LH2 complex

One of the characteristics of R palustris is the multiplicity of LH2 complexes. High-light-adapted membranes comprise 50% of standard B800-850 LH2 and 50% of specific 8800 LH2, while in low-light-adapted membranes only 10% of 8800-850 

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The structure of the LH2 complex of R. acidophila is both simple and elegant (Figure 12.17). It is a ring of nine identical units, each containing an a and a P polypeptide of 53 and 41 residues, respectively, which both span the membrane once as a helices (Figure 12.18). The two polypeptides bind a total of three chlorophyll molecules and two carotenoids. The nine heterodimeric units form a hollow cylinder with the a chains forming the inner wall and the P chains the outer wall. The hole in the middle of the cylinder is empty, except for lipid molecules from the membrane. 

Spectroscopic measurements show that the reaction center and LHl are tightly associated and therefore it is assumed that the ring of pigments in LHl surrounds the reaction center. Careful model building indicates that the hole in the middle of LHl is large enough to accommodate the whole reaction center molecule. We do not know exactly how the LH2 complexes are arranged in the membrane around the LHl-reaction center complex, but at least some of them should be in contact with the outer rim of LHl for efficient... 

In the case of the carotenoid-containing LH2 complex, the triplet states of BChl a and carotenoid (spheroidene) were generated immediately after excitation, but the triplet-state BChl a was quenched efficiently by the carotenoid so that no BChl a cation-radical was generated. Thus, the photoprotective function of the carotenoid in this antenna complex has been proven. 

The study of artificial photosynthesis has been the subject of ongoing attention for many years now due to the need for sustainable energy resources. In natural photosynthesis a lightharvesting antenna system with a large optical cross-section (for example the LH2 complex) absorbs a photon that is funneled by energy transfer (ET) to the reaction centre [1-3]. Excellent candidates to mimic the natural antenna system are molecules that efficiently absorb light and are able to transfer the captured energy to other parts of the molecule. Molecules based on Zn and free-base porphyrins are examples of compounds that can be used as models for the LID complex [4]. 

Figure 23-26 (A), (B) Arrangement of bacteriochlorophyll chromophores in the cyclic LH2 array of Rhodopseudomonas acidophila. The B850 subunits are gray while the B800 subunits are black. (C) Fluorescence-excitation spectra. Top trace, for an ensemble of LH2 complexes, other traces, for several individual LH2 complexes at 1.2K. Fine structure is evident for the B800 but not for the B850 chromophores. From van Oijen et al.299 with permission.

Jimenez R, Mourik F, Yu JY, Fleming GR. Three-pulse photon echo measurements on LH1 and LH2 complexes of Rhodobacter sphaeroides nonlinear spectroscopic probe of energy transfer. J Phys Chem B 1997 101 7350-7359. 

The Structure of the Light-harvesting LHl and LH2 Complexes of Purple Photosynthetic Bacteria ... 

Electron and atomic force microscopy has shown that the LH2 complexes from Rhodopseudomonas acidophila, Rhodovulum sulfidophilum, and Rhodobacter sphaeroides are all monomers of the basic a/3-subunits. Crystal structures of the LH2 complexes from Rps. acidophila and Rs. molischianum have also been reported. Each a/3-unit binds 3 bacteriochlorophyll and two carotenoid molecules, although in the crystal structure only one carotenoid electron density was clearly defined. The arrangement of the ajS-subunits is... 

Fig. 26.1. Structure of the LH2 complex from Rps. acidophila as determined by X-ray diffraction. Part (a) shows the whole pigment-protein complex in a top view part (b) displays the spatial arrangement of the BChl a in a tilted side-view. The B800 BChl a pigments are shown in hght gray and the B850 BChl a molecules are shown in dark gray. The numbers indicate the centre to centre distances of the pigments in A. The arrows indicate the direction of the Qy transition moments. The phytol chains of the pigments are removed for clarity. Adapted from [3]...

Fig. 26.3. Fluorescence-excitation spectra of LH2 complexes of Rp. acidophila. The top traces show the comparison between an ensemble spectrum (dotted line) and the sum of spectra recorded from nineteen individual complexes (full line). The lower trace displays the spectrum from a single LH2 complex. The spectra have been averaged over all polarizations of the incident radiation. All spectra were measured at 1.2 K at 20W/cm with LH2 dissolved in a PVA-buffer solution. Adapted from [39]...

M.Z. Papiz, S.M. Prince, T. Howard, R.J. Cogdell, N.W. Isaacs, The structure and thermal motion of the B800-850 LH2 complex from Rps. acidophila at 2.0 (A)over-circle resolution and 100 K new structural features and functionally relevant motions. J. Mol. Biol. 326, 1523-1538 (2003)... 

D. Rutkauskas, R. Novoderezkhin, R.J. Cogdell, R. van Grondelle, Fluorescence spectral fluctuations of single LH2 complexes from Rhodopseudomonas acidophila strain 10050. Biochemistry 43, 4431-4438 (2004)... 

V.I. Novoderezhkin, D. Rutkauskas, R. Van Grondelle, Multistate conformational model of a single LH2 complex quantitative picture of time-dependent spectral fluctuations. Chem. Phys. 341, 45-56 (2007)... 

Low-temperature N and CPMAS NMR has been used to analyze BChl-histidine interactions and the electronic structure of histidine residues in the light-harvesting complex II (LH2) of Rhodopseudomonas acidophila. Comparison of the 2D MAS NMR homonuclear ( C- C) dipolar correlation spectrum of [ C(6), N(3)]-histidines in the LH2 complex with model systems in the solid state reveals two different classes of electronic structures from the... 

The three-dimensional octameric structure of the LH2 of Rs. molischianum was established soon after by the X-ray work of Koepke, Hu, Muenke, Schulten and Michel at 2.4-A resolution. The three-dimensional structure of the Rs. molischianum LH2 complex as viewed from the cytoplasmic side and along the membrane plane is shown in Fig. 7, with color figures of the LH2 complex shown in Color plates 4 (A) and (B). The orientation of the BChl-a molecules ofB850 and B800 and the tilt of the carotenoid chains can be clearly seen particularly in Color plate 5 (B). 

Figure 21. Left Absorption spectra calculated for a random selection of single LH2 complexes Rps. acidophila, 77K). Right The corresponding calculated circular dichroism spectra. Only diagonal disorder is included in the site energies of the monomers, but note the dramatic effects it has on state energies and intensities.

The Structure and Function of the LH2 Complex from Rhodopseudomonas acidophila Strain 10050, with Special Reference to the Bound Carotenoid... 

A typical purple bacterial photosynthetic unit consists of two types of antenna complexes, called LHl and LH2, which transfer absorbed solar radiation to the reaction center, where the light-energy is trapped and converted into useful chemical energy (for reviews see Zuber and Cogdell, 1995 Cogdell et al., 1996 Papiz et al., 1996 Hu et al., 1998). The LHl complexes surround the RC and form the so-called core complex. The LH2 complexes are arranged more peripherally around the core complexes (Fig. 1). 

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