Synthesis, structural elucidation, and biochemical analysis of immunoactive glucuronosyl diacylglycerides of mycobacteria and corynebacteria

Article abstract of DOI:10.1021/jo302508e

Glucuronosyl diacylglycerides (GlcAGroAc₂) are functionally important glycolipids and membrane anchors for cell wall lipoglycans in the Corynebacteria. Here we describe the complete synthesis of distinct acyl-isoforms of GlcAGroAc₂ bearing both acylation patterns of (R)-tuberculostearic acid (C_19:0) and palmitic acid (C _16:0) and their mass spectral characterization. Collision-induced fragmentation mass spectrometry identified characteristic fragment ions that were used to develop rules allowing the assignment of the acylation pattern as C_19:0 (sn-1), C_16:0 (sn-2) in the natural product from Mycobacterium smegmatis, and the structural assignment of related C_18:1 (sn-1), C_16:0 (sn-2) GlcAGroAc₂ glycolipids from M. smegmatis and Corynebacterium glutamicum. A synthetic hydrophobic octyl glucuronoside was used to characterize the GDP-mannose-dependent mannosyltransferase MgtA from C. glutamicum that extends GlcAGroAc₂. This enzyme is an Mg²⁺/Mn²⁺- dependent metalloenzyme that undergoes dramatic activation upon reduction with dithiothreitol.