The crystal structure of the light-harvesting complex II (B800–850) from Rhodospirillum molischianum (original) (raw)
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Biochemistry, 1997
The core light-harvesting LH1 complex of Rhodobacter sphaeroides consists of an assembly of membrane-spanning R and polypeptides, each of which binds one bacteriochlorophyll molecule. In this study we have used site-directed mutagenesis to demonstrate that the B880 bacteriochlorophyll binding site of LH1 shows a high degree of specificity for the residue that provides the ligand to the Bchl Mg 2+ ion. R His 0 (RH 0 ) was changed to Asn, Leu, and Tyr, and His 0 ( H 0 ) to Asn, Gln, Leu, and Tyr; the mutated genes were expressed to yield both LH1-only and LH1 + RC strains, in two different carotenoid backgrounds. None of the RH 0 mutations formed an LH1 complex either in isolation or with RCs. Of the mutations of H 0 those to Asn and Gln formed LH1 complexes but in the latter case the complex was very unstable and occurred at very low cellular levels. In a similar study, the RH 0 and H 0 residues of the LH2 complex were changed to Asn. However, no complex was formed in either case. FT Raman spectroscopy of the H 0 N mutant LH1 shows perturbation of the interaction state of the keto carbonyl of one Bchl which sheds light on the possible H-bond partners for these keto oxygens. These data directly address the B880 binding site of the core LH1 complex and show that it is subtly different from the B850 binding site of the peripheral LH2 complex. A model of this binding site may be proposed from these results.
2020
We report the 2.4 Å resolution structure of the light harvesting 2 complex (LH2) from Marichromatium (Mch.) purpuratum determined by electron cryo-microscopy. The structure contains a heptameric ring that is unique among all known LH2 structures, explaining the unusual spectroscopic properties of this bacterial antenna complex. Two sets of distinct carotenoids are identified in the structure, and a network of energy transfer pathways from the carotenoids to bacteriochlorophyll a molecules is shown. The geometry imposed by the heptameric ring controls the resonant coupling of the long wavelength energy absorption band. Together, these details reveal key aspects of the assembly and oligomeric form of purple bacterial LH2 complexes that were previously inaccessible by any technique.One Sentence SummaryThe structure of a heptameric LH2 antenna complex reveals new energy transfer pathways and the basis for assembling LH rings.
The structural basis of light-harvesting in purple bacteria
Febs Letters, 2003
A typical purple bacterial photosynthetic unit consists of two types of light-harvesting complex (LH1 and LH2) together with a reaction centre. This short review presents a description of the structure of the LH2 complex from Rhodopseudomonas acidophila, which has recently been improved to a resolution of 2.0 A î [Papiz et al., J. Mol. Biol. 326 (2003) 15231 538]. We show how this structure has helped to reveal the details of the various excitation energy transfer events in which it is involved.
The Journal of Physical Chemistry B, 2001
Using time-dependent density functional theory (TDDFT), we obtained the excitation energy transfer coupling (Coulombic coupling) between the S 1 state of rhodopin glucoside (RG) and the Q y state of bacteriochlorophylls (BChl) in the light-harvesting complex II (LH2) of purple photosynthetic bacterium Rhodopseudomonas (Rps.) acidophila. Our results suggest that the small mixing of S 2 character arising from symmetry-breaking of the carotenoid plays an important role in the Coulombic coupling. As a result the carotenoid (car) S 1 couplings to chlorophylls are similar to a set of scaled down Car(S 2)-BChl(Q y) couplings. We also report results for 6,10,15,19-tetramethyl-2-cis-4,6,8,10,12,14,16,18,20-all trans-22-cis-tetracosaundecaene, the polyene backbone of RG with six methyl groups attached, in two different structures: an optimized planar structure and the crystal structure of RG with hydrogen atoms replacing the two end groups, which is distorted from its planar structure. The mixing of S 2 configuration is strictly forbidden in the planar structure due to symmetry. In this case the polyene still couples moderately strongly to the nearby BChls. In the distorted structure derived from RG crystal structure, coupling strengths and the role of S 2 character mixing are similar to those of the full RG. Using an exciton model simulation, the calculated coupling strengths yield Car(S 1)-to-BChl(Q y) excitation energy transfer times that are in good agreement with recent experimental results.
Predicting the structure of the light-harvesting complex II of rhodospirillum molischianum
Protein Science, 1995
We attempted to predict through computer modeling the structure of the light-harvesting complex II (LH-Ii) of Rhodospirillum molischianum, before the impending publication of the structure of a homologous protein solved by means of X-ray diffraction. The protein studied is an integral membrane protein of 16 independent polypeptides, 8 a-apoproteins and 8 @-apoproteins, which aggregate and bind to 24 bacteriochlorophyll-a's and 12 lycopenes. Available diffraction data of a crystal of the protein, which could not be phased due to a lack of heavy metal derivatives, served to test the predicted structure, guiding the search. In order to determine the secondary structure, hydropathy analysis was performed to identify the putative transmembrane segments and multiple sequence alignment propensity analyses were used to pinpoint the exact sites of the 20-residue-long transmembrane segment and the 4-residue-long terminal sequence at both ends, which were independently verified and improved by homology modeling. A consensus assignment for the secondary structure was derived from a combination of all the prediction methods used. Three-dimensional structures for the cy-and the @-apoprotein were built by comparative modeling. The resulting tertiary structures are combined, using X-PLOR, into an a@ dimer pair with bacteriochlorophyll-a's attached under constraints provided by site-directed mutagenesis and spectral data. The a@ dimer pairs were then aggregated into a quaternary structure through further molecular dynamics simulations and energy minimization. The structure of LH-I1 so determined is an octamer of a@ heterodimers forming a ring with a diameter of 70 A.