s Glucose Oxidase Production from Sustainable Substrates

Background: Glucose oxidase (GoxA) catalyzes the reaction from β-D-glucose to gluconic acid. It has a wide range of applications, for example as a sugar sensor for diabetes monitoring or as a prominent additive in food industry. The fungus Aspergillus niger naturally expresses and secretes GoxA. Currently, GoxA is produced by A. niger on yeast peptone dextrose media or by yeasts on media containing sugars in high concentration. Objective: Trichoderma reesei is a well-studied, saprotrophic fungus that is used for industry-scale enzyme production due to its high secretory capacity. GoxA production in T. reesei could combine two promising aspects: high expression and secretion on the one hand, and the utilization of a sustainable and inexpensive carbon source, such as wheat straw or chitin, on the other hand. Method: To evaluate if this is a feasible concept for GoxA production we applied four different expression systems: the constitutive promoter of the pyruvate kinase-encoding gene pki1 of T. reesei, the inducible promoters of the xylanase II-encoding gene xyn2 and of the cellobiohydrolase I-encoding gene cbh1, which is considered as one of the strongest promoters known in T. reesei, and finally, the promoter of the N-acetylglucosaminidase-encoding gene nag1 of Trichoderma harzianum. Result: We discovered that an engineered variant of the cbh1 promoter led to higher yields of GoxA than the wild-type promoter did. This could be demonstrated in shake flask and bioreactor cultivation experiments. The obtained yields (between 28.90 U/ml and 39.00 U/ml) from wheat straw even exceeded the ones reported for A. niger.