Vanillyl alcohol oxidases produced in Komagataella phaffii contain a highly stable non- covalently bound anionic FAD semiquinone

Vanillyl alcohol oxidase (VAO) from Penicillium simplicissimum is a covalent flavoprotein that has emerged as a promising biocatalyst for the production of aromatic fine chemicals such as vanillin, coniferyl alcohol and enantiopure 1-(4’-hydroxyphenyl) alcohols. The largescale production of this eukaryotic enzyme in Escherichia coli has remained challenging thus far. For that reason an alternative, eukaryotic expression system, Komagataella phaffii, was tested. Additionally, to produce novel VAO biocatalysts, we screened genomes for VAO homologues. One bacterial and five fungal sequences were selected for expression, using key active site residues as criteria for their selection. Expression of the putative vao genes in K. phaffii was successful, however expression levels were low (1 mg per litre of culture). Surprisingly, all purified enzymes were found to contain a highly stable, non-covalently bound anionic FAD semiquinone that could not be reduced by dithionite or cyanoborohydride. Activity experiments revealed that VAO expressed in K. phaffii does not produce vanillin because the enzyme suffers from oxidative stress.