Photosystem II Structure

PSII

Here is a very simple cartoon of PSII showing the main functional parts of the enzyme. It is comprised of more than 20 separate proteins, many of which are membrane proteins. It also contains cofactors for capturing light energy and transferring electrons from water to plastoquinone. It can be divided into two functional parts, (1) the electron transfer domain and (2) the water oxidation complex. The portion of the enzyme within the thylakoid membrane contains the components of the electron transfer pathway. The water oxidation complex, where water-splitting occurs, is found on the lumenal side of the complex.

A number of PSII structural models based on x-ray diffraction data have been reported over the last decade with increasing resolution. These models are based on the enzyme from thermophilic cyanobacteria. Each model contains about 20 protein subunits along with a slew of associated cofactors (pigments, ions, metal cofactors, lipid molecules) with a total molecular weight of ~350 kDa per monomer (single unit). This represents a lot of hard work by more than a few talented scientists. There’s even more info here: Molecule of the month at the RSCB

Below is the 1.9 Angstrom resolution structure from Umena et al 2011 PDB ID: 3ARC

It’s much more sophisticated than the cartoon drawing, no?

This view looks from the bottom of the membrane.

Here are some other side views of PSII

 

English: PDB 2axt, using pymol

English: PDB 2axt, using pymol (Photo credit: Wikipedia)

 

This image gives you a better idea of how the dimeric complex can be divided into two dimers (one is colored, the other monomer is shown in white; colored cofactors are shown in the white monomer)

PSII structure

The following image drives home just how many cofactors are associated with the PSII dimer. Only the cofactors are shown and the outline of the protein space of the structure is shown in white. Note, this is from an older structure that did not resolve as many cofactors as some of the more recent structures.

PSII cofactors

Check out these links to the scientific literature on PSII structure.

One thought on “Photosystem II Structure

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