Laccase from a Litter Degrading Fungus: Evaluation for Environmental Applications and Modification of Lignin to a Bioadhesive
Sammanfattning: Laccase, the phenol oxidizing enzyme, was first described towards the end of the 19th century from the wound sap of the lacquer tree Rhus vernicifera. The enzyme has the widest distribution in nature among the multicopper oxidases. It catalyzes single electron transfer oxidation in the presence of molecular oxygen producing water as by-product. Based on these highly desired mild oxidising conditions and wide range of substrates, immense efforts have marked the last decades on search for new enzyme sources and potential laccases that are suitable for different applications. In the present thesis, a litter degrading fungus from Galerina sp. is introduced as a laccase producer. The fungus was grown on different agricultural residues in submerged and solid state fermentations to produce high amounts of laccase. The major laccase out of four isoforms produced by the fungus was purified to homogeneity and characterized. The physical and biochemical properties are found to be similar to those of other known laccases from different fungi. The enzyme is remarkably tolerant to elevated concentrations of hydrogen peroxide, and has high affinity towards common laccase substrates. The potential of the laccase is explored for various applications with important environmental implications. Indeed, the enzyme, in both free and immobilized forms, was able to decolorize a diazo and a triphenylmethane dyes. Moreover, the Galerina sp. laccase was successfully used to demethylate hard wood Kraft lignin. Laccase mediated lignin modification is known to make it more reactive and prone to coupling. In order to understand the modifications taking place, the Galerina sp. laccase was employed to oxidize a range of lignin monomeric derivatives and facilitate their coupling to a substituted aromatic amine. The kinetics of the enzymatic oxidation was studied using calorimetry. Lignin activation by laccase has also been applied for the production of formaldehyde-free wood bioadhesives. In other words, lignin, a natural adhesive in plants, after extraction and modification, ends up as a wood adhesive outside the plants. In this context, a variety of polymeric materials (a synthetic polymer, a natural polysaccharide, and a protein) were reacted with Kraft lignin in order to form composite adhesives. The tensile strength and water resistance of the adhesive formulations were evaluated. Changes in surface properties due to the modifications were also studied by determining water sorption isotherms.
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