Structure/function/dynamics of photosystem II plastoquinone binding sites

Photosystem II (PSII) continuously attracts the attention of researchers aiming to unravel the riddle of its functioning and efficiency fundamental for all life on Earth. Besides, an increasing number of biotechnological applications have been envisaged exploiting and mimicking the unique properties of this macromolecular pigment-protein complex. The PSII organization and working principles have inspired the design of electrochemical water splitting schemes and charge separating triads in energy storage systems as well as biochips and sensors for environmental, agricultural and industrial screening of toxic compounds. An intriguing opportunity is the development of sensor devices, exploiting native or manipulated PSII complexes or ad hoc synthesized polypeptides mimicking the PSII reaction centre proteins as biosensing elements. This review offers a concise overview of the recent improvements in the understanding of structure and function of PSII donor side, with focus on the interactions of the plastoquinone cofactors with the surrounding environment and operational features. Furthermore, studies focused on photosynthetic proteins structure/function/dynamics and computational analyses aimed at rational design of high-quality bio-recognition elements in biosensor devices are discussed

The plastoquinol–plastoquinone exchange mechanism in photosystem II: insight from molecular dynamics simulations

In the photosystem II (PSII) of oxygenic photosynthetic organisms, the reaction center (RC) core mediates the light-induced electron transfer leading to water splitting and production of reduced plastoquinone molecules. The reduction of plastoquinone to plastoquinol lowers PSII affinity for the latter and leads to its release. However, little is known about the role of protein dynamics in this process. Here, molecular dynamics simulations of the complete PSII complex embedded in a lipid bilayer have been used to investigate the plastoquinol release mechanism. A distinct dynamic behavior of PSII in the presence of plastoquinol is observed which, coupled to changes in charge distribution and electrostatic interactions, causes disruption of the interactions seen in the PSII–plastoquinone complex and leads to the “squeezing out” of plastoquinol from the binding pocket. Displacement of plastoquinol closes the second water channel, recently described in a 2.9 Å resolution PSII structure (Guskov et al. in Nat Struct Mol Biol 16:334–342, 2009), allowing to rule out the proposed “alternating” mechanism of plastoquinol–plastoquinone exchange, while giving support to the “single-channel” one. The performed simulations indicated a pivotal role of D1-Ser264 in modulating the dynamics of the plastoquinone binding pocket and plastoquinol–plastoquinone exchange via its interaction with D1-His252 residue. The effects of the disruption of this hydrogen bond network on the PSII redox reactions were experimentally assessed in the D1 site-directed mutant Ser264Lys

Mutations of Photosystem II D1 Protein That Empower Efficient Phenotypes of <i>Chlamydomonas reinhardtii</i> under Extreme Environment in Space

<div><p>Space missions have enabled testing how microorganisms, animals and plants respond to extra-terrestrial, complex and hazardous environment in space. Photosynthetic organisms are thought to be relatively more prone to microgravity, weak magnetic field and cosmic radiation because oxygenic photosynthesis is intimately associated with capture and conversion of light energy into chemical energy, a process that has adapted to relatively less complex and contained environment on Earth. To study the direct effect of the space environment on the fundamental process of photosynthesis, we sent into low Earth orbit space engineered and mutated strains of the unicellular green alga, <i>Chlamydomonas reinhardtii,</i> which has been widely used as a model of photosynthetic organisms. The algal mutants contained specific amino acid substitutions in the functionally important regions of the pivotal Photosystem II (PSII) reaction centre D1 protein near the Q_B binding pocket and in the environment surrounding Tyr-161 (Y_Z) electron acceptor of the oxygen-evolving complex. Using real-time measurements of PSII photochemistry, here we show that during the space flight while the control strain and two D1 mutants (A250L and V160A) were inefficient in carrying out PSII activity, two other D1 mutants, I163N and A251C, performed efficient photosynthesis, and actively re-grew upon return to Earth. Mimicking the neutron irradiation component of cosmic rays on Earth yielded similar results. Experiments with I163N and A251C D1 mutants performed on ground showed that they are better able to modulate PSII excitation pressure and have higher capacity to reoxidize the Q_A⁻ state of the primary electron acceptor. These results highlight the contribution of D1 conformation in relation to photosynthesis and oxygen production in space.</p></div

Limits of detection of IL and selected D1 random mutants to three different herbicides, representing the lower herbicide molar concentration inducing detectable inhibition of strains PSII electron transport efficiency.

<p>Limits of detection of IL and selected D1 random mutants to three different herbicides, representing the lower herbicide molar concentration inducing detectable inhibition of strains PSII electron transport efficiency.</p

Amino acid substitutions and their localization in the D1 protein of the analysed <i>C. reinhardtii</i> random mutants survived after neutrons and protons exposures.

<p>Amino acid substitutions and their localization in the D1 protein of the analysed <i>C. reinhardtii</i> random mutants survived after neutrons and protons exposures.</p

Herbicides toxicity effects on the PSII electron transport efficiency (<i>1-V_J</i>) of IL and selected D1 random mutants, and corresponding R/S values.

<p>The <i>I₅₀</i> is the molar herbicide concentration, which induces 50% inhibition of the parameter <i>1-V_J</i>. The R/S were calculated as ratios of mutant <i>I₅₀</i> and <i>I₅₀</i> of the reference strain; the asterisks indicate values that do not differ significantly from the IL strain at p≤0.05 (Mann-Whitney U Test). We considered that R/S<0.9 values define increased herbicide sensitivity, R/S>1.1 – increased herbicide resistance and 0.9≤R/S≤1.1 – no significant alterations in the mutants herbicide sensitivity relative to the reference strain.</p

Comparison of the <i>C. reinhardtii</i> strains culture chlorophyll and optical density during growth.

<p>The development of the reference strain, IL, and D1 random mutants is presented as a ratio of the total chlorophyll mL⁻¹ and the corresponding OD₇₅₀ mL⁻¹. The cell cultures were grown for a period of 88 h in TAP medium under continues illumination of 50 µmol m⁻² s⁻¹ at 24°C and 150 rpm agitation. Average values from four experiments are presented, ±SE, n = 4. For the sake of clarity the standard error bars of the mutants values are omitted.</p

Herbicide dose-response curves of PSII electron transport efficiency.

<p>Fluorescence transients of the reference strain IL and D1 random mutants of <i>C. reinhardtii</i> were registered after 10 min of incubation with increasing concentrations of atrazine, terbutilazine or linuron. The parameter <i>1-V_J</i> was calculated as <i>1-V_J = </i>1-(F_2ms-F₀)/(F_m-F₀) according to Strasser et al. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0061851#pone.0061851-Strasser1" target="_blank">[32]</a>. Average values from three independent experiments are presented, ±SE, n = 6–9.</p