ABSTRACT

Oxygenic photosynthesis is a key biological process on Earth: it gives rise to much of its organic matter and is essential for the maintenance of most forms of life. During this process, light energy is converted into chemical energy, which is stored in the form of highly reduced organic compounds. The initial events of this transformation involve absorption of solar radiation by various pigments (predominantly chlorophylls [Chls] and carotenoids [Cars] but also by phycobilins in some organisms), and subsequent trapping of absorbed energy by a group of specialized pigment molecules (the so-called reaction center [RC] pigments) bound within membrane protein complexes termed photosystems or RC complexes. Excitation of these RC pigments leads to transmembrane charge separation and initiation of electron ·ow within the membrane to ultimately produce reduced nicotinamide adenine dinucleotide phosphate (NADPH), a source of reducing power for the cell. The electron transport chain consists of a system of electron carriers ranging from simple organic compounds

1.1 Introduction ......................................................................................................3 1.2 Structure and Function of Photosystem II ........................................................4 1.3 Photodamage to Photosystem II .......................................................................9 1.4 Photosystem II Repair Cycle .......................................................................... 10

1.4.1 Initiation of the Repair Cycle ............................................................. 11 1.4.2 Selective D1 Degradation and the Identity of the Protease ................ 12 1.4.3 Mechanism of D1 Replacement and Its Dynamics ............................. 13 1.4.4 Fate of Cofactors during D1 Replacement .......................................... 15

1.5 Conclusions and Perspectives ......................................................................... 16 Acknowledgment ..................................................................................................... 16 References ................................................................................................................ 17

(e.g., quinones) and small redox-active proteins up to multisubunit pigment-binding protein complexes, all of which are embedded in specialized photosynthetic membranes (termed thylakoids) found within cyanobacteria and the chloroplasts of plants and algae (Figure 1.1). Electron transfer is coupled to formation of a gradient of protons across the thylakoid membrane, which is utilized for ATP synthesis by the ATP synthase. NADPH and ATP generated by the “light reactions” are used in the “dark reactions” for synthesis of reduced organic compounds, mostly sugars, from CO2.