Polyketide‐Chain Branching by an Enzymatic Michael Addition

Abstract

A new "branch" for polyketide synthases was discovered in the biosynthesis of the antimitotic rhizoxin complex in the endofungal bacterium Burkholderia rhizoxinica. Genetic engineering and the structural elucidation of pathway intermediates revealed that a complex polyketide chain is branched at the beta position by an unprecedented conjugate addition of an acetyl building block to an acryloyl precursor (see scheme).

The Catalytic Diversity of Multimodular Polyketide Synthases: Natural Product Biosynthesis Beyond Textbook Assembly Rules

“…An alternative b-branching mechanism has recently been described by Hertweck and coworkers for the rhizoxin trans-AT PKS [165]. Most rhizoxins, such as 47 or rhizoxin A (13), carry an unusual b-branched d-lactone moiety (Fig.…”

“…To dissect the b-branching process via the same approach as described in Sect. 4.1, the TE domain was inactivated to promote release of individual intermediates [165]. The compounds were identified as the free acids of 60-62, which excluded an isoprenoidlike pathway starting from a b-keto precursor.…”

“…A different approach was used to study double bond formation in bacillaene (2) [26] and rhizoxin D (47) [153] biosynthesis. Piel and coworkers observed that inactivation of the TE domain in the bacillaene (2) trans-AT system resulted in the hydrolytic release of virtually all polyketide intermediates from the PKS [23].…”

Assessing Oxazole Bioisosteres as Mutasynthons on the Rhizoxin Assembly Line

“…However, neither the expected macrolides nor biosynthetic [38,39] could be detected by LC-HRMS. This finding strongly suggests that the rhi PKS module 1 does not accept heterocycles with nitrogen atoms that do not donate their lone-pair electrons into the ring, regardless of the position(s) and number of the nitrogens.…”

Acyl hydrolases from trans-AT polyketide synthases target acetyl units on acyl carrier proteins

“…This observation leads us to re-assess the principal role of AHs in PKS assembly lines as housekeeper enzymes, serving to hydrolyse erroneously loaded acetyl groups on ACPs. It should be pointed out that previous work on bacillaene and rhizoxin PKS possessing inactivated terminal thioesterase domains showed that short-and long-chain intermediates were hydrolysed off the PKS in cells [34][35][36] . This finding might be due to an additional proofreading activity present in these pathways, a different activity profile of the cognate AHs, or simply base-catalysed hydrolysis.…”