The substrate-dependent stereoselectivity of the multicopper oxidase (MCO)-catalyzed oxidative coupling in the biosynthesis of the bisnaphthopyrone viriditoxin

VdtB, the multiple-copper oxidase (MCO) from the bisnaphthopyrone (M)-viriditoxin biosynthetic pathway in Paecilomyces variotii, was shown to catalyze regioselective 6,6′-coupling of semi-viriditoxin (1). The stereoselectivity of the oxidative coupling reaction for the production of the atropisomer (M)-viriditoxin, however, was controlled by VdtD, a non-catalytic dirigent protein from the pathway. In this work, VdtB either alone or together with VdtD were investigated for its stereoselective control upon coupling of other monomeric naphthopyrone derivatives from the pathway with different minor structural variations in terms of presence/absence of O-methylation at C7-position and C3-C4 Δ2 double bond on the pyrone ring, and the different side-chain modifications. We showed that VdtB could favour either M- or P-form coupling in a substrate-dependent manner. For some substrates, VdtB could catalyze oxidative coupling in an enantiomerically selective manner. The efficiency of the VdtD in exerting stereoselective control of the oxidative coupling reaction also varies between substrates. The results point to a model whereby VdtB and VdtD form a VdtB-ligand-VdtD complex in which the stereochemical outcome of the coupling reaction depends on how the substrate interacts with both proteins, based on the substrate structure. Our findings contributed to a more comprehensive understanding of dirigent protein-mediated MCO-catalyzed stereoselective oxidative coupling reactions in fungi.