# Naphthoquinone-Based Meroterpenoids from Marine-Derived _Streptomyces_ sp. B9173
## 4 Discussion  
Marine microbial metabolites are a prevalent source of bioactive natural products and a number of entities have been identified with promising therapeutic potential [1; 28]. Taking natural naphthoquinone meroterpenoids, for example, much research has been carried out to test differences in their biological activity and elucidate their pharmacological profiles. In this work, we checked the metabolite profiles of marine-derived _Streptomyces_ sp. B9173 in MS medium, set up the large-scale fermentation, and performed subsequent isolation. Chemical investigations into the fermentation cultures led to the isolation of a group of naphthoquinone-containing compounds. Among them, **1-3** are previously unreported naphthoquinone-containing derivatives, and others are biogenetically related compounds including flaviogeranin A and substituted 5-hydroxy-1,4-naphthoquinones [10; 25; 26]. The carbon skeleton of naphthoquinone has been established from several metabolic pathways. One is the direct incorporation of chorismate, derived from the shikimate pathway via formation of 1,4-dihydroxy-2-naphthoate intermediate [29] or through a futalosine intermediate [30]. The other pathway is through a common THN intermediate which was formed by type III PKS catalyzing Claisen and aldol condensation [12]. From the point view of the fundamental biosynthetic basis, naphthoquinone-derived meroterpenoids share a common paradigm in which the THN is formed and subsequently attached with a terpenoid moiety. THN is preceded by the biosynthesis of several hybrid meroterpenoids (furaquinocin and napyradiomycin), and previous precursor incorporation studies demonstrated the polyketide biosynthetic origin of THN. Additionally, it utilizes the mevalonate pathway for the production of IPP for terpenoids [31]. As seen in flaviogeranin biosynthesis, core enzymes including type III PKS and prenyltransferases contribute to the naphthoquinone-terpenoid skeleton formation while tailoring enzymes such as methyltransferases, oxidases, etc., accounting for the structural diversity. It is conceivable that hydroxylation at the C8 results in intermediate flaviogeranin C1 (**4**) and flaviogeranin C2 (**5**). Isolation of **1** and **2** suggests that 8-amino-flaviolin serves as a common precursor and an aminotransferase catalyzes the introduction of the amino group to the C8 of naphthoquinone core. A similar aminotransferase was seen in the furaquinocin biosynthetic pathway and 8-amino-flaviolin can be detected in the heterologous expression system consisting of three contiguous genes (Type III PKS, monooxygenase, and aminotransferase) [15; 23]. Since no furaquinocin congeners with the C8 amino group have been reported until now, the isolation of the amino group containing compounds **1** and **2** in our work confirmed the \"cryptic transamination\" and indicated an independent biosynthetic pathway through the precursor 8-amino-flaviolin. The common and differentiating features of these flaviogeranin compounds are related to their bioactivities. The biological activities of four flaviogeranins (**1-3** and **5**) were tested for antibacterial activity and cytotoxicity. Both **3** and **5** show potent activity while **1** and **2**, with the amino group at the C8, exhibit weaker activity. The true function of transamination at the C8 of naphthoquinone remains unclear. It is suggested that such modification may confer self-protection for the producers or facilitate the catalytic efficiency of specific enzymes during biosynthesis. The remarkable structural variations observed among flaviogeranin congeners derive from the catalytic activities of diverse enzymes for structural decoration. Multiple modification enzymes function on the common intermediate THN to generate diverse flaviogeranin congeners. The isolated flaviogeranin congeners and intermediates further unveil the biosynthetic steps and new modification enzymes for flaviogeranin family synthesis.