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Esterases form a class of enzymes catalysing the hydrolysis of different types of esters. Steryl esterases are a sub-class of esterases that primarily catalyse the hydrolysis of fatty acid esters of sterols, i.e. steryl esters. Steryl esterases may also be active against other substrates containing ester linkages, such as triglycerides, and thus they can also be classified as lipases. Steryl esterases and lipases capable of modifying different types of esters have potential applications in the food, textile, and pulp and paper industries. In this work novel steryl esterases were characterised and their usefulness in modification of fibre products was preliminarily evaluated. Commercial lipase preparations were tested for their ability to degrade steryl esters. Lipases from yeast Candida rugosa and bacteria Chromobacterium viscosum and Pseudomonas sp. were shown to have the highest steryl esterase activities, and they were able to hydrolyse steryl esters totally in the presence of surfactant. Up to 80-90% of the steryl esters were also degraded by lipases from C. rugosa and Pseudomonas sp. in the absence of surfactant. The steryl esterase of C. rugosa lipase preparation was purified and identified as the lipase LIP3, a well-known lipase having steryl esterase activity. LIP3 was found to be highly active against plant-derived steryl esters, especially in the presence of surfactant. In aqueous dispersions, all the different steryl esters were hydrolysed equally but total hydrolysis of steryl esters was not achieved. A novel steryl esterase from filamentous fungus M. albomyces was purified and biochemically characterised. The enzyme had broad substrate specificity for different steryl esters, p-nitrophenyl esters and triglycerides. The pH optimum was dependent on the substrate and varied within the pH range 5-7. The enzyme was more stable at lower pH than at alkaline values. It had a half-life of 2 h at 70°C, and thus was rather thermostable. The enzyme was highly active on steryl esters and triglycerides in the presence of surfactant, whereas only triglycerides were degraded in the absence of surfactant. The gene encoding M. albomyces steryl esterase (ste1) was isolated in order to express the protein heterologously in Pichia pastoris and Trichoderma reesei. The mature M. albomyces steryl esterase with a length of 545 amino acids appeared to be significantly related to other lipases and esterases. The level of amino acid sequence identity of the M. albomyces steryl esterase was 47% with the C. rugosa lipase LIP3. The production level in P. pastoris was very low, and a significant proportion of the total activity was found to be present intracellularly. Heterogeneous overglycosylation of the intracellular enzyme indicated that STE1 enters the secretory pathway but is not fully secreted. The M. albomyces steryl esterase was also expressed in the filamentous fungus T. reesei under the inducible cbh1 promoter. In a laboratory-scale fermenter, the main portion of activity was in the culture supernatant and a production level of 280 mg l-l was achieved. The recombinant steryl esterase (rSTE1) was characterised and its properties were compared to those of the native enzyme. The recombinant enzyme was shown to be a dimer with a molecular weight of 120 kDa, whereas the native enzyme has a tetrameric structure with a molecular weight of 238 kDa. The recombinant enzyme was somewhat less stable at higher temperatures and had slightly lower specific activities against various substrates than the native enzyme. The effects of rSTE1 on wood extractives, as well as on polyethylene terephthalate (PET) fabric, were preliminarily evaluated. The tensile strength and hydrophilicity of the paper were increased by rSTE1 due to the hydrolysis of esters. The rSTE1 treatment increased significantly the polarity of PET fabric. Dyeing of PET with methylene blue was also slightly improved after rSTE1 treatment. Improved textile properties of PET by steryl esterase treatment showed that esterases are interesting tools for the modification of synthetic materials in addition to their natural substrates.