Biosynthesis of orthoester natural products.

Orthoesters are unique functional groups consisting of three alkoxy substituents attached to a central carbon atom, which endow orthoester-bearing compounds with structural complexity and therapeutically valuable bioactivity. As orthoester natural products are rarely encountered in nature, their biosynthesis is poorly understood, particularly regarding the generation of their orthoester functionalities. Over the past decade, the isolation of putative biosynthetic precursors of orthoester natural products in plants and animals as well as the discovery of orthoester-forming enzymes in four species of microorganisms including bacteria and fungi have shed light on the cryptic biosynthetic machinery behind orthoester biogenesis. Protein structural analyses and biochemical enzymatic investigations have since revealed a variety of unique catalytic strategies for activating substrate scaffolds and stabilizing reactive intermediates to enable the formation of highly constrained orthoester moieties. Accordingly, this review summarizes recent advances made in the biosynthesis of orthoester natural products, highlighting the structural bases of unusual enzymatic mechanisms involved in orthoester formation. Additionally, established and putative biosynthetic pathways of prominent orthoester compounds are examined to inform future studies for discovering new orthoester natural products as well as for uncovering novel biocatalysts for the generation of orthoester compounds.