Elucidation of cellobiose metabolic pathway in Trichoderma reesei

里氏木霉（Trichoderma reesei）在纤维素上生长时，可表达3种β-葡萄糖苷酶（β-glucosidase，BGL），其中一种为分泌型Cel 3A，另一种为胞内型Cel 1A和Cel 3C。为了阐明这些BGL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基因的缺陷突变体，并表征了突变体在纤维素或纤维二糖上生长时的生物量和纤维素酶和BGL活性。结果表明，Cel 3A和Cel 3C对pNP-Glc的底物选择性高于对纤维二糖的底物选择性，而Cel 1A对纤维二糖的底物选择性高于pNP-Glc。突变体的细胞内和细胞外BGL活性反映了相应酶的底物特异性。我们推测Cel 1A在纤维二糖代谢中起关键作用，BGL不直接参与纤维素酶的诱导。当T.当里氏木霉在纤维二糖上生长时，除了Cel 3A之外的其它BGL似乎也被表达和分泌。

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.