The structure and function of the LH2 (B800–850) complex from the purple photosynthetic bacterium Rhodopseudomonas acidophila strain 10050

27 pages, 11 figures, 2 tables.-- PMID: 9481143 [PubMed].The major light-absorbing pigments in purple photosynthetic bacteria are the bacteriochlorophylls (a and b) (BChl) and the carotenoids. These pigments are noncovalently attached to two types of integral membrane protein forming the reaction centers and the light-harvesting or antenna complexes (Hawthornthwaite and Cogdell, 1991; Hunter, 1995; Zuber and Cogdell, 1995). Photosynthesis in purple bacteria usually begins with the absorption of a photon in the light-harvesting system. The absorbed energy is then rapidly (in less than ~100 ps) and efficiently transferred to the reaction center (~95% quantum efficiency). In the reaction center this energy is used to drive the initial charge separation reaction and the energy is then "trapped" (Feher and Okamura, 1978; Feher et al., 1989; Deisenhofer et al., 1995). The combination of antenna complexes with a reaction center constitutes the photosynthetic unit (PSU). For most commonly studied purple bacteria the number of PSUs per cell and their size are variable. Depending on such factors as the light-intensity at which cells are grown, the size of the PSU can vary from about 30 BChls per reaction center up to 200-300 BChls per reaction center (Aagaard and Sistrom, 1972; Drews, 1985). This arrangement of reaction centers surrounded by an antenna system ensures that each reaction center is kept well supplied with incoming solar energy and effectively acts to increase their cross-sectional area for photon capture. It is interesting to note that in most species the same pigments are found in both reaction centers and antenna complexes, and it is the protein that determines which function a given pigment is destined to fulfil.When BChl a is dissolved in an organic solvent such as 7:2 v/v acetone:methanol its NIR absorption band is located at 772 nm. This is the typical Qy absorption band of monomeric BChl a. However, when the BChl a is non-covalently bound into an antenna complex, this NIR absorption band is red shifted between 800-940 nm, depending on the species (Fig. 1) (Thornber et al., 1978; Hawthornthwaite and Cogdell, 1991). In most species this red shift is associated with an increase in spectral complexity, with several peaks/shoulders clearly visible in the in vivo absorption spectrum. This red shift arises from pigment-pigment and pigment-protein interactions within the antenna complexes and is regularly used to both identify them and judge their integrity.Since they are integral membrane proteins, the isolation of a purple bacterial antenna complex begins with the solubilization of the photosynthetic membrane with a suitable detergent (Cogdell and Thorber, 1979; Hawthornthwaite and Cogdell, 1991). Very often the solubilized complexes are then initially fractionated by sucrose gradient centrifugation (Fig. 2). In most species this fractionation reveals two types of antenna complex, called LH1 and LH2. LH1 forms the so called "core" complex. It is closely associated with the reaction center and forms a stoichiometric complex with it (usually ~32 BChls per reaction center (Gall, 1995; Karrasch et al., 1995; Zuber and Cogdell, 1995). LH2, also sometimes called the "variable" or "peripheral" antenna complex, is the topic for the remainder of this review. Readers who want to obtain more information on the overall subject of the structure and function of the bacterial PSU should consult the following general reviews (Somsen et al., 1993; Blankenship et al., 1995; Loach and Parkes-Loach, 1995; Cogdell et al., 1996; Papiz et al., 1996).Some of the work described in this review was supported by grants from the BBSRC, the EU and the Human Frontiers of Science Programme. RJC would like to thank the Alexander von Humboldt Foundation for financial support during the writing of this review.Peer reviewe

Three-Dimensional Structure of the <i>Rhodobacter sphaeroides</i> RC-LH1-PufX Complex: Dimerization and Quinone Channels Promoted by PufX

Reaction center-light harvesting 1 (RC-LH1) complexes are the fundamental units of bacterial photosynthesis, which use solar energy to power the reduction of quinone to quinol prior to the formation of the proton gradient that drives ATP synthesis. The dimeric RC-LH1-PufX complex of Rhodobacter sphaeroides is composed of 64 polypeptides and 128 cofactors, including 56 LH1 bacteriochlorophyll <i>a</i> (BChl <i>a</i>) molecules that surround and donate energy to the two RCs. The 3D structure was determined to 8 Å by X-ray crystallography, and a model was built with constraints provided by electron microscopy (EM), nuclear magnetic resonance (NMR), mass spectrometry (MS), and site-directed mutagenesis. Each half of the dimer complex consists of a RC surrounded by an array of 14 LH1 αβ subunits, with two BChls sandwiched between each αβ pair of transmembrane helices. The N- and C-terminal extrinsic domains of PufX promote dimerization by interacting with the corresponding domains of an LH1 β polypeptide from the other half of the RC-LH1-PufX complex. Close contacts between PufX, an LH1 αβ subunit, and the cytoplasmic domain of the RC-H subunit prevent the LH1 complex from encircling the RC and create a channel connecting the RC Q_B site to an opening in the LH1 ring, allowing Q/QH₂ exchange with the external quinone pool. We also identified a channel that connects the two halves of the dimer, potentially forming a long-range pathway for quinone migration along rows of RC-LH1-PufX complexes in the membrane. The structure of the RC-LH1-PufX complex explains the crucial role played by PufX in dimer formation, and it shows how quinone traffic traverses the LH1 complex as it shuttles between the RC and the cytochrome <i>bc</i>₁ complex