Feruloyl esterases (FAEs, EC 220.127.116.11) represent a subclass of carboxylic acid esterases that under normal conditions catalyze the hydrolysis of the ester bond between hydroxycinnamic acids (ferulic acid, sinapic acid, caffeic acid, p-coumaric acid) and arabinose residues in plant cell walls. Based on their specificity towards monoferulates and diferulates, substitutions on the phenolic ring and on their amino acid sequence identity, they have been classified into four types (A-D). The use of FAEs as accessory enzymes for the degradation of lignocellulosic biomass and their synergism with other hemicellulases has been studied for application in many industries, such as the food, the biofuel and the paper pulp industry. In the recent years, the use of FAEs as biosynthetic tools has been underlined. Under low water content, these enzymes are able to catalyze the esterification of hydroxycinnamic acids or the transesterification of their esters resulting in compounds with altered lipophilicity, revealing a great potential for tailor-made modification of natural antioxidants for use in cosmetic, cosmeceutical and pharmaceutical industries.
The first part of the thesis is focused on the optimization of reaction conditions for the synthesis of two bioactive esters: prenyl ferulate and L-arabinose ferulate using 5 FAEs (FaeA1, FaeA2, FaeB1, FaeB2 and MtFae1a) from Myceliophthora thermophila in detergentless microemulsions. Reaction conditions were optimized investigating parameters such as the medium composition, the substrate concentration, the enzyme load, the pH, the temperature and the agitation. Regarding the synthesis of prenyl ferulate, FaeB2 offered the highest transesterification yield (71.5±0.2%) after 24 h of incubation at 30oC using 60 mM vinyl ferulate (VFA), 1 M prenol and 0.02 mg FAE/mL in a mixture comprising of 53.4: 43.4: 3.2 v/v/v n-hexane: t-butanol: 100 mM MOPS-NaOH pH 6.0. At these conditions, the competitive hydrolysis was 4-7 fold minimized. Regarding the synthesis of L-arabinose ferulate, FaeA1 offered highest transesterification yield (35.9±2.96%) after 8 h of incubation at 50oC using 80 mM VFA, 55 mM L-arabinose and 0.02 mg FAE/mL in a mixture of 19.8: 74.7: 5.5 v/v/v n-hexane: t-butanol: 100 mM MOPS-NaOH pH 8.0. It was revealed that the type B FAEs from M. thermophila show higher preference to more lipophilic acceptors like prenol, while the type A FaeA1 was more efficient in the synthesis of the more hydrophilic L-arabinose ferulate.
The second part of the thesis is focused on the investigation of the basis of the type A classification of a well-studied FAE from Aspergillus niger (AnFaeA) by comparing its activity towards methyl and arabinose hydroxycinnamate esters. For this purpose, L-arabinose ferulate and caffeate were synthesized enzymatically. kcat/Km ratios revealed that AnFaeA hydrolyzed arabinose ferulate 1600 times and arabinose caffeate 6.5 times more efficiently than methyl esters. This study demonstrated that short alkyl chain hydroxycinnamate esters which are used nowadays for FAE classification can lead to activity misclassification, while L-arabinose esters could potentially substitute synthetic esters in classification.
Luleå: Luleå University of Technology, 2017.