The Staudinger ligation of azides and phosphines has found widespread use in the field of chemical biology, but the mechanism of the transformation has not been characterized in detail. In this work, we undertook a mechanistic study of the Staudinger ligation with a focus on factors that affect reaction kinetics and on the identification of intermediates. The Staudinger ligation with alkyl azides was second-order overall and proceeded more rapidly in polar, protic solvents. Hammett analyses demonstrated that electron-donating substituents on the phosphine accelerate the overall reaction. The electronic and steric properties of the ester had no significant impact on the overall rate but did affect product ratios. Finally, the structure of an intermediate that accumulates under anhydrous conditions was identified. These findings establish a platform for optimizing the Staudinger ligation for expanded use in biological applications.
pathogenesis ͉ biochemistry ͉ glycolipid ͉ sulfation T he thick Mycobacterium tuberculosis (M. tb) cell wall consists of numerous glycolipids that are distinctive to the mycobacterial genus, including phosphatidylinositol mannosides, trehalose mycolates, and lipoarabinomannans (1). These molecules are essential for many of the characteristics that distinguish mycobacterial pathogenesis, such as the inhibition of phagosomal maturation, drug resistance, and alteration of the host immune response (2-6). A family of cell surface sulfated lipids (dubbed sulfatides) were identified in M. tb extracts and correlated to strain virulence (7-9). The most abundant sulfatide, termed Sulfolipid-1 (SL-1), consists of a trehalose core, four fatty acyl groups, and a sulfate ester (Fig. 1A) (10-13). Despite the discovery of SL-1 nearly 50 years ago, the biological function of the molecule is not known. Conflicting reports suggest a role for SL-1 in superoxide (O 2 Ϫ ) release from human neutrophils or monocytes, alteration of trehalose dimycolate toxicity, and inhibition of trehalose dimycolate-induced macrophage recruitment (14-19). The relevance of these studies to the physiological role of SL-1 in M. tb infection is debatable.Although the role of SL-1 remains elusive, advances in genetics and metabolite analysis have sped the discovery of genes, proteins, and intermediates associated with SL-1 biosynthesis (20). Currently, three proteins are known to be involved in SL-1 assembly: Stf0, Pks2, and MmpL8. The sulfotransferase Stf0 sulfates trehalose at the 2-position, forming trehalose-2-sulfate (T2S), thereby initiating SL-1 biosynthesis (21). Meanwhile, the polyketide synthase Pks2 synthesizes the phthioceranoyl and hydroxyphthioceranoyl lipids that occupy the 6-, 6Ј-, and 3Ј-positions of SL-1 (Fig. 1 A) (22). The proteins responsible for transfer of the Pks2 products and the palmitoyl group to the T2S core, and the order in which these lipids are added, have not yet been defined.Insight into the order of lipid addition came from characterization of the putative lipid transporter MmpL8 (23,24). A mutant strain, ⌬mmpL8, lacks SL-1 but accumulates the diacylated intermediate SL 1278 (named for its observed mass) inside the cell (Fig. 1B). This intermediate possesses two of the four SL-1-associated lipids: a hydroxyphthioceranoyl group at the 3Ј-position and a palmitoyl group at the 2Ј-position (24). SL 1278 was recently found to be an immunostimulant in human tuberculosis patients (25). The glycolipid is presented on the surface of M. tb-infected antigen-presenting cells by CD1b, a member of the MHC class I-like CD1 family. Intriguingly, the ⌬mmpL8 mutant, which lacks SL-1 but accumulates SL 1278 , shows attenuated virulence in mice (23,24). By contrast, a ⌬pks2 mutant, which lacks both SL-1 and SL 1278 , is indistinguishable from WT M. tb in mice and guinea pigs (23,26). These observations suggest that SL 1278 , and possibly other SL-1 intermediates, modulate M. tb pathogenesis.In our effort to define the functions of M. tb sulf...
Sulfolipid-1 (SL-1) is an abundant sulfated glycolipid and potential virulence factor found in Mycobacterium tuberculosis. SL-1 consists of a trehalose-2-sulfate (T2S) disaccharide elaborated with four lipids. We identified and characterized a conserved mycobacterial sulfotransferase, Stf0, which generates the T2S moiety of SL-1. Biochemical studies demonstrated that the enzyme requires unmodified trehalose as substrate and is sensitive to small structural perturbations of the disaccharide. Disruption of stf0 in Mycobacterium smegmatis and M. tuberculosis resulted in the loss of T2S and SL-1 formation, respectively. The structure of Stf0 at a resolution of 2.6 A reveals the molecular basis of trehalose recognition and a unique dimer configuration that encloses the substrate into a bipartite active site. These data provide strong evidence that Stf0 carries out the first committed step in the biosynthesis of SL-1 and establish a system for probing the role of SL-1 in M. tuberculosis infection.
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