Characterization of Glycolipids in Light-Harvesting Chlorosomes from the Green Photosynthetic Bacterium Chlorobium tepidum

Abstract

Abstract Glycolipids were isolated from extramembraneous light-harvesting complexes, called “chlorosomes,” in a thermophilic green photosynthetic bacterium Chlorobium tepidum. The lipids were first characterized in terms of their precise structures of both hydrophilic (saccharides) and hydrophobic (fatty acids) moieties by means of 1H and 13C NMR spectroscopy as well as high-performance liquid chromatography coupled with an evaporative light-scattering detector and an electron spray ionization mass spectrometer. The results clearly demonstrated that glycolipids having a disaccharide group, rhamnosylgalactosyldiacylglycerides, were the majority in the total glycolipid component of the chlorosomes, in addition to the second major glycolipids with a monosaccharide group, monogalactosyldiacylglycerides, which had been believed to be predominant before the report by Miller and her co-workers (Photosynth. Res.2008, 95, 191–196). We also found that these glycolipids had methylene-bridged palmitoleyl (=(Z)-9-hexadecenoyl) (17:1) and palmitoyl groups (16:0) as their predominant acyl chains on the glycerol moiety. By using enzymatically site-specific hydrolysis of the acyl chain at the sn-1 position on the glycerol, the modified palmitoleyl group was determined to be esterified at the sn-1 position. These unique glycolipids might play an important role in maintaining fluidization of chlorosomal envelopes and achieving thermal stability of chlorosomes at 45 °C.