The glucose-methanol-choline (GMC) oxidoreductase enzyme group was first introduced in 1992. All members of this group are flavin-adenine dinucleotide (FAD) dependent oxidoreductases and can be further subdivided into oxidases and dehydrogenases. Oxidases transfer electrons to molecular oxygen by producing hydrogen peroxide, which is often metabolized by lignocellulose degrading enzymes, whereas the dehydrogenases transfer electrons to e.g., hydroquinones and show only low reactivity with oxygen. Phylogenetically, they share high sequence similarity; all members contain the canonical Rossman fold. The Rossman fold is a secondary structure motif that is made up of an alpha-helix and two beta-strands and binds the catalytically active FAD. The aryl-alcohol oxidases (AAO) and the pyranose dehydrogenases (PDH) are two representative enzyme families of the GMC family. Both are monomeric enzymes containing an N-terminal catalytic FAD-binding domain and a C-terminal putative substrate-binding domain. PDHs have their catalytic cofactor covalently bound to a His residue whereas FAD is non-covalently attached to AAOs. The first AAO was isolated from various Pleurotus species and can be found in many wood degrading fungi. They typically oxidize the primary alcohol of aromatic or aliphatic unsaturated alcohols to the corresponding aldehydes. The first PDH was found in mycelial cultures of the fungi Agaricus bisporus and is also found in white-rot wood-decomposing fungi. The substrate specificity comprises a vast number of both aldoses and ketoses but also oligosaccharides. Interestingly, on a phylogenetic tree calculated in our group the two families of AAO and PDH cluster closely together, in detail PDHs might have evolved from AAOs suggesting that only small sequence changes lead to a shift of oxygen reactivity and substrate specificity. The aim of this work is to characterize several AAO/PDH enzymes, since only a few enzymes have been biochemically characterized so far. Furthermore, the analysis aims to investigate and understand the structure-function relationships of these enzymes. In addition, it seeks to explain how amino acids in the active site are responsible for oxygen activity and how the transition between AAOs and PDHs occurs. Golden gate cloning, P. pastoris expression and biochemical methods such as SDS-PAGE and Western blot are used to address these questions. Furthermore, enzyme activity was observed using three different assays. Finally, the identified enzymes were purified using IMACChromatography. Based on these results all samples were successfully transferred in E. coli and P. pastoris. From 24 constructs seven could be identified with native- and alpha signals. However, no enzymatic activity could be detected in these samples. IMAC-chromatography was used for purification and the purity was checked via SDS-PAGE.