UV-B AND UV-A RADIATION EFFECTS ON PHOTOSYNTHESIS AT THE MOLECULAR LEVEL - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 119

cycle. Previous literature data have demonstrated that the light reactions of photosynthesis,
especially those mediated by the PSII complex 9-11 , as well as the Calvin cycle 12-13 are
susceptible to UV-induced damage. UV effects on DNA and on the Calvin cycle enzymes
will be covered in other chapters of this topic, therefore we will concentrate on the effects
exerted by UV radiation on the light energy converting complex of PSII.
PSII is a water/plastoquinone oxido/reductase embedded in the thylakoid
membrane. It contains over 25 subunits and performs the exclusive task of light-driven
oxidation of water that serves as final electron source for all photosynthetic processes
(Figure 2). The electrons liberated from water are transferred to membrane soluble
plastoquinone molecules 14 . The redox cofactors of PSII electron transport are bound to
or contained by the D1 and D2 protein subunits which form the reaction center of PSII.

PSII

Stroma

D2 D1

Q A Q B

Fe

Phe

Phe

PQ

P 680

Tyr-D

Tyr-Z

Mn 23

33

Lumen

17

4H +

O 2 +

2H O
2

Figure 2. The structure and function of the Photosystem II complex . The reaction center of PSII
consists of the D1 and D2 protein subunits, which bind the redox cofactors of light-induced electron
transport: The Mn cluster of water oxidation, the redox-active tyrosine electron donors (Tyr-Z and
Tyr-D), the reaction center chlorophyll (P 680 ), the primary electron acceptor phyophytin (PheO), and
the first and second quinone electron acceptors Q A and Q B , respectively. The reaction center
heterodimer is closely associated with cytochrome b-559, and surrounded by chlorophyll binding
antenna (CP43 and CP47). The PSII complex also contains various low molecular mass polipeptides
(LMP).

Water oxidation is catalyzed by a Mn cluster, and electrons liberated during this process
are transferred to the reaction center chlorophyll, P680, via a redox active tyrosine
residue, Tyr-Z, of the D1 protein. On the acceptor side of PSII, the electron produced by
the light-induced charge separation event reduces a pheophytin molecule and then the
first, Q A , and second, Q B , plastoquinone electron acceptor. Q A is a firmly bound
component of the reaction center complex, which undergoes one-electron reduction. In
contrast, Q B is a mobile electron carrier, which takes up two electrons sequentially from