Chlorosome

In photosynthetic bacteria, (i.e., Chlorobiaceae) the bacteriochlorophyUs support photosynthesis at low light intensities, and they accomplish this activity by using a unique antenna complex known as a chlorosome in which the pigments are located. Since these bacteria are strict anaerobes, photosynthesis is nonoxygenic. ... 

Figure 23 MAS NMR spectra of intact chlorosome and of solid aggregate of BChl c...

Y. Kakitani, Y. Koyama, Y. Shimoikeda, T. Nakai, H. Utsumi, T. Shimizu and H. Nagae, Stacking of bacteriochlorophyll c macrocycles in chlorosome from CMorobium limicola as revealed by intermolecular magnetic-dipole correlation. X-ray diffraction, and quadrupole coupling in Mg NMR. Biochemistry, 2009,48, 74—86. 

Figure 23-28 (A) Model of a light-harvesting chlorosome from green photosynthetic sulfur bacteria such as Chlorobium tepidum and species of Prosthecochloris. The chlorosome is attached to the cytoplasmic membrane via a baseplate, which contains the additional antenna bacteriochlorophylls (795 BChl a) and is adjacent to the trimeric BChl protein shown in (B) and near the reaction center. After Li et al.302 and Remigy et a/.304 (B) Alpha carbon diagram of the polypeptide backbone and seven bound BChl a molecules in one subunit of the trimeric protein from the green photosynthetic bacterium Prosthecochloris. For clarity, the magnesium atoms, the chlorophyll ring substituents, and the phytyl chains, except for the first bond, are omitted. The direction of view is from the three-fold axis, which is horizontal, toward the exterior of the molecule. From Fenna and Matthews.305 See also Li et al.302...

According to the fossil records [13], the early photosensitizers were purple bacteria and green sulfur bacteria. Purple bacteria have been the subject of many structural [14] and spectroscopic studies. [15-17] Particular attention was focused on their antenna complexes (LH1, LH2), which represent the best understood system in terms of light collection and energy transfer. Recently, some interest has also been directed to green bacteria containing chlorosomes,... 

PES4S (Rhodomonas CS24) LHCII (Spinacia olearia) Chlorosome (Chi. aurantiacus)... 

Fig. 3 Schematic model of light-harvesting compartments in photosynthetic organisms and their position with respect to the membrane and the reaction centers. RC1(2) Photosystem I(II) reaction centre. Peripheral membrane antennas Chlorosome/FMO in green sulfur and nonsulfur bacteria, phycobilisome (PBS) in cyanobacteria and rhodophytes and peridinin-chlorophyll proteins (PCP) in dyno-phytes. Integral membrane accessory antennas LH2 in purple bacteria, LHC family in all eukaryotes. Integral membrane core antennas B808-867 complex in green nonsulfur bacteria, LH1 in purple bacteria, CP43/CP47 (not shown) in cyanobacteria and all eukaryotes.

The Fenna-Matthews-Olson (FMO) protein is an unusual, water-soluble chlorophyll protein found only in green sulfur bacteria. [18] It is believed to be located between the chlorosome and the cytoplasmatic membrane and functions as an excitation transfer link between the chlorosome and the reaction center. Each subunit contains 7 BChl a molecules embedded in a primarily /3 sheet structured protein. The protein has a trimeric quaternary structure, with a three-fold axis of symmetry in the center of the complex. [55] The green nonsulfur bacteria do not contain the FMO protein. In these organisms the chlorosome transfers energy directly to the integral membrane core antenna B808-865, and then to the reaction center. 

Figure 7 Rogue s gallery of structures of peripheral anteima complexes. As labelled these include Chlorosomes from green sulfur bacteria, fused antenna domains of the Photosystem I core, the CP43 and CP47 proteins of Photosystem II, the Fenna-Matthew-Olson (FMO) protein associated with chlorosomes, LHI proteins surrounding a purple bacterial photo synthetic core, the peridinin-chlorophyll a protein of dinoflagellate algae, the LHCI and LHCII proteins found in plants and many algae, and the LHII protein complex that is associated with LHI in purple bacteria...

Another antenna complex where high-resolution structural information is available is the bacteriochlorophyll a binding protein (also known as the Fenna-Matthews-Olson or FMO protein) from green sulfur bacteria. This complex serves as the bridge between the peripheral chlorosome complex and the membrane-bound reaction center complexes. In this... 

Oxygen suppresses the synthesis of BChl a and c [11,12], and under aerobic conditions C. aurantiacus switches to heteroZ-organotrophic respiration using an electron transfer system involving cytochromes of the b, c and probably a types [11,13,14]. Under these conditions synthesis of the chlorosomes is also repressed. [12]. 

Two other genera, Oscillochloris [17] and Chloronema [18], are known to contain BChl a and either BChl c ox d (Fig. 2), apparently housed in chlorosomes. 

All species contain two different chlorophylls, BChl a and an additional light-harvesting pigment, either BChl c, d ox e [40], As in Chloroflexus these light-harvesting pigments are housed in chlorosomes located adjacent to the cytoplasmic membrane [45,46]. In Chlorobium BChl c is associated with a 7.5-kDa polypeptide. (J.M. Olson and P. Roepstorff, unpublished). A small amount of energytransferring BChl a is also found in the chlorosomes [47,48], but most of the BChl a is found in a water-soluble protein associated with the cytoplasmic membrane [45]. The RC (see Fig. 4 and Ref. 36) is similar to that of PS I in cyanobacteria and chloroplasts. The primary electron donor P-840 = +0.25 V) is a BChl... 

MO. Cyts b and c BChl a, BChl c or d Plasma membrane chlorosome y Carotene... 

Except for the chlorosome and the base-plate-type antenna of chlorobineae, the bacterial antenna complexes are intra-membrane bound. 

Fig. 8. Amino acid sequence of the BChl c-binding (antenna) polypeptide of the chlorosomes from Chloroflexus aurantiacus (A) and the proposed a-helix model of this antenna polypeptide (B) with possible BChl c binding sites (Gin 12,15,22,26,33, Asn 30,41) via the central Mg atom. (C) Chlorosome subunit ( globular subunit ) (light-harvesting BChl c-protein complex of C. aurantiacus) composed of 12 polypeptide chains (a-helices) of the BChl c-binding polypeptide. Pairs of a-helices (dimeric basic units) are tilted to expose the areas for BChl c binding (hatched areas). (D) Chlorosome model (cross-section, C. aurantiacus) containing the chlorosome subunits of the rod-shaped elements in the chlorosome core region (taken from Refs. 69 and 72).

Green photosynthetic bacteria intramembrane antenna complexes, baseplate systems and the accessory antenna systems (chlorosomes)... 

In all green bacteria the main antenna system is organized in small ovoid particles, the so-called chlorosomes, located adjacent to the cell membrane. The BChl c and d pigments are associated with this accessory light-harvesting system. 

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