Abstract
Oxidative plant cell-wall processing enzymes are of great importance in biology and biotechnology. Yet, our insight into the functional interplay amongst such oxidative enzymes remains limited. Here, a phylogenetic analysis of the auxiliary activity 7 family (AA7), currently harbouring oligosaccharide flavo-oxidases, reveals a striking abundance of AA7-genes in phytopathogenic fungi and Oomycetes. Expression of five fungal enzymes, including three from unexplored clades, expands the AA7-substrate range and unveils a cellooligosaccharide dehydrogenase activity, previously unknown within AA7. Sequence and structural analyses identify unique signatures distinguishing the strict dehydrogenase clade from canonical AA7 oxidases. The discovered dehydrogenase directly is able to transfer electrons to an AA9 lytic polysaccharide monooxygenase (LPMO) and fuel cellulose degradation by LPMOs without exogenous reductants. The expansion of redox-profiles and substrate range highlights the functional diversity within AA7 and sets the stage for harnessing AA7 dehydrogenases to fine-tune LPMO activity in biotechnological conversion of plant feedstocks.
Highlights
Oxidative plant cell-wall processing enzymes are of great importance in biology and biotechnology
This notion has gained strong support by the recent discovery of lytic polysaccharide monooxygenases (LPMOs) that uniquely catalyse the oxidative cleavage of glycosidic bonds incrystalline polysaccharides such as starch[2,3,4], chitin[5], cellulose[6,7,8] and cellulose-bound hemicelluloses, e.g., xyloglucan[9] and xylan[10]
Clade I, which is the largest (34% of all sequences), is dominated by plant and fungal sequences (Fig. 1b). This clade contains plant non-carbohydrate active enzymes, e.g., the berberine bridge enzyme from Eschscholzia californica EcBBE32 and the monolignol oxidase from Arabidopsis thaliana AtBBE-like 1533, as well as the oligogalacturonide oxidase from A. thaliana AtOGOX113, none of these sequences are currently assigned into activity family 7 (AA7)
Summary
Oxidative plant cell-wall processing enzymes are of great importance in biology and biotechnology. The involvement of oxidative processes in polysaccharide degradation by fungi has been proposed by the pioneering work of Eriksson et al in 19741 This notion has gained strong support by the recent discovery of lytic polysaccharide monooxygenases (LPMOs) that uniquely catalyse the oxidative cleavage of glycosidic bonds in (semi)crystalline polysaccharides such as starch[2,3,4], chitin[5], cellulose[6,7,8] and cellulose-bound hemicelluloses, e.g., xyloglucan[9] and xylan[10]. The tertiary structures of hitherto described AA7 enzymes[15,16,17,19] share a common fold comprising an N-terminal FAD-binding domain (F domain) and a C-terminal substrate-binding domain (S domain) This fold is common within the vanillyl alcohol oxidase This fold is common within the vanillyl alcohol oxidase (VAO, EC. 1.1.3.38) super family[20] that harbours AA7
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