Photosynthetic Reaction Centre
The photosynthetic reaction centre of the purple bacterium
Rhodopseudomonas occurs in the
membranes of photosynthetic vesicles. This protein complex is composed of
4 subunits: L, M, H and a cytochrome. The L and M subunits are homologous
and each have 5 transmembrane alpha-helices. A helix length of 20-25 residues is
required to span this bacterial membrane. The H subunit, on the cytoplasmic
side of the membrane, also has a single transmembrane helix. As in
bacteriorhodopsin, the helices are tilted at an angle of 20-25 ° to the
perpendicular to the membrane. The whole complex is shown below.
A number of pigments (quinones) are bound between the helices of the photosynthetic
reaction centre. Buried helix residues
which interact
either which the pigments, or with other helices are relatively highly conserved
between different bacterial species, whereas those on the outside of the bundle
are not. This indicates the non-specific nature of the hydrophobic interactions
between the complex and the bilayer.