On the origin of vanillyl alcohol oxidases

Vanillyl alcohol oxidase (VAO) is a fungal flavoenzyme that converts a wide range of para-substituted phenols. The products of these conversions, e.g. vanillin, coniferyl alcohol and chiral aryl alcohols, are of interest for several industries. VAO is the only known fungal member of the 4-phenol oxidising (4PO) subgroup of the VAO/PCMH flavoprotein family. While the enzyme has been biochemically characterised in great detail, little is known about its physiological role and distribution in fungi. We have identified and analysed novel, fungal candidate VAOs and found them to be mostly present in Pezizomycotina and Agaricomycotina. The VAOs group into three clades, of which two clades do not have any characterised member. Interestingly, bacterial relatives of VAO do not form a single outgroup, but rather split up into two separate clades. We have analysed the distribution of candidate VAOs in fungi, as well as their genomic environment. VAOs are present in low frequency in species of varying degrees of relatedness and in regions of low synteny. These findings suggest that fungal VAOs may have originated from bacterial ancestors, obtained by fungi through horizontal gene transfer. Because the overall conservation of fungal VAOs varies between 60 and 30% sequence identity, we argue for a more reliable functional prediction using critical amino acid residues. We have defined a sequence motif P-x-x-x-x-S-x-G-[RK]-N-x-G-Y-G-[GS] that specifically recognizes 4PO enzymes of the VAO/PCMH family, as well as additional motifs that can help to further narrow down putative functions. We also provide an overview of fingerprint residues that are specific to VAOs.