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A multiblocked mutant strain (ΔAHBA and Δasm12, asm21) of Actinosynnema pretiosum, the producer of the highly toxic maytansinoid ansamitocin, has been used for the mutasynthetic production of new proansamitocin derivatives. The use of mutant strains that are blocked in the biosynthesis of an early building block as well as in the expression of two tailoring enzymes broadens the scope of chemo-biosynthetic access to new maytansinoids. Remarkably, a ring-expanded macrolactone derived from ansamitocin was created for the first time., (© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

Ansamitocins are potent antitumor agents produced by Actinosynnema pretiosum. As deduced from their structures, an N-methylation on the amide bond is required among the various modifications. The protein encoded by asm10 belongs to the SAM-dependent methyltransferase family. Through gene inactivation and complementation, asm10 was proved to be responsible for the N-methylation of ansamitocins. Asm10 is a 33.0 kDa monomer, as determined by gel filtration. By using N-desmethyl-ansamitocin P-3 as substrate, the optimal temperature and pH for Asm10 catalysis were determined to be 32 °C and 10.0, respectively. Asm10 also showed broad substrate flexibility toward other N-desmethyl-ansamycins and synthetic indolin-2-ones. Through site-directed mutagenesis, Asp154 and Leu155 of Asm10 were confirmed to be essential for its catalysis, possibly through the binding of SAM. The characterization of this unique N-methyltransferase has enriched the toolbox for engineering N-methylated derivatives from both natural and synthetic compounds; this will allow known potential drugs to be modified., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

New ansamitocin derivatives were prepared by feeding aminobenzoic acid derivatives to cultures of Actinosynnema pretiosum HGF073, a mutant strain blocked in the biosynthesis of the required 3-amino-5-hydroxybenzoic acid (AHBA) starter unit. Use of several aminobenzoic acids as precursors led to a spectrum of products, reflecting the sequence of post-PKS tailoring steps involved in the generation of ansamitocins and adding novel aspects to the published suggestion model of post-PKS tailoring logic and flexibility. The studies provide insights into the substrate flexibility of the enzymes required for ansamitocin biosynthesis in A. pretiosum, whereas preliminary biological testing of the derivatives isolated and fully characterized by NMR spectroscopy allowed structure-activity relationship assignments to be made for a variety of intermediates occurring during the post-PKS tailoring sequence in ansamitocin biosynthesis., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The biosynthesis of the antitumor antibiotic, ansamitocin, involves the assembly of a linear octaketide on the ansamitocin (asm) polyketide synthase (PKS), which is then cyclized to proansamitocin and further modified to the final product. In the first chain-extension step on the asm PKS, a stereocenter is generated which is then obliterated in a subsequent double-bond migration. The cryptic configuration at this stereocenter was determined by first synthesizing the two enantiomers of the intermediate diketide as their N-acetylcysteamine (SNAC) thioesters. These were then used to demonstrate that only the R enantiomer complements a 3-amino-5-hydroxybenzoic acid (AHBA) deficient mutant of Actinosynnema pretiosum to restore ansamitocin formation. The low efficiency of complementation by the diketide, compared to AHBA, is due to inefficient loading onto the PKS and not the inhibition of the enzyme. A presumed next chain-extension intermediate-the triketide with an unrearranged double bond-was also synthesized as its SNAC ester, but did not complement the AHBA(-) mutant.