Photosystem II: Introduction

    Photosynthesis is a major biological energy source for life on the earth. The process of photosynthesis requires a variety of energy transducing protein complexes that transport electrons and pump of protons, leading to the formation of ATP (adenosine triphosphate) and NADPH (reduced nicotinamide adenine dinucleotide phosphate). Photosystem II (PS II) is one of these protein complexes and it has been the focus of many studies. The complex traps light and uses it to reduce the Q_B plastoquinone and to synthesize molecular oxygen from water). At the core of PS II is a pair of chlorophyll a molecules ("the special pair") known as P680. Upon absorption of a photon, P680 is excited to P680*, which is rapidly transferred to a nearby pheophytin within a few picoseconds. Within 200-400 ps, the plastoquinone, Q_A accepts the electron from the pheophytin and transfers it to the Q_B plastoquinone in 200 ms. After two reductions by Q_A, the Q_B plastoquinone is converted into a plastoquinol, which is replaced by another "new" plastoquinone. Prior to the second reduction by Q_A, the positive charge or "hole" on P680⁺ must be transferred to the manganese cluster via a redox-active tyrosine, known as Y_Z, and used to oxidize water to oxygen (O₂). The oxidation of water involves 5 different oxidation states of the Manganese cluster (Mn cluster), known as S-states, and requires the absorption of four photons and the reduction of 2 Q_B plastoquinones.

Photosystem II Structure