Biosynthesis of cell envelope-associated phenolic glycolipids in Mycobacterium marinum.

Phenolic glycolipids (PGLs) are polyketide synthase-derived glycolipids unique to pathogenic mycobacteria. PGLs are found in several clinically relevant species, including various Mycobacterium tuberculosis strains, Mycobacterium leprae, and several nontuberculous mycobacterial pathogens, such as M. marinum. Multiple lines of investigation implicate PGLs in virulence, thus underscoring the relevance of a deep understanding of PGL biosynthesis. We report mutational and biochemical studies that interrogate the mechanism by which PGL biosynthetic intermediates (p-hydroxyphenylalkanoates) synthesized by the iterative polyketide synthase Pks15/1 are transferred to the noniterative polyketide synthase PpsA for acyl chain extension in M. marinum. Our findings support a model in which the transfer of the intermediates is dependent on a p-hydroxyphenylalkanoyl-AMP ligase (FadD29) acting as an intermediary between the iterative and the noniterative synthase systems. Our results also establish the p-hydroxyphenylalkanoate extension ability of PpsA, the first-acting enzyme of a multisubunit noniterative polyketide synthase system. Notably, this noniterative system is also loaded with fatty acids by a specific fatty acyl-AMP ligase (FadD26) for biosynthesis of phthiocerol dimycocerosates (PDIMs), which are nonglycosylated lipids structurally related to PGLs. To our knowledge, the partially overlapping PGL and PDIM biosynthetic pathways provide the first example of two distinct, pathway-dedicated acyl-AMP ligases loading the same type I polyketide synthase system with two alternate starter units to produce two structurally different families of metabolites. The studies reported here advance our understanding of the biosynthesis of an important group of mycobacterial glycolipids.

Oxathiazolones Selectively Inhibit the Human Immunoproteasome over the Constitutive Proteasome.

Selective inhibitors for the human immunoproteasome LMP7 (ß5i) subunit over the constitutive proteasome hold promise for the treatment of autoimmune and inflammatory diseases and hematologic malignancies. Here we report that oxathiazolones inhibit the immunoproteasome ß5i with up to 4700-fold selectivity over the constitutive proteasome, are cell permeable, and inhibit proteasomes inside cells.

Evaluation of triazolamers as active site inhibitors of HIV-1 protease.

Proteases typically recognize their peptide substrates in extended conformations. General approaches for designing protease inhibitors often consist of peptidomimetics that feature this conformation. Herein we discuss a combination of computational and experimental studies to evaluate the potential of triazole-linked beta-strand mimetics as inhibitors of HIV-1 protease activity.

Solution- and solid-phase synthesis of triazole oligomers that display protein-like functionality.

We recently developed a new class of oligomers that contain alpha-amino acid residues linked by 1,2,3-triazole groups [Angelo, N. G.; Arora, P. S. J. Am. Chem. Soc. 2005, 127, 17134-17135]. Synthesis of these oligomers involves an iterative sequence consisting of diazotransfer and Huisgen 1,3-dipolar cycloaddition steps. In this contribution, we describe an efficient one-pot, two-step sequence for the preparation of triazoles from the corresponding amino acid-derived amines and alkynes in solution. The one-pot sequence affords the desired products in significantly higher yields than our original method. We also outline a highly effective protocol for the synthesis of these triazole-based biomimetic oligomers on the solid phase. We find that amino acid derivatives and iterative formation of triazole rings require nontraditional reaction conditions for high yields.

Nonpeptidic foldamers from amino acids: synthesis and characterization of 1,3-substituted triazole oligomers.

Nonpeptidic foldamers capable of displaying protein-like functionality were prepared by swapping amide bonds with 1,2,3-triazole rings. The overall conformation of these triazole oligomers is largely dictated by dipole-dipole interactions between adjacent rings. Solution NMR studies suggest that a zigzag conformation, which closely mimics the beta-strand structure, predominates in two different tetramers.