One‐Pot Enzymatic Synthesis of Merochlorin A and B

Abstract

The polycyclic merochlorin A and B are complex halogenated meroterpenoid natural products with significant antibacterial activities that are produced by the marine bacterium Streptomyces sp. strain CNH-189. Here we employ heterologously produced enzymes and chemical synthesis to fully reconstitute the merochlorin biosynthesis in vitro. The interplay of a dedicated type III polyketide synthase, a prenyl diphosphate synthase, and an aromatic prenyltransferase allow formation of a highly unusual aromatic polyketide-terpene hybrid intermediate that features an unprecedented branched sesquiterpene moiety. As supported by in vivo experiments, this precursor is furthermore chlorinated and cyclized to merochlorin A and isomeric merochlorin B by a single vanadium-dependent haloperoxidase, thus completing the remarkably efficient pathway.

Enzymatic Halogenation and Dehalogenation Reactions: Pervasive and Mechanistically Diverse

“…Surprisingly, only four enzymes (Mcl17, 22–24) are required for the complete biosynthesis of 74 and 75 from the biosynthetic precursors malonyl-CoA, dimethylallyl pyrophosphate (DMAPP), and geranyl pyrophosphate (GPP). 230 The prenyl diphosphate synthase Mcl22 was shown to catalyze the unusual “head-to-torso” coupling of DMAPP and GPP to form the newly established sesquiterpene isosesquilavandulyl pyrophosphate. The prenyltransferase Mcl23 then attaches the isosesquilavandulyl moiety to the aromatic tetrahydroxynaphthalene scaffold, previously formed through the activity of the type III polyketide synthase Mcl17, to yield the triene intermediate pre-merochlorin ( 78 ).…”

“…The prenyltransferase Mcl23 then attaches the isosesquilavandulyl moiety to the aromatic tetrahydroxynaphthalene scaffold, previously formed through the activity of the type III polyketide synthase Mcl17, to yield the triene intermediate pre-merochlorin ( 78 ). 230 The identification of 78 suggested an oxidative cyclization event must occur in order to form the polycyclic cores of 74 and 75 . This was confirmed by in vitro characterization of Mcl24, wherein incubation with enzymatically prepared 78 afforded both 74 and 75 .…”

Oxidative Cyclization in Natural Product Biosynthesis

“…535,536 Installation of a rearranged farnesyl moiety by Mcl23 creates the substrate 811 for haloperoxidase-mediated chlorinations. 537,538 Mlc24, the vanadium-dependent chloroperoxidase, catalyzes a site-selective naphthol chlorination to 812a and initiates oxidative dearomatization to afford cation intermediate 812b . This triggers terpene cyclization to create the possible cation 813 .…”

Extreme promiscuity of a bacterial and a plant diterpene synthase enables combinatorial biosynthesis

“…However, it has recently been shown that the cisoid analog ( Z,Z,Z )-nerylneryl diphosphate (NNPP, 2 ) can serve a precursor as well (Zi et al, 2014b). This result suggests that diterpenes may be more broadly derived from various precursors with a variety of double-bonds configurations and potentially even irregularly (i.e., not just head-to-tail) joined isoprenyl units, as suggested by the production of such compounds by certain isoprenyl diphosphate synthases (Noike et al, 2008; Teufel et al, 2014). Nevertheless, among diterpenes the labdane-related super-family stands out for its sheer size, as these comprise almost 7,000 of the 12,000 known such natural products (Peters, 2010).…”