Recombinant expression of Pleurotus ostreatus laccases in Kluyveromyces lactis and Saccharomyces cerevisiae

Random mutants of a Pleurotus ostreatus laccase as new biocatalysts for industrial effluents bioremediation

AIMS

To select better performing laccase variants among the 2300 randomly mutated variants of Pleurotus ostreatus POXA1b laccase to develop improved laccase-based biocatalysts.

METHODS AND RESULTS

Screening of collections of 2300 randomly mutated variants of POXA1b was performed by assaying activity towards the phenolic substrate 2,6-dimethoxyphenol. Two new variants endowed with higher enzyme activity than the wild-type laccase were characterized, and their ability to decolourize industrial dyes with complex trisazo-, polyazo- and stilbene-type structures, in the absence of mediators, was demonstrated. One of the mutants (2L4A) was also proved to be highly stable at both acidic and alkaline pH values (displaying a half-life of around 1 month at the pH levels of both 5 and 10).

CONCLUSIONS

In comparison with the wild-type laccase, the new selected 2L4A mutant shows a significant increase in stability at acidic pH, whilst storing its high stability at alkaline pH. This variant also represents a more versatile enzyme with respect to both the variety of xenobiotics degraded and the operative conditions.

SIGNIFICANCE AND IMPACT OF THE STUDY

This work represents the first example of improvement of a basidiomycete laccase for industrial effluents bioremediation by directed evolution.

Engineering and Applications of fungal laccases for organic synthesis

Laccases are multi-copper containing oxidases (EC 1.10.3.2), widely distributed in fungi, higher plants and bacteria. Laccase catalyses the oxidation of phenols, polyphenols and anilines by one-electron abstraction, with the concomitant reduction of oxygen to water in a four-electron transfer process. In the presence of small redox mediators, laccase offers a broader repertory of oxidations including non-phenolic substrates. Hence, fungal laccases are considered as ideal green catalysts of great biotechnological impact due to their few requirements (they only require air, and they produce water as the only by-product) and their broad substrate specificity, including direct bioelectrocatalysis.

Thus, laccases and/or laccase-mediator systems find potential applications in bioremediation, paper pulp bleaching, finishing of textiles, bio-fuel cells and more. Significantly, laccases can be used in organic synthesis, as they can perform exquisite transformations ranging from the oxidation of functional groups to the heteromolecular coupling for production of new antibiotics derivatives, or the catalysis of key steps in the synthesis of complex natural products. In this review, the application of fungal laccases and their engineering by rational design and directed evolution for organic synthesis purposes are discussed.

Molecular cloning and heterologous expression of a laccase gene from Pleurotus eryngii in free and immobilized Saccharomyces cerevisiae cells

A full length cDNA encoding an extracellular laccase was isolated by reverse transcription polymerase chain reaction from the mycelia of the mushroom Pleurotus eryngii. The isolated sequence, denoted Ery3, encodes for a mature laccase isoenzyme of 531 amino acid residues with a predicted molecular weight of 56.6 kDa. All sequence motifs, being the signature sequences used to identify the laccases, were found in the Ery3 protein sequence. The Ery3 cDNA was expressed in Saccharomyces cerevisiae and the effects of copper concentration and cultivation temperature were investigated. S. cerevisiae cells were immobilized in calcium alginate gel and the optimal immobilization parameters for the enhanced production of laccase were determined. The immobilization was most effective with 3% sodium alginate, 0.1 M calcium chloride and an initial biomass of 4.5 x 10(8) cells. The enzyme yield obtained with immobilized cells (139 mU ml(-1)) showed a 1.6-fold increase compared to the highest yield obtained with free cells. The alginate beads showed good stability and retained 84% capacity of enzyme production after seven repeated cycles of batch fermentation. The immobilization system proved to increase the proteolytic stability of the recombinant Ery3 protein. To our knowledge, this is the first report on S. cerevisiae whole-cell immobilization for recombinant laccase production.

Heterologous expression of heterodimeric laccase from Pleurotus ostreatus in Kluyveromyces lactis

Among the laccases produced by the white-rot fungus Pleurotus ostreatus, there are two closely related atypical isoenzymes, POXA3a and POXA3b. These isoenzymes are endowed with quaternary structure, consisting of two subunits very different in size. The POXA3 large subunit is clearly homologous to other known laccases, while the small subunit does not show significant homology with any protein in data banks. To investigate on the singular structure of the POXA3 complex, a new system for recombinant expression of heterodimer proteins in the yeast Kluyveromyces lactis has been set up. A unique expression vector has been used and the cDNAs encoding the two subunits have been cloned under the control of the same bi-directionally acting promoter. Expression of the large subunit alone and co-expression of both subunits in the same host have been demonstrated and the properties of the recombinant proteins have been compared. Clones expressing the large subunit alone exhibited always notably lower activity than those expressing both subunits. In addition to the activity increase, the presence of the small subunit led to a significant increase of laccase stability. Therefore, a role of the small subunit in POXA3 stabilisation is suggested.

Laccase production at reactor scale by filamentous fungi

Laccases have received much attention from researchers during the past decades due to their broad substrate specificity and to the fact that they use molecular oxygen as the final electron acceptor instead of hydrogen peroxide as used by peroxidases. This makes laccases highly interesting for a wide variety of processes, such as textile dye decolouration, pulp bleaching, effluent detoxification, biosensors and bioremediation. The successful application of laccases to the above-mentioned processes requires the production of large quantities of enzyme at low cost. Filamentous fungi are able to produce laccases in high amounts, however, an efficient production system at bioreactor scale is still lacking. This is mainly due to the fact that laccase production by wild-type strains of filamentous fungi is linked to secondary metabolism, which implies that the following drawbacks must be overcome: uncontrolled fungal growth, the formation of polysaccharides around mycelia and the secretion of certain compounds (i.e. proteases) that inactivate laccases. This review summarizes the current status of laccase production by wild-type strains of filamentous fungi at the bioreactor scale.

Heterologous protein production in the yeastKluyveromyces lactis

Kluyveromyces lactis is both scientifically and biotechnologically one of the most important non-Saccharomyces yeasts. Its biotechnological significance builds on its history of safe use in the food industry and its well-known ability to produce enzymes like lactase and bovine chymosin on an industrial scale. In this article, we review the various strains, genetic techniques and molecular tools currently available for the use of K. lactis as a host for protein expression. Additionally, we present data illustrating the recent use of proteomics studies to identify cellular bottlenecks that impede heterologous protein expression.