A cinnamoyl esterase from Aspergillus niger can break plant cell wall cross-links without release of free diferulic acids

A cinnamoyl esterase, ferulic acid esterase A, from Aspergillus niger relea ses ferulic acid and 5-5- and 8-O-4-dehydrodiferulic acids from plant cell walls. The breakage of one or both ester bonds from dehydrodimer cross-link s between plant cell wall polymers is essential for optimal action of carbo hydrases on these substrates, but it is not known if cinnamoyl esterases ca n break these cross-links by cleaving one of the ester linkages which would not release the free dimer. It is difficult to determine the mechanism of the reaction on complex substrates, and so we have examined the catalytic p roperties of ferulic acid esterase A from Aspergillus niger using a range o f synthetic ethyl esterified dehydrodimers (5-5-, 8-5-benzofuran and 8-O-4- ) and two 5-5-diferulate oligosaccharides. Our results show that the estera se is able to cleave the three major dehydrodiferulate crosslinks present i n plant cell walls. The enzyme is highly specific at hydrolysing the 5-5- a nd the 8-5-benzofuran diferulates but the 8-O-4-is a poorer substrate. The hydrolysis of dehydrodiferulates to free acids occurs in two discrete steps , one involving dissociation of a monoesterified intermediate which is nega tively charged at the pH of the reaction. Although ferulic acid esterase A was able to release monoesters as products of reactions with all three form s of diesters, only the 5-5- and the 8-O-4-monoesters were substrates for t he enzyme, forming the corresponding free diferulic acids. The esterase can not hydrolyse the second ester bond from the 8-5-benzofuran monoester and t herefore, ferulic acid esterase A does not form 8-5-benzofuran diferulic ac id. Therefore, ferulic acid esterase A from Aspergillus niger contributes t o total plant cell wall degradation by cleaving at least one ester bond fro m the diferulate cross links that exist between wall polymers but does not always release the free acid product.