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Light harvesting in photosystem II.

van Amerongen H, Croce R - Photosyn. Res. (2013)

Bottom Line: The whole system consists of many subunits and appears to be modular, i.e., both its composition and organization depend on environmental conditions, especially on the quality and intensity of the light.It will become clear that time-resolved fluorescence data can provide invaluable information about the organization and functioning of thylakoid membranes.At the end, an overview will be given of unanswered questions that should be addressed in the near future.

View Article: PubMed Central - PubMed

Affiliation: Laboratory of Biophysics, Wageningen University, P. O. Box 8128, 6700 ET, Wageningen, The Netherlands, herbert.vanamerongen@wur.nl.

ABSTRACT
Water oxidation in photosynthesis takes place in photosystem II (PSII). This photosystem is built around a reaction center (RC) where sunlight-induced charge separation occurs. This RC consists of various polypeptides that bind only a few chromophores or pigments, next to several other cofactors. It can handle far more photons than the ones absorbed by its own pigments and therefore, additional excitations are provided by the surrounding light-harvesting complexes or antennae. The RC is located in the PSII core that also contains the inner light-harvesting complexes CP43 and CP47, harboring 13 and 16 chlorophyll pigments, respectively. The core is surrounded by outer light-harvesting complexes (Lhcs), together forming the so-called supercomplexes, at least in plants. These PSII supercomplexes are complemented by some "extra" Lhcs, but their exact location in the thylakoid membrane is unknown. The whole system consists of many subunits and appears to be modular, i.e., both its composition and organization depend on environmental conditions, especially on the quality and intensity of the light. In this review, we will provide a short overview of the relation between the structure and organization of pigment-protein complexes in PSII, ranging from individual complexes to entire membranes and experimental and theoretical results on excitation energy transfer and charge separation. It will become clear that time-resolved fluorescence data can provide invaluable information about the organization and functioning of thylakoid membranes. At the end, an overview will be given of unanswered questions that should be addressed in the near future.

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Picosecond kinetics of isolated PSII core complexes from Thermosynechococcus, reconstructed from (Miloslavina et al. 2006) (black solid) and (van der Weij-de Wit et al. 2011). The decay curve presented in (Miloslavina et al. 2006) was reconstructed based on the DAS shown in Fig. 7 of that work, and only τ1–τ5 are included in the calculation. The decay curve from (van der Weij-de Wit et al. 2011) was reconstructed based on the compartmental scheme shown in Fig. 6 in that article and the initial excitation fractions therein. Excitation wave lengths were 663 and 400 nm, respectively, but these differences are not expected to significantly influence the overall kinetics. The dotted line represents the fluorescence kinetics of PSII core in vivo for a Synechocystis mutant (excitation wavelength 400 nm) (Tian et al. 2013)
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Fig3: Picosecond kinetics of isolated PSII core complexes from Thermosynechococcus, reconstructed from (Miloslavina et al. 2006) (black solid) and (van der Weij-de Wit et al. 2011). The decay curve presented in (Miloslavina et al. 2006) was reconstructed based on the DAS shown in Fig. 7 of that work, and only τ1–τ5 are included in the calculation. The decay curve from (van der Weij-de Wit et al. 2011) was reconstructed based on the compartmental scheme shown in Fig. 6 in that article and the initial excitation fractions therein. Excitation wave lengths were 663 and 400 nm, respectively, but these differences are not expected to significantly influence the overall kinetics. The dotted line represents the fluorescence kinetics of PSII core in vivo for a Synechocystis mutant (excitation wavelength 400 nm) (Tian et al. 2013)

Mentions: In Fig. 3, the reconstructed picosecond fluorescence kinetics of the PSII core from Thermosynechococcus from two different studies are shown (Miloslavina et al. 2006; van der Weij-de Wit et al. 2011) and the results are nearly identical. Accurate data fitting requires five or more exponentials but two direct observations stand out. Charge separation occurs with an average time constant τ below 100 ps, leading to the relatively fast disappearance of the (fluorescence) signal. Since this time constant is much shorter than the normal excited-state lifetime of chlorophyll (typically several ns), this guarantees a high quantum efficiency of charge separation (ϕCS). The corresponding value is above 0.95, using the well-known relation ϕCS = 1 – τ/τChl (Croce and van Amerongen 2011), where τChl is the average lifetime of the excited Chl in PSII in the absence of charge separation. The exact value for this parameter is unknown but a recent study led to a value of ~2 ns (Belgio et al. 2012). The kinetics also shows a small contribution of a long-lived component which is usually ascribed to the fact that charge separation is partly reversible. The amplitude and lifetime of this component depend on the competition between secondary charge separation in the RC (forward electron transfer from the primary electron acceptor) and back transfer of the electron from primary acceptor to primary donor.Fig. 3


Light harvesting in photosystem II.

