Selective Binding of Carotenoids with a Shorter Conjugated Chain to the LH2 Antenna Complex and Those with a Longer Conjugated Chain to the Reaction Center from Rubrivivax gelatinosus

Intramolecular charge-transfer enhances energy transfer efficiency in carotenoid-reconstituted light-harvesting 1 complex of purple photosynthetic bacteria

In bacterial photosynthesis, the excitation energy transfer (EET) from carotenoids to bacteriochlorophyll a has a significant impact on the overall efficiency of the primary photosynthetic process. This efficiency can be enhanced when the involved carotenoid has intramolecular charge-transfer (ICT) character, as found in light-harvesting systems of marine alga and diatoms. Here, we provide insights into the significance of ICT excited states following the incorporation of a higher plant carotenoid, β-apo-8'-carotenal, into the carotenoidless light-harvesting 1 (LH1) complex of the purple photosynthetic bacterium Rhodospirillum rubrum strain G9+. β-apo-8'-carotenal generates the ICT excited state in the reconstituted LH1 complex, achieving an efficiency of EET of up to 79%, which exceeds that found in the wild-type LH1 complex.

Photoprotection in intact cells of photosynthetic bacteria: quenching of bacteriochlorophyll fluorescence by carotenoid triplets

Upon high light excitation in photosynthetic bacteria, various triplet states of pigments can accumulate leading to harmful effects. Here, the generation and lifetime of flash-induced carotenoid triplets (³Car) have been studied by observation of the quenching of bacteriochlorophyll (BChl) fluorescence in different strains of photosynthetic bacteria including Rvx. gelatinosus (anaerobic and semianaerobic), Rsp. rubrum, Thio. roseopersicina, Rba. sphaeroides 2.4.1 and carotenoid- and cytochrome-deficient mutants Rba. sphaeroides Ga, R-26, and cycA, respectively. The following results were obtained: (1) ³Car quenching is observed during and not exclusively after the photochemical rise of the fluorescence yield of BChl indicating that the charge separation in the reaction center (RC) and the carotenoid triplet formation are not consecutive but parallel processes. (2) The photoprotective function of ³Car is not limited to the RC only and can be described by a model in which the carotenoids are distributed in the lake of the BChl pigments. (3) The observed lifetime of ³Car in intact cells is the weighted average of the lifetimes of the carotenoids with various numbers of conjugated double bonds in the bacterial strain. (4) The lifetime of ³Car measured in the light is significantly shorter (1-2 μs) than that measured in the dark (2-10 μs). The difference reveals the importance of the dynamics of ³Car before relaxation. The results will be discussed not only in terms of energy levels of the ³Car but also in terms of the kinetics of transitions among different sublevels in the excited triplet state of the carotenoid.

Redesign, reconstruction, and directed extension of the Brevibacterium linens C40 carotenoid pathway in Escherichia coli

In this study, the carotenoid biosynthetic pathways of Brevibacterium linens DSMZ 20426 were reconstructed, redesigned, and extended with additional carotenoid-modifying enzymes of other sources in a heterologous host Escherichia coli. The modular lycopene pathway synthesized an unexpected carotenoid structure, 3,4-didehydrolycopene, as well as lycopene. Extension of the novel 3,4-didehydrolycopene pathway with the mutant Pantoea lycopene cyclase CrtY(2) and the Rhodobacter spheroidene monooxygenase CrtA generated monocyclic torulene and acyclic oxocarotenoids, respectively. The reconstructed beta-carotene pathway synthesized an unexpected 7,8-dihydro-beta-carotene in addition to beta-carotene. Extension of the beta-carotene pathway with the B. linens beta-ring desaturase CrtU and Pantoea beta-carotene hydroxylase CrtZ generated asymmetric carotenoid agelaxanthin A, which had one aromatic ring at the one end of carotene backbone and one hydroxyl group at the other end, as well as aromatic carotenoid isorenieratene and dihydroxy carotenoid zeaxanthin. These results demonstrate that reconstruction of the biosynthetic pathways and extension with promiscuous enzymes in a heterologous host holds promise as a rational strategy for generating structurally diverse compounds that are hardly accessible in nature.

Purification and partial characterization of a lutein-binding protein from human retina

Dietary intake of lutein and zeaxanthin appears to be advantageous for protecting human retinal and macular tissues from degenerative disorders such as age-related macular degeneration. Selective concentration of just two of the many dietary carotenoids suggests that uptake and transport of these xanthophyll carotenoids into the human foveal region are mediated by specific xanthophyll-binding proteins such as GSTP1 which has previously been identified as the zeaxanthin-binding protein of the primate macula. Here, a membrane-associated human retinal lutein-binding protein (HR-LBP) was purified from human peripheral retina using ion-exchange chromatography followed by size-exclusion chromatography. After attaining 83-fold enrichment of HR-LBP, this protein exhibited a significant bathochromic shift of approximately 90 nm in association with lutein, and equilibrium binding studies demonstrated saturable, specific binding toward lutein with a K(D) of 0.45 muM. Examination for cross-reactivity with antibodies raised against known lutein-binding proteins from other organisms revealed consistent labeling of a major protein band of purified HR-LBP at approximately 29 kDa with an antibody raised against silkworm (Bombyx mori) carotenoid-binding protein (CBP), a member of steroidogenic acute regulatory (StAR) protein family with significant homology to many human StAR proteins. Immunolocalization with antibodies directed against either CBP or GSTP1 showed specific labeling of rod and cone inner segments, especially in the mitochondria-rich ellipsoid region. There was also strong labeling of the outer plexiform (Henle fiber) layer with anti-GSTP1. Such localizations compare favorably with the distribution of macular carotenoids as revealed by resonance Raman microscopy. Our results suggest that HR-LBP may facilitate lutein's localization to a region of the cell subject to considerable oxidative stress.

Composition and localization of bacteriochlorophyll a intermediates in the purple photosynthetic bacterium Rhodopseudomonas sp. Rits

Rhodopseudomonas sp. Rits is a recently isolated new species of photosynthetic bacteria and found to accumulate a significantly high amount of bacteriochlorophyll (BChl) a intermediates possessing non-, di- and tetra-hydrogenated geranylgeranyl groups at the 17-propionate as well as normal phytylated BChl a (Mizoguchi T et al. (2006) FEBS Lett 580:137-143). A phylogenetic analysis showed that this bacterium was closely related to Rhodopseudomonas palustris. The strain Rits synthesizes light-harvesting complexes 2 and 4 (LH2/4), as peripheral antennas, as well as the reaction center and light-harvesting 1 core complex (RC-LH1 core). The amounts of these complexes were dependent upon the incident light intensities, which was also a typical behavior of Rhodopseudomonas palustris. HPLC analyses of extracted pigments indicated that all four BChls a were associated with the purified photosynthetic pigment-protein, as complexes described above. The results suggested that this bacterium could use these pigments as functional molecules within the LH2/4 and RC-LH1 core. Pigment compositional analyses in several purple photosynthetic bacteria showed that such BChl a intermediates were always detected and were more widely distributed than expected. Long chains in the propionate moiety of BChl a would be one of the important factors for assembly of LH systems in purple photosynthetic bacteria.