Identification of a preferential substrate in glucose dehydrogenase from Pycnoporus cinnabarinus: a "sugar sweet" structural approach

Fungus-derived glucose dehydrogenases (GDHs) are FAD-dependent enzymes belonging to the glucose-methanol-choline (GMC) oxidoreductase superfamily. GDHs are the most popular enzymes used in glucose sensor strips manufactured for glycemic control by diabetic patients. These enzymes are classified in the “Auxiliary activities” of the Carbohydrate-Active Enzymes (CAZy) database, since they act together with laccases in the process of lignocellulose degradation. Among fungi, the Basidiomycota Pycnoporus cinnabarinus is able to completely degrade lignin. We solved the crystal structure of GDH from P. cinnabarinus in the ligand-free form and in complex with glucose. The glucose-bound structure revealed two glucose molecules in close proximity to the FAD cofactor. Their orientation guided us to investigate the activity of the enzyme towards the disaccharide laminaribiose. Structural data on the laminaribiose-bound GDH showed a well-defined orientation of the disaccharide in the active site. Remarkably, the functional analysis revealed that the preferential substrate of this enzyme is indeed laminaribiose, with a 90% higher activity compared to glucose. Since beta-1,3 glucans are products of lignocellulose degradation, our structure-function analysis is consistent with the hypothesis of a fine tuning of the gene regulation and expression to adapt fungal organisms to the diversity of their natural substrates.