van Amerongen H, Croce R - Photosyn. Res. (2013)

Picosecond kinetics of isolated PSII core complexes from Thermosynechococcus, reconstructed from (Miloslavina et al. 2006) (black solid) and (van der Weij-de Wit et al. 2011). The decay curve presented in (Miloslavina et al. 2006) was reconstructed based on the DAS shown in Fig. 7 of that work, and only τ1–τ5 are included in the calculation. The decay curve from (van der Weij-de Wit et al. 2011) was reconstructed based on the compartmental scheme shown in Fig. 6 in that article and the initial excitation fractions therein. Excitation wave lengths were 663 and 400 nm, respectively, but these differences are not expected to significantly influence the overall kinetics. The dotted line represents the fluorescence kinetics of PSII core in vivo for a Synechocystis mutant (excitation wavelength 400 nm) (Tian et al. 2013)
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3824292&req=5

Fig3: Picosecond kinetics of isolated PSII core complexes from Thermosynechococcus, reconstructed from (Miloslavina et al. 2006) (black solid) and (van der Weij-de Wit et al. 2011). The decay curve presented in (Miloslavina et al. 2006) was reconstructed based on the DAS shown in Fig. 7 of that work, and only τ1–τ5 are included in the calculation. The decay curve from (van der Weij-de Wit et al. 2011) was reconstructed based on the compartmental scheme shown in Fig. 6 in that article and the initial excitation fractions therein. Excitation wave lengths were 663 and 400 nm, respectively, but these differences are not expected to significantly influence the overall kinetics. The dotted line represents the fluorescence kinetics of PSII core in vivo for a Synechocystis mutant (excitation wavelength 400 nm) (Tian et al. 2013)
Mentions: In Fig. 3, the reconstructed picosecond fluorescence kinetics of the PSII core from Thermosynechococcus from two different studies are shown (Miloslavina et al. 2006; van der Weij-de Wit et al. 2011) and the results are nearly identical. Accurate data fitting requires five or more exponentials but two direct observations stand out. Charge separation occurs with an average time constant τ below 100 ps, leading to the relatively fast disappearance of the (fluorescence) signal. Since this time constant is much shorter than the normal excited-state lifetime of chlorophyll (typically several ns), this guarantees a high quantum efficiency of charge separation (ϕCS). The corresponding value is above 0.95, using the well-known relation ϕCS = 1 – τ/τChl (Croce and van Amerongen 2011), where τChl is the average lifetime of the excited Chl in PSII in the absence of charge separation. The exact value for this parameter is unknown but a recent study led to a value of ~2 ns (Belgio et al. 2012). The kinetics also shows a small contribution of a long-lived component which is usually ascribed to the fact that charge separation is partly reversible. The amplitude and lifetime of this component depend on the competition between secondary charge separation in the RC (forward electron transfer from the primary electron acceptor) and back transfer of the electron from primary acceptor to primary donor.Fig. 3

Bottom Line: The whole system consists of many subunits and appears to be modular, i.e., both its composition and organization depend on environmental conditions, especially on the quality and intensity of the light.It will become clear that time-resolved fluorescence data can provide invaluable information about the organization and functioning of thylakoid membranes.At the end, an overview will be given of unanswered questions that should be addressed in the near future.

View Article: PubMed Central - PubMed

Affiliation: Laboratory of Biophysics, Wageningen University, P. O. Box 8128, 6700 ET, Wageningen, The Netherlands, herbert.vanamerongen@wur.nl.

ABSTRACT
Water oxidation in photosynthesis takes place in photosystem II (PSII). This photosystem is built around a reaction center (RC) where sunlight-induced charge separation occurs. This RC consists of various polypeptides that bind only a few chromophores or pigments, next to several other cofactors. It can handle far more photons than the ones absorbed by its own pigments and therefore, additional excitations are provided by the surrounding light-harvesting complexes or antennae. The RC is located in the PSII core that also contains the inner light-harvesting complexes CP43 and CP47, harboring 13 and 16 chlorophyll pigments, respectively. The core is surrounded by outer light-harvesting complexes (Lhcs), together forming the so-called supercomplexes, at least in plants. These PSII supercomplexes are complemented by some "extra" Lhcs, but their exact location in the thylakoid membrane is unknown. The whole system consists of many subunits and appears to be modular, i.e., both its composition and organization depend on environmental conditions, especially on the quality and intensity of the light. In this review, we will provide a short overview of the relation between the structure and organization of pigment-protein complexes in PSII, ranging from individual complexes to entire membranes and experimental and theoretical results on excitation energy transfer and charge separation. It will become clear that time-resolved fluorescence data can provide invaluable information about the organization and functioning of thylakoid membranes. At the end, an overview will be given of unanswered questions that should be addressed in the near future.

Show MeSH
Related in: MedlinePlus