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1.
Nat Commun ; 11(1): 2481, 2020 05 18.
Artigo em Inglês | MEDLINE | ID: mdl-32424145

RESUMO

Photosynthetic light-harvesting complexes (LHCs) play a pivotal role in collecting solar energy for photochemical reactions in photosynthesis. One of the major LHCs are fucoxanthin chlorophyll a/c-binding proteins (FCPs) present in diatoms, a group of organisms having important contribution to the global carbon cycle. Here, we report a 2.40-Å resolution structure of the diatom photosystem I (PSI)-FCPI supercomplex by cryo-electron microscopy. The supercomplex is composed of 16 different FCPI subunits surrounding a monomeric PSI core. Each FCPI subunit showed different protein structures with different pigment contents and binding sites, and they form a complicated pigment-protein network together with the PSI core to harvest and transfer the light energy efficiently. In addition, two unique, previously unidentified subunits were found in the PSI core. The structure provides numerous insights into not only the light-harvesting strategy in diatom PSI-FCPI but also evolutionary dynamics of light harvesters among oxyphototrophs.


Assuntos
Diatomáceas/metabolismo , Complexos de Proteínas Captadores de Luz/química , Complexos de Proteínas Captadores de Luz/metabolismo , Complexo de Proteína do Fotossistema I/química , Complexo de Proteína do Fotossistema I/metabolismo , Clorofila/metabolismo , Proteínas de Ligação à Clorofila/química , Proteínas de Ligação à Clorofila/ultraestrutura , Transferência de Energia , Complexos de Proteínas Captadores de Luz/ultraestrutura , Modelos Moleculares , Complexo de Proteína do Fotossistema I/ultraestrutura , Ligação Proteica , Subunidades Proteicas/metabolismo , Relação Estrutura-Atividade
2.
Nat Commun ; 11(1): 1361, 2020 03 13.
Artigo em Inglês | MEDLINE | ID: mdl-32170184

RESUMO

Grana are a characteristic feature of higher plants' thylakoid membranes, consisting of stacks of appressed membranes enriched in Photosystem II (PSII) and associated light-harvesting complex II (LHCII) proteins, together forming the PSII-LHCII supercomplex. Grana stacks undergo light-dependent structural changes, mainly by reorganizing the supramolecular structure of PSII-LHCII supercomplexes. LHCII is vital for grana formation, in which also PSII-LHCII supercomplexes are involved. By combining top-down and crosslinking mass spectrometry we uncover the spatial organization of paired PSII-LHCII supercomplexes within thylakoid membranes. The resulting model highlights a basic molecular mechanism whereby plants maintain grana stacking at changing light conditions. This mechanism relies on interactions between stroma-exposed N-terminal loops of LHCII trimers and Lhcb4 subunits facing each other in adjacent membranes. The combination of light-dependent LHCII N-terminal trimming and extensive N-terminal α-acetylation likely affects interactions between pairs of PSII-LHCII supercomplexes across the stromal gap, ultimately mediating membrane folding in grana stacks.


Assuntos
Complexos de Proteínas Captadores de Luz/metabolismo , Complexo de Proteína do Fotossistema II/metabolismo , Plantas/metabolismo , Proteínas Quinases/metabolismo , Tilacoides/metabolismo , Proteínas de Ligação à Clorofila/metabolismo , Embriófitas , Luz , Complexos de Proteínas Captadores de Luz/química , Espectrometria de Massas/métodos , Modelos Moleculares , Complexo de Proteína do Fotossistema II/química , Proteínas de Plantas/metabolismo , Conformação Proteica , Proteínas Quinases/química , Proteômica
3.
Nat Commun ; 11(1): 1460, 2020 03 19.
Artigo em Inglês | MEDLINE | ID: mdl-32193383

RESUMO

Since the discovery of quantum beats in the two-dimensional electronic spectra of photosynthetic pigment-protein complexes over a decade ago, the origin and mechanistic function of these beats in photosynthetic light-harvesting has been extensively debated. The current consensus is that these long-lived oscillatory features likely result from electronic-vibrational mixing, however, it remains uncertain if such mixing significantly influences energy transport. Here, we examine the interplay between the electronic and nuclear degrees of freedom (DoF) during the excitation energy transfer (EET) dynamics of light-harvesting complex II (LHCII) with two-dimensional electronic-vibrational spectroscopy. Particularly, we show the involvement of the nuclear DoF during EET through the participation of higher-lying vibronic chlorophyll states and assign observed oscillatory features to specific EET pathways, demonstrating a significant step in mapping evolution from energy to physical space. These frequencies correspond to known vibrational modes of chlorophyll, suggesting that electronic-vibrational mixing facilitates rapid EET over moderately size energy gaps.


