Cyanobacterial photosystem II
Thermosynechococcus elongatus (cyanobacteria)
Photosynthesis uses light energy to produce organic compounds from CO2. Plants carry out this process in chloroplasts of cells. Photosynthetic bacteria, on the other hand, do this in a photosynthetic organ, a vesicle called chromatophore. Photosynthesis involves two systems, photosystem I and II (PSI and PSII), and via the two systems, the electron from a water molecule is ultimately transferred to NADP+. Many proteins and chemical compounds participate in these systems; cytochrome and non-heme proteins act in the electron transfer, and proteins containing metal ions such as Mn and Ca ions act in the oxidation. In the photosynthesis of plants and cyanobacteria, electrons are obtained from the photoinduced water oxidation and molecular dioxygens are released. Simultaneously, CO2 is reduced with the energy generated by the hydrolysis of ATP, synthesized by the photophosphorylation reaction, and converted to biomass, food and fuel. The PSII, which plays the central role in these reactions, is essential for maintaining the biosphere.
Recently the 3D-structure of the PSII complex has been determined and the details of the water oxidation and the oxygen release in oxygen-evolving center (OEC) have been revealed. This is a very large dimeric complex. Each monomer contains 19 protein subunits, 36 chlorophylls, 7 carotenoids and one OEC. The OEC contains a Mn3CaO4 cluster. Because a fourth Mn ion (Mn4) is linked to O4 of the cluster by a bridge and is close to the Ca ion, a possible site for the oxidation reaction is formed between Mn4 and Ca. At this site, a water molecule may bind to Mn4, and an oxygen molecule will be released via a formation of an intermediate. The electron obtained from a water molecule is first transferred to P680. It is then quickly transferred to its final acceptor, plastoquinone Qb near the stromal surface, relayed through chlorophyll, pheophytin and plastoquinone Qa in thylakoid of chloroplast. The electron is subsequently transferred to the next system, releasing Qb from PSII.
Protein Data Bank (PDB)
Ferreira, K.N. Iverson, T.M. Maghlaoui, K. Barber, J. Iwata, S.; "Architecture of the Photosynthetic Oxygen-Evolving Center"; Science; (2004) 303:1831-1838 PubMed:14764885.
author: Yuko Tsuchiya