Structure and function of wild-type and subunit-depleted photosystem I in Synechocystis

The ability of photosynthetic organisms to use the sun's light as a sole source of energy sustains life on our planet. Photosystems I (PSI) and II (PSII) are large, multi-subunit, pigment–protein complexes that enable photosynthesis, but this intriguing process remains to be explained fully. Currently, crystal structures of these complexes are available for thermophilic prokaryotic cyanobacteria. The mega-Dalton trimeric PSI complex from thermophilic cyanobacterium, Thermosynechococcus elongatus, was solved at 2.5?Å resolution with X-ray crystallography. That structure revealed the positions of 12 protein subunits (PsaA-F, PsaI-M, and PsaX) and 127 cofactors.Although mesophilic organisms perform most of the world's photosynthesis, no well-resolved trimeric structure of a mesophilic organism exists. Our research model for a mesophilic cyanobacterium was Synechocystis sp. PCC6803. This study aimed to obtain well-resolved crystal structures of [1] a monomeric PSI with all subunits, [2] a trimeric PSI with a reduced number of subunits, and [3] the full, trimeric wild-type PSI complex. We only partially succeeded with the first two structures, but we successfully produced the trimeric PSI structure at 2.5?Å resolution. This structure was comparable to that of the thermophilic species, but we provided more detail. The PSI trimeric supercomplex consisted of 33 protein subunits, 72 carotenoids, 285 chlorophyll a molecules, 51 lipids, 9 iron-sulfur clusters, 6 plastoquinones, 6 putative calcium ions, and over 870 water molecules.This study showed that the structure of the PSI in Synechocystis sp. PCC6803 differed from previously described PSI structures. These findings have broadened our understanding of PSI structure.     

The slow S to M rise of chlorophyll a fluorescence reflects transition from state 2 to state 1 in the green alga Chlamydomonas reinhardtii

Photosynthesis Research - The green alga Chlamydomonas (C.) reinhardtii is a model organism for photosynthesis research. State transitions regulate redistribution of excitation energy between...     

Anaerobiosis Induced State Transition: A Non Photochemical Reduction of PQ Pool Mediated by NDH in Arabidopsis thaliana

Background

Non photochemical reduction of PQ pool and mobilization of LHCII between PSII and PSI are found to be linked under abiotic stress conditions. The interaction of non photochemical reduction of PQ pool and state transitions associated physiological changes are critically important under anaerobic condition in higher plants.

Methodology/Findings

The present study focused on the effect of anaerobiosis on non-photochemical reduction of PQ pool which trigger state II transition in Arabidopsis thaliana. Upon exposure to dark-anaerobic condition the shape of the OJIP transient rise is completely altered where as in aerobic treated leaves the rise is unaltered. Rise in F _o and F _J was due to the loss of oxidized PQ pool as the PQ pool becomes more reduced. The increase in F_o′ was due to the non photochemical reduction of PQ pool which activated STN7 kinase and induced LHCII phosphorylation under anaerobic condition. Further, it was observed that the phosphorylated LHCII is migrated and associated with PSI supercomplex increasing its absorption cross-section. Furthermore, evidences from crr2-2 (NDH mutant) and pgr5 mutants (deficient in non NDH pathway of cyclic electron transport) have indicated that NDH is responsible for non photochemical reduction of the PQ pool. We propose that dark anaerobic condition accelerates production of reducing equivalents (such as NADPH by various metabolic pathways) which reduce PQ pool and is mediated by NDH leading to state II transition.

Conclusions/Significance

Anaerobic condition triggers non photochemical reduction of PQ pool mediated by NDH complex. The reduced PQ pool activates STN7 kinase leading to state II transition in A. thaliana.     

Unraveling the binding mechanism of asiatic acid with human serum albumin and its biological implications

Asiatic acid (AsA), a naturally occurring pentacyclictriterpenoid found in Centella asiatica, plays a major role in neuroprotection, anticancer, antioxidant, and hepatoprotective activities. Human serum albumin (HSA), a blood plasma protein, participates in the regulation of plasma osmotic pressure and transports endogenous and exogenous substances. The study undertaken to analyze the drug-binding mechanisms of HSA is crucial in understanding the bioavailability of drugs. In this study, we analyzed the cytotoxic activity of AsA on HepG2 (human hepatocellular carcinoma) cell lines and its binding, conformational, docking, molecular simulation studies with HSA under physiological pH 7.2. These studies revealed a clear decrease in the viability of HepG2 cells upon exposure to AsA in a dose-dependent manner with an IC50 of 45?μM. Further studies showed the quenching of intrinsic fluorescence of HSA by AsA with a binding constant of K_AsA?=?3.86?±?0.01?×?10⁴?M^?1, which corresponds to the free energy of (ΔG) ?6.3?kcal?M^?1 at 25?°C. Circular dichroism (CD) studies revealed that there is a clear decrease in the α-helical content from 57.50?±?2.4 to 50%?±?2.3 and an increase in the β-turns from 25?±?0.65 to 29%?±?0.91 and random coils from 17.5%?±?0.95 to 21%?±?1.2, suggesting partial unfolding of HSA. Autodock studies revealed that the AsA is bound to the subdomain IIA with hydrophobic and hydrophilic interactions. From molecular dynamics, simulation data (RMSD, Rg and RMSF) emphasized the local conformational changes and rigidity of the residues of both HSA and HSA–AsA complexes.