Mechanisms of glycosyl transferases and hydrolases

Diferulic acid forms cross-links in naturally occurring plant cell wall polymers such as arabinoxylans and pectins. We have used model ethyl esterified substrates to find enzymes able to break these cross-links. A tannase from Aspergillus oryzae exhibited esterase activity on several synthetic ethyl esterified diferulates. The efficiency of this esterase activity on most diferulates is low compared to that of a cinnamoyl esterase, FAEA, from Aspergillus niger. Of the diferulate substrates assayed, tannase was most efficient at hydrolysing the first ester bond of the 5¯5- type of dimer. Importantly and unlike the cinnamoyl esterase, tannase from A. oryzae is able to hydrolyse both ester bonds from the 8¯5-benzofuran dimer, thus forming the corresponding free acid product. These results suggest that tannases may contribute to plant cell wall degradation by cleaving some of the cross-links existing between cell wall polymers.