Plastocyanin

• UniProt: P00289

Plastocyanin, sometimes referred to as Plastocyan is a small copper protein in algae, green plants and some of cyanobacteria, which plays an important role in photosynthesis. It transports electrons from the cytochrome b6f complex to photosystem I. In eukaryotes plastocyanin located in the lumen of the thylakoids.

Plastocyanin one of the blue copper proteins found in plants, bacteria and archaea.

Structure

Plastocyanin is constructed in spinach and most other organisms in its mature form, 10.5 kDa heavy, as well as 99 amino acids. It binds as a cofactor coordinated to a copper atom, which changes by electron attachment between oxidation states CuII and CuI. The protein can thus accommodate only one electron, and return it. The tertiary structure of the protein is a β -barrel.

Function

The light response of the photosynthesis is a sequence of electron transitions that occur in plants in the thylakoid of chloroplasts. The protein complex Photosystem II gains of the use of light energy electrons from the water. Electrons are initially at the small molecule plastoquinone, and then via the cytochrome b6f complex on plastocyanin and finally the photosystem I to NADP transfer. Plastocyanin is a soluble electron carrier, diffuses in the lumen of the thylakoid.

The electron is transferred from cytochrome f, a subunit of the cytochrome b6f complex directly on plastocyanin. In some cyanobacteria and algae plastocyanin is replaced by the small, iron-containing protein cytochrome c6. Plastocyanin and cytochrome c6 interact directly with photosystem I and reduce its photooxidiertes reaction center P700 .

Expression and Topogenese

Plastocyanin is encoded in plants and green algae in the core of the gene petE whose cytosolic -translated product is a prepro- apoprotein with an N-terminal transit peptide that is cleaved off after import into the chloroplast by a peptidase. This intermediate form is then transported via the SecA - transport in the thylakoid lumen, where it binds the copper atom.

Pete gene expression is regulated in part by the cellular sugar level ( sugar sensing), the expression is inhibited by the accumulation of sugars. Furthermore repressed also a reduction in the plastoquinone pool and the thioredoxin system and the quality of the incident light (or a change from preferentially photosystem II excitation light to photosystem I excitation light ) the petE expression.

In addition, the petE expression is suppressed by the presence of reactive oxygen species. For retrograde signal paths are required to regulate the gene expression of nuclear -encoded plastid genes in response to signals. In the mentioned studies, however, only the petE gene expression and accumulation of the protein was not analyzed, which must not necessarily coincide.

Interaction with the photosynthetic complexes

Plastocyanin with cytochrome f forms a solid transition complex, creating a very fast electron transfer is possible within 35-350 microseconds. Arginine and Lysine in the region between the large and small domain of cytochrome f bind plastocyanin possibly via electrostatic forces. The reduced plastocyanin interacts directly with in higher plants photosystem I and with it forms a transition complex. Then, the complex undergoes a conformational change and an electron is transferred to P700 . The electron transfer from plastocyanin to P700 is fast with a duration 10 to 20 microseconds, since reduced plastocyanin is bound more strongly than oxidized plastocyanin. In the crystal structures of photosystem I is a flat region can be seen on the lumenal side of photosystem that is probably the binding site for plastocyanin. Luminal loops of PsaA and PSAB contribute to the efficient binding of plastocyanin. In plants is further achieved by additional amino acids at the N-terminus of a much stronger PsaF plastocyanin bond, as in most cyanobacteria - these interacts plastocyanin by collision, and not specifically with photosystem I; the measured transfer speeds of up to two orders of magnitude smaller. However, is forming cyanobacteria in a fixed transition complex between photosystem I and the alternative electron carrier cytochrome c6.