Thermostability Of Photosystem Research Articles

Thermostability of Photosystem I Trimers and Monomers from the Cyanobacterium Arthrospira platensis

The correlation between changes in the circular dichroism (CD) spectra of antenna pigments and the photooxidation kinetics of the primary electron donor (P700) in trimeric and monomeric complexes of photosystem 1 (PS1) after incubation at elevated temperatures was studied to assess the thermostability of photosystem I (PSI) complexes from the cyanobacterium Arthrospira platensis. It was shown that heating of the monomers and trimers to 60 and to 70°C, respectively, caused a 10% decrease in the amplitude of the CD signals of antenna chlorophyll. Thermal disturbance of the spatial organization of the antenna chlorophyll correlated with a lower concentration of photoactive P700. A decrease in the initial rate of P700 photooxidation was observed already after 40°C for both monomers and trimers. This decrease occurred simultaneously with a decrease in the intensity of the carotenoid CD band of trimers but not monomers, which is probably due to the different carotenoid compositions of trimers and monomers. It is assumed that the slowdown of P700 photooxidation kinetics in PS1 complexes at temperatures up to 50–60°C is associated not with impairment of energy migration to the reaction center (P700) but rather with an initial delay of P700 photooxidation due to the changes in the state of the acceptor part of PS1, in particular, the secondary acceptor phylloquinone.

Thermostability of photosystem I trimers and monomers from the cyanobacterium Thermosynechococcus elongatus

The performance of solar energy conversion into alternative energy sources in artificial systems highly depends on the thermostability of photosystem I (PSI) complexes Terasaki et al. (2007), Iwuchukwu et al. (2010), Kothe et al. (2013) . To assess the thermostability of PSI complexes from the thermophilic cyanobacterium Thermosynechococcus elongatus heating induced perturbations on the level of secondary structure of the proteins were studied. Changes were monitored by Fourier transform infrared (FT-IR) spectra in the mid-IR region upon slow heating (1°C per minute) of samples in D2O phosphate buffer (pD 7.4) from 20°C to 100°C. These spectra showed distinct changes in the Amide I region of PSI complexes as a function of the rising temperature. Absorbance at the Amide I maximum of PSI monomers (centered around 1653cm−1), gradually dropped in two temperature intervals, i.e. 60–75 and 80–90°C. In contrast, absorbance at the Amide I maximum of PSI trimers (around 1656cm−1) dropped only in one temperature interval 80–95°C. The thermal profile of the spectral shift of α-helices bands in the region 1656–1642cm−1 confirms the same two temperature intervals for PSI monomers and only one interval for trimers. Apparently, the observed absorbance changes at the Amide I maximum during heating of PSI monomers and trimers are caused by deformation and unfolding of α-helices. The absence of absorbance changes in the interval of 20–65°C in PSI trimers is probably caused by a greater stability of protein secondary structure as compared to that in monomers. Upon heating above 80°C a large part of α-helices both in trimers and monomers converts to unordered and aggregated structures. Spectral changes of PSI trimers and monomers heated up to 100°C are irreversible due to protein denaturation and non-specific aggregation of complexes leading to new absorption bands at 1618–1620cm−1. We propose that monomers shield the denaturation sensitive sides at the monomer/monomer interface within a trimer, making the oligomeric structure more stable against thermal stress.

Enhanced photosystem 2 thermostability during leaf growth of Elm (Ulmus pumila) seedlings

We examined photosynthetic activities and thermostability of photosystem 2 (PS2) in leaves of elm (Ulmus pumila) seedlings from initiation to full expansion. During leaf development, net photosynthetic rate (P N) increased gradually and reached the maximum when leaves were fully developed. In parallel with the increase of P N, chlorophyll (Chl) content was significantly elevated. Chl a fluorescence measurements showed that the maximum quantum yield of PS2 (ϕPS2), the efficiency a trapped exciton, moved an electron into the electron transport chain further than QA − (Ψo), and the quantum yield of electron transport beyond QA (ϕEo) increased gradually. These results were independently confirmed by our low irradiance experiments. When subjected to progressive heat stress, the young leaves exhibited considerably lower ϕPS2 and higher minimal fluorescence (F0) than the mature leaves, revealing the highly sensitive nature of PS2 under heat in the newly initiating leaves. Further analysis showed that PS2 structure in the newly initiating leaves was strongly altered under heat, as evidenced by the increased fluorescence signals at the position of the K step. We therefore demonstrated an inhibition in the oxygen-evolving complex (OEC) in the young leaves. This resulted in decrease in amount of the functional PS2 reaction centres and relative increase in the PS2 reaction centres with inhibited electron transport at the acceptor side under heat. We suggest that the enhanced thermostability of PS2 during leaf development is associated with improved OEC stability.