Assuntos
Transferência de Energia , Complexos de Proteínas Captadores de Luz/química , Teoria Quântica , Elétrons , Complexos de Proteínas Captadores de Luz/metabolismo , Modelos Químicos , Folhas de Planta/citologia , Análise Espectral , Tilacoides/metabolismo
4.
Biochim Biophys Acta Bioenerg ; 1861(7): 148191, 2020 07 01.
Artigo em Inglês | MEDLINE | ID: mdl-32201306

RESUMO

Light-harvesting complex II (LHCII) from the marine green macroalga Bryopsis corticulans is spectroscopically characterized to understand the structural and functional changes resulting from adaptation to intertidal environment. LHCII is homologous to its counterpart in land plants but has a different carotenoid and chlorophyll (Chl) composition. This is reflected in the steady-state absorption, fluorescence, linear dichroism, circular dichroism and anisotropic circular dichroism spectra. Time-resolved fluorescence and two-dimensional electronic spectroscopy were used to investigate the consequences of this adaptive change in the pigment composition on the excited-state dynamics. The complex contains additional Chl b spectral forms - absorbing at around 650 nm and 658 nm - and lacks the red-most Chl a forms compared with higher-plant LHCII. Similar to plant LHCII, energy transfer between Chls occurs on timescales from under hundred fs (mainly from Chl b to Chl a) to several picoseconds (mainly between Chl a pools). However, the presence of long-lived, weakly coupled Chl b and Chl a states leads to slower exciton equilibration in LHCII from B. corticulans. The finding demonstrates a trade-off between the enhanced absorption of blue-green light and the excitation migration time. However, the adaptive change does not result in a significant drop in the overall photochemical efficiency of Photosystem II. These results show that LHCII is a robust adaptable system whose spectral properties can be tuned to the environment for optimal light harvesting.


Assuntos
Clorófitas/metabolismo , Transferência de Energia , Complexos de Proteínas Captadores de Luz/metabolismo , Dicroísmo Circular , Espectrometria de Fluorescência , Temperatura , Fatores de Tempo
5.
Biochim Biophys Acta Bioenerg ; 1861(5-6): 148186, 2020 06 01.
Artigo em Inglês | MEDLINE | ID: mdl-32171793

RESUMO

The light-harvesting complexes II (LHCIIs) of spinach and Bryopsis corticulans as a green alga are similar in structure, but differ in carotenoid (Car) and chlorophyll (Chl) compositions. Carbonyl Cars siphonein (Spn) and siphonaxanthin (Spx) bind to B. corticulans LHCII likely in the sites as a pair of lutein (Lut) molecules bind to spinach LHCII in the central domain. To understand the light-harvesting and photoprotective properties of the algal LHCII, we compared its excitation dynamics and relaxation to those of spinach LHCII been well documented. It was found that B. corticulans LHCII exhibited a substantially longer chlorophyll (Chl) fluorescence lifetime (4.9 ns vs 4.1 ns) and a 60% increase of the fluorescence quantum yield. Photoexcitation populated 3Car* equally between Spn and Spx in B. corticulans LHCII, whereas predominantly at Lut620 in spinach LHCII. These results prove the functional differences of the LHCIIs with different Car pairs and Chl a/b ratios: B. corticulans LHCII shows the enhanced blue-green light absorption, the alleviated quenching of 1Chl*, and the dual sites of quenching 3Chl*, which may facilitate its light-harvesting and photoprotection functions. Moreover, for both types of LHCIIs, the triplet excitation profiles revealed the involvement of extra 3Car* formation mechanisms besides the conventional Chl-to-Car triplet transfer, which are discussed in relation to the ultrafast processes of 1Chl* quenching. Our experimental findings will be helpful in deepening the understanding of the light harvesting and photoprotection functions of B. corticulans living in the intertidal zone with dramatically changing light condition.


