Elucidation of cellobiose metabolic pathway in Trichoderma reesei

里氏木霉（Trichoderma reesei）以产生大量纤维素酶而闻名，当生长在纤维素上时，它表达三种β-葡萄糖苷酶（BGLs），其中一种是分泌的Cel3A，另一种是细胞内的Cel1A和Cel3C。为了阐明这些BGLs的生理作用，我们研究了异源宿主产生的重组酶的酶学性质。此外，我们构建了相应的BGL基因缺陷突变体，并表征了突变体在纤维素或纤维素二糖上生长时的生物量、纤维素酶和BGL活性。结果表明，相比于纤维素二糖，Cel3A和Cel3C更倾向于pNP-Glc作为底物，而Cel1A对纤维素二糖的活性高于pNP-Glc。突变体的细胞内和细胞外BGL活性反映了各自酶的底物特异性。我们假设Cel1A在纤维素糖代谢中起关键作用，而BGLs不会直接参与纤维素酶诱导。当T. reesei在纤维素二糖上生长时，除Cel3A外，其他BGL(s)似乎也有表达和分泌。

Trichoderma reesei, which is well known to produce large amount of cellulases, expresses threeβ-glucosidases(BGLs), one of which is secreted Cel3A and the other are intracellular Cel1A and Cel3C, when grown on cellulose. To elucidate the physiological roles of these BGLs, we investigated the enzymatic properties of the recombinant enzymes produced by heterologous hosts. Furthermore, we constructed the deficient mutants of the corresponding BGL genes and characterized the amount of biomass and the cellulase and BGL activities of the mutants when grown on cellulose or cellobiose. The results demonstrated that Cel3A and Cel3C preferred pNP-Glc to cellobiose as the substrate, whereas Cel1A had higher activity for cellobiose than that for pNP-Glc. The intracellular and extracellular BGL activities of the mutants reflected the substrate specificity of the respective enzymes. We assumed that Cel1A plays the key role in cellobiose metabolism and the BGLs would not directly involve in cellulase induction. When T. reesei is grown on cellobiose, the other BGL(s) besides Cel3A seems to be expressed and secreted.