Dihydrokalafungin (Antibiotic YS-02931K-β) is a benzoisochromanequinone that used as a natural substrate in enzyme assays of the ActVB-ActVA system[1].
The quinone form of Dihydrokalafungin is not oxidized by the ActVA-ActVB system, whereas the corresponding hydroquinone is an excellent substrate[1].
References:
[1]. Julien Valton, et al. An aromatic hydroxylation reaction catalyzed by a two-component FMN-dependent Monooxygenase. The ActVA-ActVB system from Streptomyces coelicolor. J Biol Chem. 2006 Jan 6;281(1):27-35.