Assuntos
Clorófitas/metabolismo , Complexos de Proteínas Captadores de Luz/metabolismo , Água do Mar , Carotenoides/metabolismo , Clorofila/metabolismo , Cinética , Espectrometria de Fluorescência , Spinacia oleracea/metabolismo
6.
Biochim Biophys Acta Bioenerg ; 1861(5-6): 148184, 2020 06 01.
Artigo em Inglês | MEDLINE | ID: mdl-32179058

RESUMO

The Photosystem I (PSI) reaction center in cyanobacteria is comprised of ~96 chlorophyll (Chl) molecules, including six specialized Chl molecules denoted Chl1A/Chl1B (P700), Chl2A/Chl2B, and Chl3A/Chl3B that are arranged in two branches and function in primary charge separation. It has recently been proposed that PSI from Chroococcidiopsis thermalis (Nürnberg et al. (2018) Science 360, 1210-1213) and Fischerella thermalis PCC 7521 (Hastings et al. (2019) Biochim. Biophys. Acta 1860, 452-460) contain Chl f in the positions Chl2A/Chl2B. We tested this proposal by exciting RCs from white-light grown (WL-PSI) and far-red light grown (FRL-PSI) F. thermalis PCC 7521 with femtosecond pulses and analyzing the optical dynamics. If Chl f were in the position Chl2A/Chl2B in FRL-PSI, excitation at 740 nm should have produced the charge-separated state P700+A0- followed by electron transfer to A1 with a τ of ≤25 ps. Instead, it takes ~230 ps for the charge-separated state to develop because the excitation migrates uphill from Chl f in the antenna to the trapping center. Further, we observe a strong electrochromic shift at 685 nm in the final P700+A1- spectrum that can only be explained if Chl a is in the positions Chl2A/Chl2B. Similar arguments rule out the presence of Chl f in the positions Chl3A/Chl3B; hence, Chl f is likely to function solely as an antenna pigment in FRL-PSI. We additionally report the presence of an excitonically coupled homo- or heterodimer of Chl f absorbing around 790 nm that is kinetically independent of the Chl f population that absorbs around 740 nm.


Assuntos
Clorofila/análogos & derivados , Cianobactérias/metabolismo , Cianobactérias/efeitos da radiação , Complexos de Proteínas Captadores de Luz/metabolismo , Luz , Complexo de Proteína do Fotossistema I/metabolismo , Clorofila/metabolismo , Espectrometria de Fluorescência
7.
Biochim Biophys Acta Bioenerg ; 1861(5-6): 148115, 2020 06 01.
Artigo em Inglês | MEDLINE | ID: mdl-32204904

RESUMO

Green plants protect against photodamage by dissipating excess energy in a process called non-photochemical quenching (NPQ). In vivo, NPQ is activated by a drop in the luminal pH of the thylakoid membrane that triggers conformational changes of the antenna complexes, which activate quenching channels. The drop in pH also triggers de-epoxidation of violaxanthin, one of the carotenoids bound within the antenna complexes, into zeaxanthin, and so the amplitude of NPQ in vivo has been shown to increase in the presence of zeaxanthin. In vitro studies on light-harvesting complex II (LHCII), the major antenna complex in plants, compared different solubilization environments, which give rise to different levels of quenching and so partially mimic NPQ in vivo. However, in these studies both completely zeaxanthin-independent and zeaxanthin-dependent quenching have been reported, potentially due to the multiplicity of solubilization environments. Here, we characterize the zeaxanthin dependence of the photophysics in LHCII in a near-physiological membrane environment, which produces slightly enhanced quenching relative to detergent solubilization, the typical in vitro environment. The photophysical pathways of dark-adapted and in vitro de-epoxidized LHCIIs are compared, representative of the low-light and high-light conditions in vivo, respectively. The amplitude of quenching as well as the dissipative photophysics are unaffected by zeaxanthin at the level of individual LHCIIs, suggesting that zeaxanthin-dependent quenching is independent of the channels induced by the membrane. Furthermore, our results demonstrate that additional factors beyond zeaxanthin incorporation in LHCII are required for full development of NPQ.


