Structural enzymology of a <i>Fusarium graminearum</i> aldehyde oxidase reveals a distinct active-site and reactivity versus its paralog galactose oxidase

Copper radical oxidases (CROs), which comprise Auxiliary Activity Family 5 (AA5) in the Carbohydrate-Active Enzymes (CAZy) classification, have a long history of study due to their unique catalytic mechanism and biotechnological applications. The majority of mechanistic and structural insights into CRO function have been obtained from studies on the galactose 6-oxidase from the fungal phytopathogen Fusarium graminearum (FgrGalOx) of AA5 subfamily 2 (AA5_2). In contrast, enzyme structure/function studies of CROs from subfamily 1, comprising glyoxal oxidases, are limited. Here, we report the biochemical characterisation of the individual AA5_1 members from F. graminearum and Colletotrichum graminicola, which exhibit predominant activities on aldehydes, such as methylglyoxal, and enantioselectivity for d-glyceraldehyde. Electron paramagnetic resonance indicated that the AA5_1 aldehyde oxidases possessed similar copper coordination geometry to AA5_2 CROs, including a canonical cross-linked Tyr-Cys residue. However, the X-ray crystal structure of the F. graminearum aldehyde oxidase-the first of a fungal AA5_1 CRO-strikingly revealed that a key radical-stabilising tryptophan side chain in the second coordination sphere is provided by a different position in the polypeptide chain and exists in a flipped orientation vis-à-vis AA5_2 members. Quantum mechanics/molecular mechanics (QM/MM) calculations demonstrated that, in contrast to the AA5_2 GalOx, the AA5_1 aldehyde oxidase does not delocalise spin density onto the second-sphere tryptophan as a consequence of this alternative active-site arrangement. Together, these data provide new molecular insight into catalytic selectivity among the distinct subfamilies of alcohol- and aldehyde-specific CROs, which will facilitate elucidation of their biological roles and inform their application as biocatalysts.