scholarly journals The terminal phycobilisome emitter, LCM: A light-harvesting pigment with a phytochrome chromophore

2015 ◽  
Vol 112 (52) ◽  
pp. 15880-15885 ◽  
Author(s):  
Kun Tang ◽  
Wen-Long Ding ◽  
Astrid Höppner ◽  
Cheng Zhao ◽  
Lun Zhang ◽  
...  
Keyword(s):  
Crystal Structure ◽  
Transient Absorption ◽  
Light Harvesting ◽  
Reaction Centers ◽  
Protein Fold ◽  
Light Harvesting Complexes ◽  
Exciton Coupling ◽  
Femtosecond Transient Absorption ◽  
Orange Carotenoid Protein ◽  
Sensory Photoreceptors

Photosynthesis relies on energy transfer from light-harvesting complexes to reaction centers. Phycobilisomes, the light-harvesting antennas in cyanobacteria and red algae, attach to the membrane via the multidomain core-membrane linker, LCM. The chromophore domain of LCM forms a bottleneck for funneling the harvested energy either productively to reaction centers or, in case of light overload, to quenchers like orange carotenoid protein (OCP) that prevent photodamage. The crystal structure of the solubly modified chromophore domain from Nostoc sp. PCC7120 was resolved at 2.2 Å. Although its protein fold is similar to the protein folds of phycobiliproteins, the phycocyanobilin (PCB) chromophore adopts ZZZssa geometry, which is unknown among phycobiliproteins but characteristic for sensory photoreceptors (phytochromes and cyanobacteriochromes). However, chromophore photoisomerization is inhibited in LCM by tight packing. The ZZZssa geometry of the chromophore and π-π stacking with a neighboring Trp account for the functionally relevant extreme spectral red shift of LCM. Exciton coupling is excluded by the large distance between two PCBs in a homodimer and by preservation of the spectral features in monomers. The structure also indicates a distinct flexibility that could be involved in quenching. The conclusions from the crystal structure are supported by femtosecond transient absorption spectra in solution.

scholarly journals A photosynthetic antenna complex foregoes unity carotenoid-to-bacteriochlorophyll energy transfer efficiency to ensure photoprotection

2020 ◽  
Vol 117 (12) ◽  
pp. 6502-6508 ◽  
Author(s):  
Dariusz M. Niedzwiedzki ◽  
David J. K. Swainsbury ◽  
Daniel P. Canniffe ◽  
C. Neil Hunter ◽  
Andrew Hitchcock
Keyword(s):  
Energy Transfer ◽  
Transient Absorption ◽  
Light Harvesting ◽  
Protein Complexes ◽  
Fluorescence Emission ◽  
Energy Transfer Efficiency ◽  
Transfer Efficiency ◽  
Light Harvesting Complexes ◽  
Antenna Complex ◽  
Pigment Protein Complexes

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) fromRhodobacter sphaeroidescontaining theN= 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 S1state 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 BChlsa. These measurements show that energy transfer proceeds with an efficiency of ∼100%, primarily via the S1→ Qxroute because the S1→ S0fluorescence emission of ζ-carotene overlaps almost perfectly with the Qxabsorption band of the BChls. However, transient absorption measurements performed on microsecond timescales reveal that, unlike the nativeN≥ 9 carotenoids normally utilized in light-harvesting complexes, ζ-carotene does not quench excited triplet states of BChla, likely due to elevation of the ζ-carotene triplet energy state above that of BChla. These findings provide insights into the coevolution of photosynthetic pigments and pigment–protein complexes. We propose that theN≥ 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.

scholarly journals The Molecular Mechanisms of Photoactivation of Orange Carotenoid Protein Revealed by Molecular Dynamics

2020 ◽  
Author(s):  
Mattia Bondanza ◽  
Lorenzo Cupellini ◽  
Pietro Faccioli ◽  
Benedetta Mennucci
Keyword(s):  
Structural Changes ◽  
Molecular Mechanisms ◽  
Light Harvesting ◽  
Experimental Studies ◽  
Green Light ◽  
Enhanced Sampling ◽  
Light Harvesting Complexes ◽  
Effective Combination ◽  
Large Conformational Change ◽  
Orange Carotenoid Protein

Light-harvesting in photosynthesis is accompanied by photoprotective processes. In cyanobacteria, the photoprotective role is played by a specialized complex, the Orange Carotenoid Protein which is activated by strong blue-green light. This photoactivation involves a unique series of structural changes which terminate with an opening of the complex into two separated domains, one of which acts as a quencher for the light-harvesting complexes. Many experimental studies have tried to reveal the molecular mechanisms through which the energy absorbed by the carotenoid finally leads to the large conformational change of the complex. Here for the first time, these mechanisms are revealed by simulating at atomistic level the whole dynamics of the complex through an effective combination of enhanced sampling techniques. On the basis of our findings, we can conclude that the carotenoid does not act as a spring that, releasing its internal strain, induces the dissociation, as it was previously proposed but as a "latch" locking together the two domains. The photochemically triggered displacement of the carotenoid breaks this balance, allowing the complex to dissociate. <br>

scholarly journals Light’s interaction with pigments in chloroplasts: The murburn perspective, with special relevance to carotenoids

2020 ◽  
Author(s):  
Kelath Murali Manoj ◽  
Afsal Manekkathodi
Keyword(s):  
Light Harvesting ◽  
Reaction Centers ◽  
Oxygenic Photosynthesis ◽  
Physiological Data ◽  
Light Harvesting Complexes ◽  
Light Reaction ◽  
Historical Perspectives ◽  
Electron Emitters ◽  
Theoretical Foundations ◽  
Special Relevance

The prevailing understanding on photolytic photophosphorylation, the light reaction of oxygenic photosynthesis, considers the vast majority of the diverse pigments, chlorophyll binding proteins (CBPs) and light harvesting complexes (LHCs) as photon-energy relaying facets; only the two photosystems’ (PS) reaction centers’ chlorophyll a couplets are deemed to serve as photo-excitable electron emitters. Highlighting the historical perspectives involved, we present reasons why this conventional perception is unmet by theoretical foundations, unsupported by molecular awareness on the various pigments and unverified by physiological data available on chloroplasts. Further, we propose a simple diffusible reactive oxygen species (DROS)-based mechanism for correlating the functions of various light harvesting LHCs and CBPs with the reaction centers of PS I &amp; II.

Enhanced Photocurrent Generation by Photosynthetic Bacterial Reaction Centers through Molecular Relays, Light-Harvesting Complexes, and Direct Protein–Gold Interactions

Langmuir ◽  
2011 ◽  
Vol 27 (16) ◽  
pp. 10282-10294 ◽  
Author(s):  
Mart-Jan den Hollander ◽  
J. Gerhard Magis ◽  
Philipp Fuchsenberger ◽  
Thijs J. Aartsma ◽  
Michael R. Jones ◽  
...  
Keyword(s):  
Light Harvesting ◽  
Reaction Centers ◽  
Light Harvesting Complexes ◽  
Photocurrent Generation ◽  
Bacterial Reaction Centers
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