Disentangling the low-energy states of the major light-harvesting complex of plants and their role in photoprotection (original) (raw)
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Crystal structure of plant light-harvesting complex shows the active, energy-transmitting state
Embo Journal, 2009
Plants dissipate excess excitation energy as heat by nonphotochemical quenching (NPQ). NPQ has been thought to resemble in vitro aggregation quenching of the major antenna complex, light harvesting complex of photosystem II (LHC-II). Both processes are widely believed to involve a conformational change that creates a quenching centre of two neighbouring pigments within the complex. Using recombinant LHC-II lacking the pigments implicated in quenching, we show that they have no particular role. Single crystals of LHC-II emit strong, orientationdependent fluorescence with an emission maximum at 680 nm. The average lifetime of the main 680 nm crystal emission at 100 K is 1.31 ns, but only 0.39 ns for LHC-II aggregates under identical conditions. The strong emission and comparatively long fluorescence lifetimes of single LHC-II crystals indicate that the complex is unquenched, and that therefore the crystal structure shows the active, energy-transmitting state of LHC-II. We conclude that quenching of excitation energy in the lightharvesting antenna is due to the molecular interaction with external pigments in vitro or other pigment-protein complexes such as PsbS in vivo, and does not require a conformational change within the complex.
Controlled Disorder in Plant Light-Harvesting Complex II Explains Its Photoprotective Role
Biophysical Journal, 2012
The light-harvesting antenna of photosystem II (PSII) has the ability to switch rapidly between a state of efficient light use and one in which excess excitation energy is harmlessly dissipated as heat, a process known as qE. We investigated the single-molecule fluorescence intermittency of the main component of the PSII antenna (LHCII) under conditions that mimic efficient use of light or qE, and we demonstrate that weakly fluorescing states are stabilized under qE conditions. Thus, we propose that qE is explained by biological control over the intrinsic dynamic disorder in the complex-the frequencies of switching establish whether the population of complexes is unquenched or quenched. Furthermore, the quenched states were accompanied by two distinct spectral signatures, suggesting more than one mechanism for energy dissipation in LHCII.
Single Molecule Spectroscopy on the Light-Harvesting Complex II of Higher Plants
Biophysical Journal, 2001
Spectroscopic and polarization properties of single light-harvesting complexes of higher plants (LHC-II) were studied at both room temperature and T Ͻ 5 K. Monomeric complexes emit roughly linearly polarized fluorescence light thus indicating the existence of only one emitting state. Most probably this observation is explained by efficient triplet quenching restricted to one chlorophyll a (Chl a) molecule or by rather irreversible energy transfer within the pool of Chl a molecules. LHC-II complexes in the trimeric (native) arrangement bleach in a number of steps, suggesting localization of excitations within the monomeric subunits. Interpretation of the fluorescence polarization properties of trimers requires the assumption of transition dipole moments tilted out of the symmetry plane of the complex. Low-temperature fluorescence emission of trimers is characterized by several narrow spectral lines. Even at lowest excitation intensities, we observed considerable spectral diffusion most probably due to low temperature protein dynamics. These results also indicate weak interaction between Chls belonging to different monomeric subunits within the trimer thus leading to a localization of excitations within the monomer. The experimental results demonstrate the feasibility of polarization sensitive studies on single LHC-II complexes and suggest an application for determination of the Chl transition-dipole moment orientations, a key issue in understanding the structure-function relationships.
Proceedings of the National Academy of Sciences, 2011
The light-harvesting complexes of photosystem I and II (Lhca's and Lhcb's) of plants display a high structural homology and similar pigment content and organisation. Yet the spectroscopic properties of these complexes, and accordingly their functionality, differ substantially. This difference is primarily due to the charge-transfer (CT) character of a chlorophyll dimer in all Lhca's, which mixes with the excitonic states of these complexes, while this CT character is generally absent in Lhcb's. By means of single-molecule spectroscopy near room temperature we demonstrate that the presence or absence of such a CT state in Lhca's and Lhcb's can occasionally be reversed, i.e., these complexes are able to interconvert conformationally to quasistable spectral states that resemble the Lhc's of the other photosystem. The high structural similarity of all the Lhca and Lhcb proteins suggests that the stable conformational states that give rise to the mixed CT-excitonic state are similar for all of these proteins, and similarly for the conformations that involve no CT state. This indicates that the specific functions related to Lhca and Lhcb complexes are realised by different stable conformations of a single generic protein structure. We propose that this functionality is modulated and controlled by the protein environment.
