Feruloyl esterases - Evaluation of their potential for biotechnological applications

Abstract: Owing to the current efforts to find sustainable alternatives to petrochemical based industries and technologies, enzymatic degradation and valorization of plant biomass has been attracting interest. Due to the complexity of plant biomass, an array of enzymes is required to hydrolyze it, including esterases. Among the esterases involved, feruloyl esterases, which are able to release ferulic acid, were of special interest in this work. Industrial processes aim for enzymes to be as efficient as possible in the designed process conditions, i.e. able to perform chemical reactions for as long as possible at the lowest possible cost. Several strategies can be employed to reach these goals, such as (i) finding novel enzymes with the desired properties, (ii) optimizing enzyme production, or (iii) immobilizing enzyme for improved stability or reusability. These strategies were applied in this work to investigate the potential of some feruloyl esterases for industrial applications. Based on functional annotations, targets originating from microorganisms found in diverse ecological niches were selected. In one study, five putative feruloyl esterases/tannases from two Aspergillus fungi were selected. In another study, two multi-domain enzymes displaying two predicted esterase domains from the polysaccharide utilization loci of bacteria in the Bacteroidetes phylum were investigated. The enzymes displayed differences in their preferred reaction conditions (pH, temperature), molecular weights, predicted isoelectric points, as well as substrate preferences. The impact of the production host on the final enzyme properties was investigated in an additional study. We demonstrated that in the case of glycosylated enzymes, careful selection of the production host is crucial for thermostability. Studying immobilization, data showed that the best immobilization yield and the best immobilized enzyme performance were not achieved in the same conditions for any of the enzyme-support couples tested. Investigations of immobilized enzyme transesterification or hydrolysis activities clearly demonstrated that immobilization does affect the catalytic activity of enzymes. In the current status of our knowledge, the way an enzyme is affected by immobilization is not predictable. Increased knowledge about esterase structures, reaction mechanisms and surface properties may however allow such predictions in the future. This thesis contributes to increasing the available information about esterases, and in particular feruloyl esterases.