| 1. |
- Oquist, Gunnar, 1941-, et al.
(författare)
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The cyanobacterium Synechococcus modulates Photosystem II function in response to excitation stress through D1 exchange
- 1995
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Ingår i: Photosynthesis Research. - 0166-8595 .- 1573-5079. ; 46:1-2, s. 151-158
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Tidskriftsartikel (refereegranskat)abstract
- In this minireview we discuss effects of excitation stress on the molecular organization and function of PS II as induced by high light or low temperature in the cyanobacterium Synechococcus sp. PCC 7942. Synechococcus displays PS II plasticity by transiently replacing the constitutive D1 form (D1:1) with another form (D1:2) upon exposure to excitation stress. The cells thereby counteract photoinhibition by increasing D1 turn over and modulating PS II function. A comparison between the cyanobacterium Synechococcus and plants shows that in cyanobacteria, with their large phycobilisomes, resistance to photoinhibition is mainly through the dynamic properties ( D1 turnover and quenching) of the reaction centre. In contrast, plants use antenna quenching in the light-harvesting complex as an important means to protect the reaction center from excessive excitation.
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| 2. |
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| 3. |
- Björn, Lars Olof
(författare)
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Effect of back reactions in the S-cycle on photosynthesis in very weak light
- 1995
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Ingår i: Photosynthesis Research. - 0166-8595. ; 46:1-2, s. 203-206
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Tidskriftsartikel (refereegranskat)abstract
- Photosynthesis in light which is so weak that excitation rates are comparable to the decay rates of S-states is modelled. It is found that oxygen evolution rate varies in a non-linear way with Photosystem II excitation rate in this region, but asymptotically approaches a linear relationship (with a slope of one oxygen molecule per four excitations) as excitation rate increases. The asymptote gives an intercept (zero oxygen evolution rate) on the excitation rate axis. The question is raised, whether this model is compatible with photoautotrophy in a red alga found at 268 m depth and able to carry out photosynthesis in extremely weak light. It is found that no unorthodox mechanisms for the S-cycle in this alga have to be assumed to explain its performance.
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| 4. |
- Bratt, Charlotte Eva, et al.
(författare)
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Regulation of violaxanthin de-epoxidase activity by pH and ascorbate concentration
- 1995
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Ingår i: Photosynthesis Research. - 0166-8595. ; 45:2, s. 169-175
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Tidskriftsartikel (refereegranskat)abstract
- The activity of violaxanthin de-epoxidase has been studied both in isolated thylakoids and after partial purification, as a function of pH and ascorbate concentration. We demonstrate that violaxanthin de-epoxidase has a Km for ascorbate that is strongly dependent on pH, with values of 10, 2.5, 1.0 and 0.3 mM at pH 6.0, 5.5, 5.0 and 4.5, respectively. These values can be expressed as a single Km±0.1±0.02 mM for the acid form of ascorbate. Release of the protein from the thylakoids by sonication was also found to be strongly pH dependent with a cooperativity of 4 with respect to protons and with an inflexion point at pH 6.7. These results can explain some of the discrepancies reported in the literature and provide a more consistent view of zeaxanthin formation in vivo.
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| 5. |
- Hoganson, C W, et al.
(författare)
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A hydrogen-atom abstraction model for the function of Y-Z in photosynthetic oxygen evolution
- 1995
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Ingår i: Photosynthesis Research. - 0166-8595. ; 46:1-2, s. 177-184
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Tidskriftsartikel (refereegranskat)abstract
- Recent magnetic-resonance work on Y-Z suggests that this species exhibits considerable motional flexibility in its functional site and that its phenol oxygen is not involved in a well-ordered hydrogen-bond interaction (Tang et al., submitted; Tommos et al., in press). Both of these observations are inconsistent with a simple electron-transfer function for this radical in photosynthetic water oxidation. By considering the roles of catalytically active amino acid radicals in other enzymes and recent data on the water-oxidation process in Photosystem II, we rationalize these observations by suggesting that Y-Z functions to abstract hydrogen atoms from aquo- and hydroxy-bound manganese ions in the (Mn)(4) cluster on each S-state transition. The hydrogen-atom abstraction process may occur either by sequential or concerted kinetic pathways. Within this model, the (Mn)(4)/Y-Z center forms a single catalytic center that comprises the Oxygen Evolving Complex in Photosystem II.
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