Characterization of a cellobiohydrolase (MoCel6A) produced by Magnaporthe oryzae

Three GH-6 family cellobiohydrolases are expected in the genome of Magnaporthe grisea based on the complete genome sequence. Here, we demonstrate the properties, kinetics, and substrate specificities of a Magnaporthe oryzae GH-6 family cellobiohydrolase (MoCel6A). In addition, the effect of cellobiose on MoCel6A activity was also investigated. MoCel6A contiguously fused to a histidine tag was overexpressed in M. oryzae and purified by affinity chromatography. MoCel6A showed higher hydrolytic activities on phosphoric acid-swollen cellulose (PSC), β-glucan, and cellooligosaccharide derivatives than on cellulose, of which the best substrates were cellooligosaccharides. A tandemly aligned cellulose binding domain (CBD) at the N terminus caused increased activity on cellulose and PSC, whereas deletion of the CBD (catalytic domain only) showed decreased activity on cellulose. MoCel6A hydrolysis of cellooligosaccharides and sulforhodamine-conjugated cellooligosaccharides was not inhibited by exogenously adding cellobiose up to 438 mM, which, rather, enhanced activity, whereas a GH-7 family cellobiohydrolase from M. oryzae (MoCel7A) was severely inhibited by more than 29 mM cellobiose. Furthermore, we assessed the effects of cellobiose on hydrolytic activities using MoCel6A and Trichoderma reesei cellobiohydrolase (TrCel6A), which were prepared in Aspergillus oryzae. MoCel6A showed increased hydrolysis of cellopentaose used as a substrate in the presence of 292 mM cellobiose at pH 4.5 and pH 6.0, and enhanced activity disappeared at pH 9.0. In contrast, TrCel6A exhibited slightly increased hydrolysis at pH 4.5, and hydrolysis was severely inhibited at pH 9.0. These results suggest that enhancement or inhibition of hydrolytic activities by cellobiose is dependent on the reaction mixture pH.