Assuntos
Membrana Celular/metabolismo , Membrana Celular/efeitos da radiação , Complexos de Proteínas Captadores de Luz/metabolismo , Luz , Zeaxantinas/metabolismo , Carotenoides/metabolismo , Clorofila/metabolismo , Transferência de Energia , Fluorescência , Concentração de Íons de Hidrogênio , Modelos Moleculares , Spinacia oleracea/metabolismo , Zeaxantinas/química
8.
Nat Commun ; 11(1): 1295, 2020 03 10.
Artigo em Inglês | MEDLINE | ID: mdl-32157079

RESUMO

Plants prevent photodamage under high light by dissipating excess energy as heat. Conformational changes of the photosynthetic antenna complexes activate dissipation by leveraging the sensitivity of the photophysics to the protein structure. The mechanisms of dissipation remain debated, largely due to two challenges. First, because of the ultrafast timescales and large energy gaps involved, measurements lacked the temporal or spectral requirements. Second, experiments have been performed in detergent, which can induce non-native conformations, or in vivo, where contributions from homologous antenna complexes cannot be disentangled. Here, we overcome both challenges by applying ultrabroadband two-dimensional electronic spectroscopy to the principal antenna complex, LHCII, in a near-native membrane. Our data provide evidence that the membrane enhances two dissipative pathways, one of which is a previously uncharacterized chlorophyll-to-carotenoid energy transfer. Our results highlight the sensitivity of the photophysics to local environment, which may control the balance between light harvesting and dissipation in vivo.


Assuntos
Carotenoides/metabolismo , Membrana Celular/metabolismo , Clorofila/metabolismo , Transferência de Energia , Complexos de Proteínas Captadores de Luz/metabolismo , Nanoestruturas/química , Complexos de Proteínas Captadores de Luz/química , Conformação Proteica
9.
Proc Natl Acad Sci U S A ; 117(12): 6502-6508, 2020 03 24.
Artigo em Inglês | MEDLINE | ID: mdl-32139606

RESUMO

Carotenoids play a number of important roles in photosynthesis, primarily providing light-harvesting and photoprotective energy dissipation functions within pigment-protein complexes. The carbon-carbon double bond (C=C) conjugation length of carotenoids (N), generally between 9 and 15, determines the carotenoid-to-(bacterio)chlorophyll [(B)Chl] energy transfer efficiency. Here we purified and spectroscopically characterized light-harvesting complex 2 (LH2) from Rhodobacter sphaeroides containing the N = 7 carotenoid zeta (ζ)-carotene, not previously incorporated within a natural antenna complex. Transient absorption and time-resolved fluorescence show that, relative to the lifetime of the S1 state of ζ-carotene in solvent, the lifetime decreases ∼250-fold when ζ-carotene is incorporated within LH2, due to transfer of excitation energy to the B800 and B850 BChls a These measurements show that energy transfer proceeds with an efficiency of ∼100%, primarily via the S1 → Qx route because the S1 → S0 fluorescence emission of ζ-carotene overlaps almost perfectly with the Qx absorption band of the BChls. However, transient absorption measurements performed on microsecond timescales reveal that, unlike the native N ≥ 9 carotenoids normally utilized in light-harvesting complexes, ζ-carotene does not quench excited triplet states of BChl a, likely due to elevation of the ζ-carotene triplet energy state above that of BChl a These findings provide insights into the coevolution of photosynthetic pigments and pigment-protein complexes. We propose that the N ≥ 9 carotenoids found in light-harvesting antenna complexes represent a vital compromise that retains an acceptable level of energy transfer from carotenoids to (B)Chls while allowing acquisition of a new, essential function, namely, photoprotective quenching of harmful (B)Chl triplets.


