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 120

Q A before leaving its binding site formed by the D1 protein. The catalytic site of water
oxidation is composed of four manganese ions bound by the D1 protein. During four
subsequent photoacts the Mn cluster accumulates four oxidising equivalents, which are
utilised in the oxidation of water into protons and molecular oxygen. This side-product
of PSII function has been the source of atmospheric oxygen during the course of
evolution, and hence is the origin of the UV-B screening ozone veil that protects life on
Earth 15 .

3. The mechanism of UV-induced damage of PSII

Damage by UV-B radiation

The mechanism of damage induced by UV-B light to the electron transport and
protein structure of PSII has been addressed by a number of studies both in isolated and
intact systems. According to an experimentally well supported model summarized in
Figure 3, the primary UV-damage occurs at the donor side of PSII, at the Mn cluster of
water oxidation 9-11,16 . However, UV-B induced modification or loss in the function of
the Q A and Q B quinone electron acceptors 10,17-19 , the Tyr-D and Tyr-Z donors 10,20 have

D2

D1 D2

D1

Q A Q B

Damage
of the Mn
cluster

Q A Q B

Fe

Fe

Phe

Phe

Phe Phe

P 680

P 680

Tyr-D

Tyr-Z

Tyr-Z Tyr-D

Mn Mn

Damage
of quinones
and tyrosines

Repair

D2 D1

D2

D1

Q A

Q B

Fe

Phe Phe

P 680

Tyr-D

Tyr-Z

Damage
of the D1 and D2
proteins

Mn

Figure 3. The sequence of UV radiation induced damaging events in PSII. The primary effect of
UV radiation is the inactivation of the Mn cluster of the water-oxidizing complex. This is followed by
the damage of quinone electron acceptors and tyrosine donors. Finally, both the D1 and D2 reaction
center subunits are degraded. In intact cells the damage can be repaired via resynthesis of the
damaged subunits.