Investigation of Atypical Transformations During the Biosynthesis of Marine Cyanobacterial Natural Products.

Abstract

Naturally occurring chemicals isolated from plants and fungi have been used as medicines for millennia. Biochemical and genetic analysis of these chemically fertile organisms has led to the elucidation of protein-megasynthases responsible for the stepwise condensation of simple chemical subunits in the assembly of biologically-significant small molecules with intriguing chemical architecture. The breadth of biological activity is directly related to the vast sampling of chemical space, achieved through this combinatorial biosynthetic approach. Indeed, structural heterogeneity can be introduced through the incorporation of enzymes catalyzing distinctive biochemical transformations during initiation, elongation and termination steps of biosynthesis. Within the context of the marine cyanobacterial natural products, the linear lipopeptide curacin A is striking not only for its incredibly potent anticancer properties, but also the unique mode by which is constructed. The 2-methyl-cyclopropyl moiety, critical for pharmacological activity, is installed via a rare six-domain beta-branching insertional protein cassette. A portion of this work is focused on parsing the mechanism by which non-catalytic proteins are involved in enhancing the biosynthetic flux through this pathway. Herein we have demonstrated that a C-terminal flanking domain (Cd) serves to not only enhance intramodular interaction, by also stabilizes intermodular protein engagement. The unusual activated-elimination chain termination strategy, carried out by the curacin sulfotransferase (ST) and thioesterase (TE), is also examined for potential development as a novel bio-fuels platform. Homologous proteins in olefin synthase gene cluster from a related cyanobacterium are also characterized. Additionally, chemical and biochemical activation of fatty acid substrates for processing by the tandem functioning ST-TE enzyme pair are studied. Finally, the biosynthetic origins and preliminary biochemical characterization of proteins responsible for the antimalarial compound carmabin A are also examined.