Assuntos
Proteínas de Bactérias/metabolismo , Bacterioclorofilas/metabolismo , Carotenoides/metabolismo , Complexos de Proteínas Captadores de Luz/metabolismo , Proteínas de Bactérias/química , Carotenoides/química , Transferência de Energia , Cinética , Complexos de Proteínas Captadores de Luz/química , Fotossíntese , Rhodobacter sphaeroides/química , Rhodobacter sphaeroides/metabolismo
10.
J Phys Chem Lett ; 11(5): 1636-1643, 2020 Mar 05.
Artigo em Inglês | MEDLINE | ID: mdl-32013435

RESUMO

High efficiency of light harvesting in photosynthetic pigment-protein complexes is governed by evolutionary-perfected protein-assisted tuning of individual pigment properties and interpigment interactions. Due to the large number of spectrally overlapping pigments in a typical photosynthetic complex, experimental methods often fail to unambiguously identify individual chromophore properties. Here, we report a first-principles-based modeling protocol capable of predicting properties of pigments in protein environment to a high precision. The technique was applied to successfully uncover electronic properties of the Fenna-Matthews-Olson (FMO) pigment-protein complex. Each of the three subunits of the FMO complex contains eight strongly coupled bacteriochlorophyll a (BChl a) pigments. The excitonic structure of FMO can be described by an electronic Hamiltonian containing excitation (site) energies of BChl a pigments and electronic couplings between them. Several such Hamiltonians have been developed in the past based on the information from various spectroscopic measurements of FMO; however, fine details of the excitonic structure and energy transfer in FMO, especially assignments of short-lived high-energy sites, remain elusive. Utilizing polarizable embedding quantum mechanics/molecular mechanics with the effective fragment potentials, we computed the electronic Hamiltonian of FMO that is in general agreement with previously reported empirical Hamiltonians and quantitatively reproduces experimental absorption and circular dichroism spectra of the FMO protein. The developed computational protocol is sufficiently simple and can be utilized for predictive modeling of other wild-type and mutated photosynthetic pigment-protein complexes.


Assuntos
Proteínas de Bactérias/metabolismo , Complexos de Proteínas Captadores de Luz/metabolismo , Simulação de Dinâmica Molecular , Teoria Quântica , Proteínas de Bactérias/química , Bacterioclorofila A/química , Bacterioclorofila A/metabolismo , Chlorobi/metabolismo , Dicroísmo Circular , Transferência de Energia , Gases/química , Complexos de Proteínas Captadores de Luz/química , Fotossíntese
12.
Nat Commun ; 11(1): 662, 2020 01 31.
Artigo em Inglês | MEDLINE | ID: mdl-32005811

RESUMO

The photosynthetic apparatus of higher plants can dissipate excess excitation energy during high light exposure, by deactivating excited chlorophylls through a mechanism called nonphotochemical quenching (NPQ). However, the precise molecular details of quenching and the mechanism regulating the quenching level are still not completely understood. Focusing on the major light-harvesting complex LHCII of Photosystem II, we show that a charge transfer state involving Lutein can efficiently quench chlorophyll excitation, and reduce the excitation lifetime of LHCII to the levels measured in the deeply quenched LHCII aggregates. Through a combination of molecular dynamics simulations, multiscale quantum chemical calculations, and kinetic modeling, we demonstrate that the quenching level can be finely tuned by the protein, by regulating the energy of the charge transfer state. Our results suggest that a limited conformational rearrangement of the protein scaffold could act as a molecular switch to activate or deactivate the quenching mechanism.


Assuntos
Carotenoides/metabolismo , Clorofila/metabolismo , Plantas/metabolismo , Carotenoides/química , Clorofila/química , Transporte de Elétrons , Cinética , Luz , Complexos de Proteínas Captadores de Luz/química , Complexos de Proteínas Captadores de Luz/metabolismo , Fotossíntese , Complexo de Proteína do Fotossistema II/química , Complexo de Proteína do Fotossistema II/metabolismo , Plantas/química , Plantas/efeitos da radiação
13.
Biochim Biophys Acta Bioenerg ; 1861(3): 148156, 2020 03 01.
Artigo em Inglês | MEDLINE | ID: mdl-31987813

