Interrupted adenylation domains are enigmatic fusions, in which one enzyme is inserted into another to form a highly unusual bifunctional enzyme. We present the first crystal structure of an interrupted adenylation domain that reveals a unique embedded methyltransferase. The structure and functional data provide insight into how these enzymes N-methylate amino acid precursors en route to nonribosomal peptides.

Stereodefined carbon-halogen bonds are ubiquitous in nature with several natural products exhibiting this motif.[1] While the biogenetic origins of this unique chiral functionality has been a subject of several investigations in the past, [2] attempts by organic chemists to forge the carbon-halogen bond stereoselectively have largely been unsuccessful. This problem has come into focus only recently. Several elegant reports of asymmetric halogenations of alkenes and alkynes followed by an intramolecular attack of a pendant nucleophile have appeared in the literature in the last decade. Kang et al. reported a cobalt-salen catalyzed iodoetherification reaction.[3a] An asymmetric fluorocyclization of allyl silanes mediated by a cinchona alkaloid dimer was reported by Gouverneur and co-workers.[3b] Tang and co-workers disclosed an asymmetric bromolactonization of enynes catalyzed by a cinchona alkaloid derived urea; other bromolactonizations have also appeared following the disclosure of their report.[3c-e] More recently, Veitch and Jacobsen reported an asymmetric iodolactonization reaction mediated by chiral thiourea catalysts.[3f] Polyene cyclizations induced by chiral halonium ions have also been realized as reported by the research groups of Ishihara and Snyder.[4a-c] However, given the fledgling nature of this research area, one may find it easy to highlight the many drawbacks and limitations even in the present state of the art-for example, the relatively large catalyst loadings (superstoichiometric quantities in some cases) to achieve meaningful levels of enantioselectivity or the lack of a robust catalytic system that can catalyze a number of diverse reactions rather than one specific reaction. Moreover, efficient asymmetric chlorocyclizations have remained underdeveloped. This situation is attributable, at least in part, to the highly reactive nature of chloronium ions, which are known to exist in equilibrium with the corresponding carbocation rather than exclusively as cyclic chloronium ions, [5a-c] thus making the development of chlorocyclizations a formidable challenge.Our research group has recently reported the catalytic asymmetric chlorolactonization of alkenoic acids.[7] Herein, we disclose the efficient halocyclization of unsaturated amides to furnish chiral heterocycles. Furthermore, these heterocycles have been transformed into useful chiral building blocks such as amino alcohols.Chiral heterocycles such as oxazolines and dihydrooxazines are commonly encountered motifs in natural products, [8a] molecules of pharmaceutical interest, [8b] and in several chiral ligands.[8c] Their syntheses, however, usually employ stoichiometric quantities of chiral amino alcohols. With only one precedented method to access these molecules in a catalytic asymmetric fashion, [6] we were intrigued by the possibility of one-step access to these versatile chiral heterocycles by a catalytic asymmetric halocyclization of easily accessed unsaturated amides.We chose the conversion of benzamide 1 into oxazoline 2 as our init...

Dimethylation of amino acids consists of an interesting and puzzling series of events that could be achieved, during nonribosomal peptide biosynthesis, either by a single adenylation (A) domain interrupted by a methyltransferase (M) domain or by the sequential action of two of such independent enzymes. Herein, to establish the method by which Nature N,S-dimethylates l-Cys, we studied its formation during thiochondrilline A biosynthesis by evaluating TioS(AMAT) and TioN(AMA). This study not only led to identification of the exact pathway followed in Nature by these two enzymes for N,S-dimethylation of l-Cys, but also revealed that a single interrupted A domain can N,N-dimethylate amino acids, a novel phenomenon in the nonribosomal peptide field. These findings offer important and useful insights for the development and engineering of novel interrupted A domain enzymes to serve, in the future, as tools for combinatorial biosynthesis.

Tuberculosis (TB) remains one of the leading causes of mortality worldwide. Hence, the identification of highly effective antitubercular drugs with novel modes of action is crucial. In this paper, we report the discovery and development of pyrrolo[1,5-a]pyrazine-based analogues as highly potent inhibitors of the Mycobacterium tuberculosis (Mtb) acetyltransferase enhanced intracellular survival (Eis), whose up-regulation causes clinically observed resistance to the aminoglycoside (AG) antibiotic kanamycin A (KAN). We performed a structure–activity relationship (SAR) study to optimize these compounds as potent Eis inhibitors both against purified enzyme and in mycobacterial cells. A crystal structure of Eis in complex with one of the most potent inhibitors reveals that the compound is bound to Eis in the AG binding pocket, serving as the structural basis for the SAR. These Eis inhibitors have no observed cytotoxicity to mammalian cells and are promising leads for the development of innovative AG adjuvant therapies against drug-resistant TB.

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