Structure of the light-harvesting bacteriochlorophyll c assembly in chlorosomes from Chlorobium limicola determined by solid-state NMR

Abstract

We have determined the atomic structure of the bacteriochlorophyll c (BChl c) assembly in a huge light-harvesting organelle, the chlorosome of green photosynthetic bacteria, by solid-state NMR. Previous electron microscopic and spectroscopic studies indicated that chlorosomes have a cylindrical architecture with a diameter of Ϸ10 nm consisting of layered BChl molecules. Assembly structures in huge noncrystalline chlorosomes have been proposed based mainly on structure-dependent chemical shifts and a few distances acquired by solid-state NMR, but those studies did not provide a definite structure. Our approach is based on 13 C dipolar spindiffusion solid-state NMR of uniformly 13 C-labeled chlorosomes under magic-angle spinning. Approximately 90 intermolecular COC distances were obtained by simultaneous assignment of distance correlations and structure optimization preceded by polarization-transfer matrix analysis. It was determined from the Ϸ90 intermolecular distances that BChl c molecules form piggybackdimer-based parallel layers. This finding rules out the well known monomer-based structures. A molecular model of the cylinder in the chlorosome was built by using this structure. It provided insights into the mechanisms of efficient light harvesting and excitation transfer to the reaction centers. This work constitutes an important advance in the structure determination of huge intact systems that cannot be crystallized.spin diffusion ͉ distance analysis ͉ photosynthesis ͉ antenna complex ͉ excitation transfer P hotosynthesis is the primary energy source for all living organisms. Chlorophyll-protein complexes capture light energy in most photosynthetic systems. Their structures are well known (1, 2). However, there are other light-harvesting devices called chlorosomes, which contain bacteriochlorophyll (BChl) assemblies. No protein is present in the BChl assemblies in sharp contrast to the light-harvesting chlorophyll-protein complexes mentioned above. Chlorosomes are found only in green sulfur bacteria and green filamentous bacteria. They catch weak light in an environment (3). A green sulfur bacterium species found in a deep-sea hydrothermal vent (4), for example, uses the dim light of geothermal radiation for photosynthesis. The atomic structure of chlorosomes has not been determined, which has impeded structure-based study of their functions. Because the design of chlorosomes is completely different from that of other light-harvesting devices, elucidation of their structure provides insight into the light-harvesting mechanism involved.Freeze-fracture electron microscopy revealed that chlorosomes were oblong bodies filled with several rod-shaped elements and were attached to the cytoplasmic side of cell membranes (5, 6). The rod elements of Chlorobium limicola, the target of this work, are composed of BChl c. Light energy captured by BChl c in the rod elements is transferred to the reaction centers in the cytoplasmic membrane through BChl a in baseplate proteins (7). The BChl c molecule has two stereoisomers...

Chlorobaculum tepidum regulates chlorosome structure and function in response to temperature and electron donor availability

“…Single chlorosomes contain *200,000 BChl c F molecules that self-aggregate into dimers, parallel dimer layers, and higher order rod elements, the major internal structural unit (Egawa et al 2007;Montaño et al 2003). BChl c molecules exist as a set of homologs which differ in the number and identity of alkyl groups on positions 8 and 12 of the chlorin ring (Nozawa et al 1991) that can be resolved by HPLC (Persson et al 2000).…”

Anisotropic distribution of emitting transition dipoles in chlorosome from Chlorobium tepidum: fluorescence polarization anisotropy study of single chlorosomes

“…There are still arguments over several types of structural models for the major pigment aggregates, which we designate hereafter as (BChl-c) n . The traditional rod model is based on early observations using freezefracture electron microscopy (EM) (Staehelin et al 1978(Staehelin et al , 1980Holzwarth and Schaffner 1994;Saga and Tamiaki 2006;Egawa et al 2007). The lamellae model, on the other hand, was recently proposed according to observations using X-ray solution scattering and cryo-EM for intact chlorosomes (Pšenčík et al 2004(Pšenčík et al , 2006.…”

α-Helices direct excitation energy flow in the Fenna–Matthews–Olson protein

“…Under physiological conditions, this water-soluble PPC is situated between the so-called baseplate protein of the major antenna system of green sulfur bacteria, the chlorosomes (23,24), and the reaction center complex (25,26). Here, it mediates excitation energy transfer from the bacteriochlorophyll c rods to the reaction center pigments (Fig.…”