RESUMO

In plants and green algae, light-harvesting complexes (LHCs) are a large family of chlorophyll binding proteins functioning as antennae, collecting solar photons and transferring the absorbed energy to the photosynthetic reaction centers, where light to chemical energy conversion begins. Although LHCs are all highly homologous in their structure and display a variety of common features, each complex finds a specific location and task in the energy transport. One example is CP29, which occupies a pivotal position in Photosystem II, bridging the peripheral antennae to the core. The design principles behind this specificity, however, are still unclear. Here, a synergetic approach combining steady-state and ultrafast spectroscopy, mutational analysis and structure-based exciton modeling allows uncovering the energy landscape of the chlorophylls bound to this complex. We found that, although displaying an overall highly conserved exciton structure very similar to that of other LHCs, CP29 possesses an additional terminal emitter domain. The simultaneous presence of two low energy sites facing the peripheral antennae and the core, allows CP29 to efficiently work as a conduit in the energy flux. Our results show that the LHCs share a common solid architecture but have finely tuned their structure to carry out specific functions.


Assuntos
Complexos de Proteínas Captadores de Luz/metabolismo , Complexo de Proteína do Fotossistema II/metabolismo , Plantas/metabolismo , Plantas/efeitos da radiação , Luz Solar , Clorofila/metabolismo , Transferência de Energia , Modelos Moleculares , Mutação/genética , Termodinâmica
14.
Biochim Biophys Acta Bioenerg ; 1861(4): 148038, 2020 04 01.
Artigo em Inglês | MEDLINE | ID: mdl-31229568

RESUMO

Photosynthesis is a fundamental biological process involving the conversion of solar energy into chemical energy. The initial photochemical and photophysical events of photosynthesis are mediated by photosystem II (PSII) and photosystem I (PSI). Both PSII and PSI are multi-subunit supramolecular machineries composed of a core complex and a peripheral antenna system. The antenna system serves to capture light energy and transfer it to the core efficiently. Both PSII and PSI in the green lineage (plants and green algae) and PSI in red algae have an antenna system comprising a series of chlorophyll- and carotenoid-binding membrane proteins belonging to the light-harvesting complex (LHC) superfamily, including LHCII and LHCI. However, the antenna size and subunit composition vary considerably in the two photosystems from diverse organisms. On the basis of the plant and algal LHCII and LHCI structures that have been solved by X-ray crystallography and single-particle cryo-electron microscopy we review the detailed structural features and characteristic pigment properties of these LHCs in PSII and PSI. This article is part of a Special Issue entitled Light harvesting, edited by Dr. Roberta Croce.


Assuntos
Complexos de Proteínas Captadores de Luz/química , Sequência de Aminoácidos , Apoproteínas/química , Apoproteínas/metabolismo , Chlamydomonas reinhardtii/metabolismo , Clorofila/metabolismo , Complexos de Proteínas Captadores de Luz/metabolismo , Modelos Moleculares , Complexo de Proteína do Fotossistema I/metabolismo , Complexo de Proteína do Fotossistema II/química , Complexo de Proteína do Fotossistema II/metabolismo , Subunidades Proteicas/química , Subunidades Proteicas/metabolismo , Rodófitas/metabolismo
15.
Biochim Biophys Acta Bioenerg ; 1861(4): 148047, 2020 04 01.
Artigo em Inglês | MEDLINE | ID: mdl-31306623

RESUMO

Cyanobacteria and red-algae share a common light-harvesting complex which is different than all other complexes that serve as photosynthetic antennas - the Phycobilisome (PBS). The PBS is found attached to the stromal side of thylakoid membranes, filling up most of the gap between individual thylakoids. The PBS self assembles from similar homologous protein units that are soluble and contain conserved cysteine residues that covalently bind the light absorbing chromophores, linear tetra-pyrroles. Using similar construction principles, the PBS can be as large as 16.8 MDa (68×45×39nm), as small as 1.2 MDa (24 × 11.5 × 11.5 nm), and in some unique cases smaller still. The PBS can absorb light between 450 nm to 650 nm and in some cases beyond 700 nm, depending on the species, its composition and assembly. In this review, we will present new observations and structures that expand our understanding of the distinctive properties that make the PBS an amazing light harvesting system. At the end we will suggest why the PBS, for all of its excellent properties, was discarded by photosynthetic organisms that arose later in evolution such as green algae and higher plants.


