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 QB 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, QA accepts the electron from the pheophytin and transfers it to the QB plastoquinone in 200 ms. After two reductions by QA, the QB plastoquinone is converted into a plastoquinol, which is replaced by another "new" plastoquinone. Prior to the second reduction by QA, the positive charge or "hole" on P680+ must be transferred to the manganese cluster via a redox-active tyrosine, known as YZ, 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 QB plastoquinones.
Photosystem II Structure
Structurally PS II is comprised of >25 polypeptides and is surrounded by a variety of chlorophyll a and chlorophyll b binding proteins, known as light harvesting complexes, which funnel light into the complex. At the enzymatic heart of the complex are two polypeptides known as the D1 and D2 proteins. The "special pair," P680, is found between these proteins. The pheophytin and QA are found on the D2 protein, and the redox-active tyrosine, YZ, and the QB plastoquinone are found on the D1 protein. In the D2 protein, the QA plastoquinone remains relatively fixed compared to the QB plastoquinone, which is free to move in and out of the D1 protein at the "QB site". Inhibitors of PS II electron transport generally bind at the "QB site", preventing the reduction and binding of the QB plastoquinone.
Photosystem II Sites of InterestPhotosystem II is based on coordinates from Dr. Jonathan Nugent. The D1 subunit is colored dark blue, D2 subunit red, chlorophylls green, pheophytins yellow, QA light blue, QB orange, bicarbonate grey (Updated 8/31/99).