Synthesis, Structural Elucidation, And Biochemical Analysis of Immunoactive Glucuronosyl Diacylglycerides of Mycobacteria and Corynebacteria

Glucuronosyl diacylglycerides (GlcAGroAc(2)) 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 GlcAGroAc(2) 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) GlcAGroAc(2) glycolipids from M. smegmatis and Corynebacterium glutamicum. A synthetic hydrophobic octyl glucuronoside was used to characterize the GDP-mannose-dependent mannosyltransferase MgtA from C. g/utamicum that extends GlcAGroAc(2). This enzyme is an Mg2+/Mn2+-dependent metalloenzyme that undergoes dramatic activation upon reduction with dithiothreitol.