Reaction mechanism proposed for TbSLS3. A, binding of PE is oriented by interaction of the substrate primary amine group with the carboxylate group of D172. B, displacement of diacylglycerol is promoted by acid-base catalysis provided by the conserved HHD triad. C, binding of ceramide and nucleophilic attack of the hydroxyl group with activation by acid-base catalysis. NBD moiety of NBD-C6-CM would be located three methylene groups beyond R2, so is located distal from the active site. D, formation of EPC.