Assuntos
Ficobilissomas/metabolismo , Proteínas de Bactérias/química , Proteínas de Bactérias/metabolismo , Transferência de Energia , Complexos de Proteínas Captadores de Luz/metabolismo , Modelos Moleculares , Processos Fotoquímicos , Ficobilissomas/química
16.
Biochim Biophys Acta Bioenerg ; 1861(4): 148078, 2020 04 01.
Artigo em Inglês | MEDLINE | ID: mdl-31476286

RESUMO

We describe a molecular mechanism tuning the functional properties of chlorophyll a (Chl-a) molecules in photosynthetic antenna proteins. Light-harvesting complexes from photosystem II in higher plants - specifically LHCII purified with α- or ß-dodecyl-maltoside, along with CP29 - were probed by low-temperature absorption and resonance Raman spectroscopies. We show that hydrogen bonding to the conjugated keto carbonyl group of protein-bound Chl-a tunes the energy of its Soret and Qy absorption transitions, inducing red-shifts that are proportional to the strength of the hydrogen bond involved. Chls-a with non-H-bonded keto C131 groups exhibit the blue-most absorption bands, while both transitions are progressively red-shifted with increasing hydrogen-bonding strength - by up 382 & 605 cm-1 in the Qy and Soret band, respectively. These hydrogen bonds thus tune the site energy of Chl-a in light-harvesting proteins, determining (at least in part) the cascade of energy transfer events in these complexes.


Assuntos
Clorofila A/metabolismo , Complexos de Proteínas Captadores de Luz/metabolismo , Clorofila A/química , Ligação de Hidrogênio , Análise Espectral Raman
17.
Biochim Biophys Acta Bioenerg ; 1861(2): 148139, 2020 02 01.
Artigo em Inglês | MEDLINE | ID: mdl-31825812

RESUMO

An aerial green alga, Prasiola crispa (Lightf.) Menegh, which is known to form large colonies in Antarctic habitats, is subject to severe environmental stresses due to low temperature, draught and strong sunlight in summer. A considerable light-absorption by long-wavelength chlorophylls (LWC) at around 710 nm, which seem to consist of chlorophyll a, was detected in thallus of P. crispa harvested at a terrestrial environment in Antarctica. Absorption level at 710 nm against that at 680 nm was correlated with fluorescence emission intensity at 713 nm at room temperature and the 77 K fluorescence emission band from LWC was found to be emitted at 735 nm. We demonstrated that the LWC efficiently transfer excitation energy to photosystem II (PSII) reaction center from measurements of action spectra of photosynthetic oxygen evolution and P700 photo-oxidation. The global quantum yield of PSII excitation in thallus by far-red light was shown to be as high as by orange light, and the excitation balance between PSII and PSI was almost same in the two light sources. It is thus proposed that the LWC increase the photosynthetic productivity in the lower parts of overlapping thalli and contribute to the predominance of alga in the severe environment.


Assuntos
Clorofila A/metabolismo , Clorófitas/metabolismo , Luz , Complexo de Proteína do Fotossistema II/metabolismo , Regiões Antárticas , Complexos de Proteínas Captadores de Luz/metabolismo , Oxigênio/metabolismo , Complexo de Proteína do Fotossistema I/metabolismo , Espectrometria de Fluorescência
18.
Biochim Biophys Acta Bioenerg ; 1861(4): 148035, 2020 04 01.
Artigo em Inglês | MEDLINE | ID: mdl-31226317