Origin of the 701-nm fluorescence emission of the Lhca2 subunit of higher plant photosystem I
Journal of Biological Chemistry, 2004
Photosystem I of higher plants is characterized by red-shifted spectral forms deriving from chlorophyll chromophores. Each of the four Lhca1 to -4 subunits exhibits a specific fluorescence emission spectrum, peaking at 688, 701, 725, and 733 nm, respectively. Recent analysis revealed the role of chlorophyll-chlorophyll interactions of the red forms in Lhca3 and Lhca4, whereas the basis for the fluorescence emission at 701 nm in Lhca2 is not yet clear. We report a detailed characterization of the Lhca2 subunit using molecular biology, biochemistry, and spectroscopy and show that the 701-nm emission form originates from a broad absorption band at 690 nm. Spectroscopy on recombinant mutant proteins assesses that this band represents the low energy form of an excitonic interaction involving two chlorophyll a molecules bound to sites A5 and B5, the same protein domains previously identified for Lhca3 and Lhca4. The resulting emission is, however, substantially shifted to higher energies. These results are discussed on the basis of the structural information that recently became available from x-ray crystallography (Ben Shem, A., Frolow, F., and Nelson, N. (2003) Nature 426, 630 -635). We suggest that, within the Lhca subfamily, spectroscopic properties of chromophores are modulated by the strength of the excitonic coupling between the chromophores A5 and B5, thus yielding fluorescence emission spanning a large wavelength interval. It is concluded that the interchromophore distance rather than the transition energy of the individual chromophores or the orientation of transition vectors represents the critical factor in determining the excitonic coupling in Lhca pigment-protein complexes.
The low energy emitting states of the Lhca4 subunit of higher plant photosystem I
FEBS Letters, 2005
The selectively red excited emission spectrum, at room temperature, of the in vitro reconstituted Lhca4, has a pronounced non-equilibrium distribution, leading to enhanced emission from the directly excited low-energy pigments. Two different emitting forms (or states), with maximal emission at 713 and 735 nm (F713 and F735) and unusual spectral properties, have been identified. Both high-energy states are populated when selective excitation is into the F735 state and the fluorescence anisotropy spectrum attains the value of 0.3 in the wavelength region where both emission states are present. This indicates that the two states are on the same Lhca4 complex and have transition dipoles with similar orientation.
Biochemistry, 2000
A preparation consisting of isolated dimeric peripheral antenna complexes from green plant photosystem I (light-harvesting complex I or LHCI) has been characterized by means of (polarized) steadystate absorption and fluorescence spectroscopy at low temperatures. We show that this preparation can be described reasonably well by a mixture of two types of dimers. In the first dimer about 10% of all Q y absorption of the chlorophylls arises from two chlorophylls with absorption and emission maxima at about 711 and 733 nm, respectively, whereas in the second about 10% of the absorption arises from two chlorophylls with absorption and emission maxima at about 693 and 702 nm, respectively. The remaining chlorophylls show spectroscopic properties comparable to those of the related peripheral antenna complexes of photosystem II. We attribute the first dimer to a heterodimer of the Lhca1 and Lhca4 proteins and the second to a hetero-or homodimer of the Lhca2 and/or Lhca3 proteins. We suggest that the chlorophylls responsible for the 733 nm emission (F-730) and 702 nm emission (F-702) are excitonically coupled dimers and that F-730 originates from one of the strongest coupled pair of chlorophylls observed in nature.
Functional architecture of the major light-harvesting complex from higher plants
Journal of Molecular Biology, 2001
Light-harvesting complexes (Lhc) catalyse sunlight harvesting for photosynthesis as well as other essential functions, including photoprotection by quenching of harmful chlorophyll triplet states and prevention of photoinhibition by dissipation of excitation energy in excess. In addition, folding of Lhc proteins depends on the availability of both xanthophylls and carotenoids, thus preventing the potential formation of harmful chlorophyll-protein complexes lacking photoprotectors. We have used the mutation analysis in order to study the association of the different functions to three protein domains, each composed of a xanthophyll molecule and of neighbour chlorophylls a and b, within the major antenna complex of photosystem II, i.e. LHCII. We have found that the xanthophyll to chlorophyll energy transfer is a shared property of the whole pigmentprotein complex, and occurs with similar ef®ciency in each of the three structural domains. Photoprotection by quenching of chlorophyll triplets is catalysed mainly by lutein bound to site L1, and occurs via energy transfer from chlorophylls A1 and B1. This domain is essential for pigment-induced protein folding.