Chlorophyll Triplet Quenching and Photoprotection in the Higher Plant Monomeric Antenna Protein Lhcb5

Abstract

In oxygenic photosynthetic organisms, chlorophyll triplets are harmful excited states readily reacting with molecular oxygen to yield the reactive oxygen species (ROS) singlet oxygen. Carotenoids have a photoprotective role in photosynthetic membranes by preventing photoxidative damage through quenching of chlorophyll singlets and triplets. In this work we used mutation analysis to investigate the architecture of chlorophyll triplet quenching sites within Lhcb5, a monomeric antenna protein of Photosystem II. The carotenoid and chlorophyll triplet formation as well as the production of ROS molecules were studied in a family of recombinant Lhcb5 proteins either with WT sequence, mutated into individual chlorophyll binding residues or refolded in vitro to bind different xanthophyll complements. We observed a site-specific effect in the efficiency of chlorophyll-carotenoid triplet-triplet energy transfer. Thus chlorophyll (Chl) 602 and 603 appear to be particularly important for triplet-triplet energy transfer to the xanthophyll bound into site L2. Surprisingly, mutation on Chl 612, the chlorophyll with the lower energy associated and in close contact with lutein in site L1, had no effect on quenching chlorophyll triplet excited states. Finally, we present evidence for an indirect role of neoxanthin in chlorophyll triplet quenching and show that quenching of both singlet and triplet states is necessary for minimizing singlet oxygen formation.

Light-Harvesting Complex Protein LHCBM9 Is Critical for Photosystem II Activity and Hydrogen Production in Chlamydomonas reinhardtii

“…It could be asked whether the lower yield of 1 O 2 derives from the lower level of 1 Chl* excited states and, thus, from an increased probability of 3 Chl* formation. We observed that the decrease of 1 O 2 formation in LHCBM9, as compared with LHCBM2/3, is stronger than the decrease of fluorescence yield, suggesting that additional mechanisms are also involved such as improved 3 Chl* quenching and/or 1 O 2 scavenging by bound carotenoids (Ballottari et al, 2013). Second, in sulfur deficiency, PSII supercomplexes containing LHCBM9 are found in a more dissipative state compared with PSII supercomplexes purified from kd-L9.1/2, thus decreasing overexcitation of PSII reaction centers and photoinhibition (Figure 4, Table 1).…”

Light-Harvesting Complex Stress-Related Proteins Catalyze Excess Energy Dissipation in Both Photosystems of Physcomitrella patens

“…Although lhcsr KO exhibited prompt recovery in the dark, npq4 did not, implying photoinhibition ( Figure 4A). In order to discriminate between genuine qE and slower inhibitory components, namely, qZ or qI (Dall'Osto et al, 2005;Kalituho et al, 2007;Ballottari et al, 2013), we isolated the rapid qE component of NPQ by freezing samples at different times up to 6 min under HL and following further incubation in dark for up to 5 min to allow for selective relaxation of only qE (Figures 5E and 5F; Supplemental Figure 6). Dark-recovered (5 min) minus light-only (6 min) difference spectra, depicted in Figure 6A, show that whereas lhcsr KO chloroplasts underwent quenching of only the PSII component, both psbs KO and wild-type chloroplasts quenched both PSI and PSII emissions.…”

“…Excess photons increase the amount of singlet chlorophyll ( 1 Chl*) and, thus, the probability for formation of triplet chlorophyll ( 3 Chl*) and singlet oxygen ( 1 O 2 ), with consequent photoinhibition that limits growth. Oxygenic organisms have evolved different photoprotective mechanisms in order to avoid the formation of reactive oxygen species, including triplet quenching (Dall'Osto et al, 2012;Ballottari et al, 2013), reactive oxygen species scavenging (Baroli et al, 2003;Dall'Osto et al, 2010), and alternative electron transport pathways (Cardol et al, 2011). In addition to these constitutive mechanisms, a rapidly inducible process known as nonphotochemical quenching (NPQ) is activated within seconds upon exposure to excess light and then catalyzes thermal dissipation within the photosystem II (PSII) antenna system (Niyogi and Truong, 2013;de Bianchi et al, 2010).…”

“…Its protonation causes a conformational change that is propagated to LHC proteins of PSII, leading to dissociation of outer antenna complexes from PSII supercomplexes and clustering of peripheral LHCII (Bonente et al, 2008a;Betterle et al, 2009;Johnson et al, 2011). This causes quenching at two sites: Q1 (zeaxanthin-independent) located in LHCII clusters and Q2 (zeaxanthin-dependent) within supercomplexes (Ballottari et al, 2013).…”

Photoprotection and triplet energy transfer in higher plants: the role of electronic and nuclear fluctuations

“…Singlet excited Chl * can decay into triplets (3 Chl * ) which sensitize molecular oxygen to form singlet oxygen, which induces damage in its local environment by destroying lipids and nucleic acids and proteins. [1], [2], [3] Cars are able to accept the triplets from chlorophylls (chls) by triplet energy transfer (TET), and dissipate the excess energy to heat. [4] The efficiency of Chl triplet quenching is 95% in antenna complexes of Photosystem II in higher plants, and the timescale of TET from Chls to Cars has been found to be faster than 500 ps in the major light-harvesting complex of Photosystem II (LHCII).…”