Feruloyl esterases substrate specificity

Feruloyl esterase activity was first detected in culture filtrates of Strepto-myces olivochromogenes (49), and has thereafter also been reported for some hemicellulolytic fungi (Table III). A partially purified feruloyl esterase from S. commune liberated hardly any ferulic acid without the presence of xylanase (65). Very recently a feruloyl esterase was purified from Aspergillus oryzae (Tenkanen, M. Schuseil, J. Puls, J. Poutanen, K., /. Biotechnol, in press). The enzyme is an acidic monomeric protein having an isoelectric point of 3.6 and a molecular weight of 30 kDa. It has wide substrate specificity, liberating ferulic, p-coumaric, and acetic acids from steam-extracted wheat straw arabinoxylan. 

Several feruloyl esterases have been purified and characterized (Table 1). However, comparison of their properties is difficult as the range of natural and synthetic substrates used to characterize these enzymes is diverse and the enzyme assays are not unifomi. The substrates range in size and complexity from small, soluble esters such as feruloylated oligosaccharides isolated from plant cell walls and phenolic acid methyl esters or synthetic feruloylated arabinosides to larger, more complex and often less soluble substrates such as feruloylated polymeric plant cell wall fractions (28). The only criterion used in all cases is the release of free ferulic acid or another hydroxycinnamic acid by hydrolysis of an ester bond. Specificity, as defined by Ae rate of catalysis (kca divided by the Michaelis constant gives the best indication of preferred substrates. However, hydrolysis of polymeric substrates is more complicated since not all of the esterified substituents are chemically equal, and effects such as decreased solubility and steric hindrance further complicates any results obtained. Therefore, these data should not be extrapolated to obtain kinetic constants. 

The use of small, soluble substrates allows the determination of kinetic constants, giving some information on the affinity (from Km values) and catalytic efficiency (kcat Vmax / m)- Substrates in question are effectively two components joined by an ester bond the phenolic component and the sugar moiety. Specificity for both of these components defines the overall catalytic rate of the reaction. The selectivity for each component gives important information for the classification on feruloyl esterases (12). 

Feruloyl esterases show intriguing differences in substrate specificity and sequence structure. The main reason for the recent increase in interest in these enzymes is their potential application in the production of ferulic acid. Ferulic acid is an important precursor in the flavor industry (77) and has multiple uses as an ultraviolet light protection agent in sun creams and cosmetics (72) by suppressing inflammatory responses and skin tumor development (75). Feruloyl esterases can also be used to selectively remove ferulic acid from agro-industrial waste products on an industrial scale 74). Ferulic acid is thought to be involved... 

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