Combinatorial biosynthesis of reduced polyketides

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Abstract
The reduced polyketides are a medicinally important group of metabolites, which are assembled in bacteria on gigantic multienzyme polyketide synthases (PKSs). These PKSs typically contain a single enzymatic domain for each step in polyketide construction, and so appear to be 'modular', on the genetic level at least. The division-of-labour organization of the PKSs suggests that they might be genetically engineered to produce polyketide analogues, with potential as drug candidates. The idea of rearranging PKS components in as many ways as possible to generate large libraries of new compounds is known as 'combinatorial biosynthesis'. Although genetic engineering has been shown repeatedly to work in practice, generating over 200 novel structures, many experiments produce low yields of the expected novel polyketides, or fail entirely. The goal of truly combinatorial engineering, though theoretically achievable, remains distant. Therefore, a central goal in the field is to obtain a significantly deeper understanding of the operation and three-dimensional architecture of the PKSs to increase the efficiency of engineering experiments. Sequencing of additional gene clusters has revealed that PKSs are far more diverse and complex than initially appreciated. Other important research areas include developing heterologous hosts for polyketide production, which are capable of post-translational modification of PKS proteins and which contain all of the necessary small-molecule precursors; improving the genetic tools required to manipulate and transfer large PKS genes and gene clusters; and identifying PKS domains and post-PKS tailoring enzymes which are inherently suited to combinatorial applications, due to their relaxed substrate specificities. Significant progress has been made in all of these areas; of particular note are the first high-resolution structures of individual PKS components and the metabolic engineering of E. coli for polyketide production. Advances in engineering strategies have also resulted in a biological route to the important antiparasitic agent ivermectin.