|Christoph Mahlert and Mohamed A. Marahiel
Faculty of Chemistry, University of Marburg, Hans-Meerwein-Straße, 35032 Marburg, Germany
In the last decades an alarming increase in microbial resistance to conventional antibiotics was ascertained. Secondary metabolites produced by large, multimodular nonribosomal peptide synthetases (NRPS) harbour a widespread spectrum of potential antimicrobial agents. Within this large group of NRPS products promising candidates for new antibiotics are acidic lipopeptides including daptomycin which was approved for clinical treatment of a wide array of infections caused by grampositive bacteria. Another example is the calcium dependent antibiotic (CDA) produced by Streptomyces coelicolor. This cyclic lipopeptide contains several nonproteinogenic amino acids including L-3-methylglutamic (mGlu) acid and D-3-hydroxyasparagine (hAsn). The methylation was proven to be important for antimicrobial activity.
Here we present the biochemical characterization of the tailoring enzymes GlmT and AsnO responsible for methylation and hydroxylation of Glu10 and Asn9 of CDA, respectively. By biochemical and structural investigations, we are able to explain the strategy of these enzymes to functionalise the unactivated β-Carbon atoms of the target amino acids. Furthermore, we show that both enzymes are acting prior to peptide assembly by the NRPSs. The results presented here open new ways for the synthesis of β-functionalised building blocks and for the engineering of acidic lipopeptide biosynthesis.