RESUMO

Proper assembly of plant photosystem II, in the appressed region of thylakoids, allows for both efficient light harvesting and the dissipation of excitation energy absorbed in excess. The core moiety of wild type supercomplex is associated with monomeric antennae that, in turn, bind peripheral trimeric LHCII complexes. Acclimation to light environment dynamics involves structural plasticity within PSII-LHCs supercomplexes, including depletion in LHCII and CP24. Here, we report on the acclimation of NoM, an Arabidopsis mutant lacking monomeric LHCs but retaining LHCII trimer. Lack of monomeric LHCs impaired the operation of both photosynthetic electron transport and state transitions, despite the fact that NoM underwent a compensatory over-accumulation of the LHCII complement compared to the wild type. Mutant plants displayed stunted growth compared to the wild type when probed over a range of light conditions. When exposed to short-term excess light, NoM showed higher photosensitivity and enhanced singlet oxygen release than the wild type, whereas long-term acclimation under stress conditions was unaffected. Analysis of pigment-binding supercomplexes showed that the absence of monomeric LHCs did affect the macro-organisation of photosystems: large PSI-LHCII megacomplexes were more abundant in NoM, whereas the assembly of PSII-LHCs supercomplexes was impaired. Observation by electron microscopy (EM) and image analysis of thylakoids highlighted impaired granal stacking and membrane organisation, with a heterogeneous distribution of PSII and LHCII compared to the wild type. It is concluded that monomeric LHCs are critical for the structural and functional optimisation of the photosynthetic apparatus.


Assuntos
Transferência de Energia , Complexos de Proteínas Captadores de Luz/metabolismo , Complexo de Proteína do Fotossistema II/metabolismo , Tilacoides/metabolismo , Arabidopsis/crescimento & desenvolvimento , Arabidopsis/metabolismo , Arabidopsis/efeitos da radiação , Biomassa , Luz , Mutação/genética , Oxirredução/efeitos da radiação , Fotossíntese/efeitos da radiação , Pigmentos Biológicos/metabolismo , Espectrometria de Fluorescência , Tilacoides/efeitos da radiação , Tilacoides/ultraestrutura
19.
Biochim Biophys Acta Bioenerg ; 1861(4): 148050, 2020 04 01.
Artigo em Inglês | MEDLINE | ID: mdl-31326408

RESUMO

During the past two decades, two-dimensional electronic spectroscopy (2DES) and related techniques have emerged as a potent experimental toolset to study the ultrafast elementary steps of photosynthesis. Apart from the highly engaging albeit controversial analysis of the role of quantum coherences in the photosynthetic processes, 2DES has been applied to resolve the dynamics and pathways of energy and electron transport in various light-harvesting antenna systems and reaction centres, providing unsurpassed level of detail. In this paper we discuss the main technical approaches and their applicability for solving specific problems in photosynthesis. We then recount applications of 2DES to study the exciton dynamics in plant and photosynthetic light-harvesting complexes, especially light-harvesting complex II (LHCII) and the fucoxanthin-chlorophyll proteins of diatoms, with emphasis on the types of unique information about such systems that 2DES is capable to deliver. This article is part of a Special Issue entitled Light harvesting, edited by Dr. Roberta Croce.


Assuntos
Elétrons , Transferência de Energia , Complexos de Proteínas Captadores de Luz/metabolismo , Plantas/metabolismo , Análise Espectral , Temperatura
20.
Biochim Biophys Acta Bioenerg ; 1861(4): 148049, 2020 04 01.
Artigo em Inglês | MEDLINE | ID: mdl-31386831

RESUMO

Light-harvesting is a crucial step of photosynthesis. Its mechanisms and related energetics have been revealed by a combination of experimental investigations and theoretical modeling. The success of theoretical modeling is largely due to the application of atomistic descriptions combining quantum chemistry, classical models and molecular dynamics techniques. Besides the important achievements obtained so far, a complete and quantitative understanding of how the many different light-harvesting complexes exploit their structural specificity is still missing. Moreover, many questions remain unanswered regarding the mechanisms through which light-harvesting is regulated in response to variable light conditions. Here we show that, in both fields, a major role will be played once more by atomistic descriptions, possibly generalized to tackle the numerous time and space scales on which the regulation takes place: going from the ultrafast electronic excitation of the multichromophoric aggregate, through the subsequent conformational changes in the embedding protein, up to the interaction between proteins.


Assuntos
Complexos de Proteínas Captadores de Luz/metabolismo , Luz , Simulação de Dinâmica Molecular , Processos Fotoquímicos/efeitos da radiação
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