Xylan-degrading enzymes from Penicillium pinophilum : purification and characterization of a feruloyl/ρ-coumaroylester esterase
Growth of Penicillium pinophilum in solid state culture on oat straw and wheat bran mixtures was found suitable for the production of xylan-degrading enzymes. An extracellular phenolic acid esterase and a xylanase were purified to homogeneity by a combination of anion-exchange and hydrophobic interaction chromatography. The physicochemical properties and the mode of action of the esterase on various xylan polysaccharides were investigated. The esterase had an apparent molecular mass of 57 kDa by SDS-PAGE and an isoelectric pH of 4.6. The enzyme released ferulic and ρ-coumaric acid from methyl esters of the acids: temperature and pH optima were 55°C and 6.0, respectively. Cu^2+ and Fe^2+ were inhibitory, but Zn^2+ , Ca^2+ , Ag^+ and Mg^2&43 stimulated activity. The enzyme had a higher affinity towards the methyl ester of ρ-coumaric acid than towards the methyl ester of ferulic acid: the apparent Km for the ferulic acid ester was 0.14 mM and 0.08 mM for ρ-coumaric acid ester; V_max values were 32.57 and 37.96 μmol min-1 mg-1 respectively. The enzyme was also able to release all the alkali-extractable feruloyl and 80% of the ρ-coumaroyl groups from a water-soluble wheat straw xylan without degrading the polysaccharide, but it showed no capacity to release ferulic and ρ-coumaric acid from a grass cell wall preparation in the absence of other xylanolytic enzymes. Synergistic effects were observed between the esterase and other xylan-degrading enzymes in hydrolysing the ferulic and, to a lesser extent, ρ-coumaric acid esters from both wheat straw xylan and the grass cell walls preparation. However, the release of ester-linked phenolic acids by the esterase did not enhance the hydrolysis of wheat straw xylan to reducing sugars by the xylanase purified from the same fungus.