Laura-Leena Kiiskinen

Crystal Structure of a Laccase from Melanocarpus Albomyces with an Intact Trinuclear Copper Site

We have crystallized the ascomycete laccase from Melanocarpus albomyces with all four coppers present and determined the crystal structure at 2.4 Å resolution. The enzyme is heavily glycosylated and consists of three cupredoxin-like domains, similar to those found in the Cu-depleted basidiomycete laccase from Coprinus cinereus. However, there are significant differences in the loops forming the substrate-binding pocket. In addition, the crystal structure of the M. albomyces laccase revealed elongated electron density between all three coppers in the trinuclear copper site, suggesting that an oxygen molecule binds with a novel geometry. This oxygen, required in the reaction, may enter the trinuclear site through the tunnel, which is open in the structure of the C. cinereus laccase. In contrast, the C-terminus on the M. albomyces laccase forms a plug that blocks this access.

Screening for novel laccase-producing microbes

Aims:  To discover novel laccases potential for industrial applications.

Molecular Cloning and Expression in Saccharomyces cerevisiae of a Laccase Gene from the Ascomycete Melanocarpus albomyces

ABSTRACT The lac1 gene encoding an extracellular laccase was isolated from the thermophilic fungus Melanocarpus albomyces. This gene has five introns, and it encodes a protein consisting of 623 amino acids. The deduced amino acid sequence of the laccase was shown to have high homology with laccases from other ascomycetes. In addition to removal of a putative 22-amino-acid signal sequence and a 28-residue propeptide, maturation of the translation product of lac1 was shown to involve cleavage of a C-terminal 14-amino-acid extension. M. albomyces lac1 cDNA was expressed in Saccharomyces cerevisiae under the inducible GAL1 promoter. Extremely low production was obtained with the expression construct containing laccase cDNA with its own signal and propeptide sequences. The activity levels were significantly improved by replacing these sequences with the prepro sequence of the S. cerevisiae α-factor gene. The role of the C-terminal extension in laccase production in S. cerevisiae was also studied. Laccase production was increased sixfold with the modified cDNA that had a stop codon after the native processing site at the C terminus.

Laccase from Melanocarpus albomyces binds effectively to cellulose

A laccase from the thermophilic fungus Melanocarpus albomyces was shown to bind to softwood and pure microcrystalline cellulose. The binding isotherm fitted well the Langmuir type one‐site binding model. The adsorption parameters indicated that M. albomyces laccase binds with high affinity to cellulose with a relatively low maximum binding capacity, as compared to the values for various cellulases. The binding was shown to be reversible and not influenced by non‐specific protein or 0.1–0.5 M Na2SO4. No binding was detected with laccases from Trametes hirsuta or Mauginiella sp., which suggests that binding to cellulose is typical for only some laccases.