Photosystem II

Parent
Photosystem
Subordinate
Photosystem II reaction center light harvesting complex II
Gene Ontology
QuickGO

Scheme

The photosystem II (PSII hereinafter) is part of the photosynthetic system from plants, algae and cyanobacteria. It is a protein complex that is embedded in the thylakoid membrane of the chloroplasts , protrudes into the stroma or lumen on both sides and in which the first step of the light reaction takes place. In this step, the reaction center Plastoquinone to plastoquinol reduced and pigments absorbed light is used as the energy required source. The energy of the light is thus stored in the form of a reduction potential. This reduction potential is required for the further steps of the photosynthesis-light reaction that take place in photosystem I (PSI).

structure

Photosystem II embedded in the thylakoid membrane, from the side, according to PDB 2AXT

The PSII is a heterodimer of the two proteins D1 and D2, which are homologous to one another . This heterodimer forms depending on the kind of a from about 20 subunits assembled protein complex in which the relevant for the light absorption light collecting pigments are embedded. The protein structure holds the light harvesting pigments in place so that the energy can be transferred from pigment to pigment as efficiently as possible. A PSII monomer (D1 or D2) contains about 35 molecules of chlorophyll a and 11 molecules of β-carotene as light harvesting pigments . In the center of the PSII monomer is the so-called special pair, two closely spaced chlorophylls to which all of the absorbed energy is transferred. Due to their close spatial proximity to each other, their energy level is lowered by excitonic interactions, so that they have their absorption maximum at 680 nm. For this reason they are also known as P680.

In the immediate vicinity of the special pair there are a pheophytin and a plastoquinone molecule, via which the energy is transferred to another plastoquinone molecule. This is reduced and its reduction potential is used for the further steps of the light reaction. The now positively charged special pair withdraws electrons from water molecules via a neighboring manganese complex, so that it is back in its original state and can absorb energy again.

Structure of the photosystem II super complex , according to PDB 2AXT 2BHW

A PSII dimer is surrounded by six light harvesting complex II trimers (LHCII) and the smaller chlorophyll proteins CP24, CP26, and CP29. These so-called light-harvesting antennas absorb energy via the light-harvesting pigments they contain and pass it on to the PSII dimer. The super complex acts like a giant funnel that focuses all the absorbed energy on the special pairs .

Running light reaction

The overall balance of the reaction taking place in PSII is:

${\displaystyle 2\ {\text{H}}_{2}{\text{O}}\ +\ 2\ {\text{PQ}}\ +\ 4\ {\text{H}}_{\text{Stroma}}^{+}\longrightarrow {\text{O}}_{2}\ +\ 2\ {\text{PQH}}_{2}\ +\ 4\ {\text{H}}_{\text{Lumen}}^{+}}$

A total of four photons must be absorbed for this reaction.

Manganese complex

The oxidation of the water takes place at the manganese complex . The exact structure of the complex has not yet been finally determined, but it is known that it consists of four manganese, four oxygen and one calcium atoms. Manganese has the ability to take on many different oxidation states. This means that the complex can be oxidized in four steps (by donating electrons to the special pair ) and finally oxidizing two molecules of water to oxygen and releasing four protons. The four protons released reduce the pH value on the thylakoid lumen side. Overall, a proton gradient is generated across the thylakoid membrane.

literature

• O. Sozer, M. Kis, Z. Gombos, B. Ughy: Proteins, glycerolipids and carotenoids in the functional photosystem II architecture. In: Front Biosci. tape 16 , 2011, doi : 10.2741 / 3710 , PMID 21196193 (English).
• PJ Nixon, F. Michoux, J. Yu, M. Boehm, J. Komenda: Recent advances in understanding the assembly and repair of photosystem II . In: Annals of Botany . tape 106 , no. 1 , June 22, 2010, p. 1-16 , PMID 20328950 .

Individual evidence

1. ↑ AW Rutherford, P. Faller: Photosystem II: evolutionary perspectives. In: Philosophical transactions of the Royal Society of London. Series B, Biological sciences. Volume 358, number 1429, January 2003, ISSN  0962-8436 , pp. 245-253, doi: 10.1098 / rstb.2002.1186 . PMID 12594932 , PMC 1693113 (free full text).
2. ↑ B. Loll, J. Kern, W. Saenger, A. Zouni, J. Biesiadka: Towards complete cofactor arrangement in the 3.0 Å resolution structure of photosystem II. In: Nature. 438, 2005, pp. 1040-1044.
3. ↑ D. Voet, JG Voet, CW Pratt: Textbook of Biochemistry. Wiley-VCH, Weinheim 2002.
4. ↑ B. Alberts, D. Bray, J. Lewis, M. Raff, K. Roberts, JD Watson: Molecular Biology of the Cell. Garland Publishing, New York 2004.