************ The 8th Lab Seminar ***************************************

Title : Vinylogous chain branching catalysed by a dedicated polyketide 
synthase module
Tom Bretschneider, Joel B. Heim, Daniel Heine, Robert Winkler, 
Benjamin Busch, Bjo(..)rn Kusebauch, Thilo Stehle, Georg Zocher and 
Christian Hertweck: Nature, 502, 124-128 (2013)

Speaker : Ikuo KURISAKI

Date : Thu. Oct. 24th, 1:30 pm

Place : Seminar room of 4F, Graduate School of Information Science

Abstract :
Bacteria use modular polyketide synthases (PKSs) to assemble complex 
polyketides, many of which are leads for the development of clinical 
drugs, in particular anti-infectives and anti-tumoral agents1. Because 
these multifarious compounds are notoriously difficult to synthesize, 
they are usually produced by microbial fermentation. During the past two 
decades, an impressive body of knowledge on modular PKSs2, 3 has been 
gathered that not only provides detailed insight into the biosynthetic 
pathways but also allows the rational engineering of enzymatic processing 
lines to yield structural analogues4, 5. Notably, a hallmark of all PKS 
modules studied so far is the head-to-tail fusion of acyl and malonyl 
building blocks, which leads to linear backbones. Yet, structural 
diversity is limited by this uniform assembly mode. Here we demonstrate 
a new type of PKS module from the endofungal bacterium Burkholderia 
rhizoxinica that catalyses a Michael-type acetyl addition to generate a 
branch in the carbon chain. In vitro reconstitution of the entire PKS 
module, X-ray structures of a ketosynthase-branching didomain and 
mutagenesis experiments revealed a crucial role of the ketosynthase 
domain in branching the carbon chain. We present a trapped intermediary 
state in which acyl carrier protein and ketosynthase are covalently 
linked by the branched polyketide and suggest a new mechanism for chain 
alkylation, which is functionally distinct from terpenoid-like ƒÀ-
branching. For the rice seedling blight toxin rhizoxin, one of the 
strongest known anti-mitotic agents, the non-canonical polyketide 
modification is indispensable for phytotoxic and anti-tumoral activities.
 We propose that the formation of related pharmacophoric groups follows 
the same general scheme and infer a unifying vinylogous branching 
reaction for PKS modules with a ketosynthase-branching-acyl-carrier-
protein architecture. This study unveils the structure and function of a 
new PKS module that broadens the biosynthetic scope of polyketide 
biosynthesis and sets the stage for rationally creating structural 
diversity.

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