ssl1.