Bacterial light harvesting complex, LH2

Bacterial light harvesting complex, LH2 from Rhodopseudomonas acidophila

The structure of bacterial light harvesting LH2 complex from Rhodopseudomonas acidophila

The structure has been determined at 2.5 Å resolution by X-ray diffraction studies by Richard Cogdell and colleagues (1), who kindly provided the coordinates for this picture.

A single a, b-subunit pair. The bacteriochlorophyll (BChl) of B800 is shown as dark green, the B850 pair of BChls are shown as cyan and purple, carotenoids are orange and yellow, water oxygens are red; a-subunit is blue, b-subunit is pale green.

BChls of B850, with histidine ligands.
BChl B800, showing the water molecule (red O) which acts as a bridging ligand between b-chain His 12 and the Mg of the BChl.
Carotenoids (2 rhodopin-glucoside, one incomplete).
The protein, colored by amino acid type. The ball and stick "tail" is the formylmethionine at the N-terminus.
Add BChls of B850.
Add BChl B800. With both B850 and B800 selected, you can see the "hook-and-eye" arrangement of the BChl phytol sidechains
Add carotenoids.
Restore wireframe view.

The interaction between a,b-pairs

In the biologically relevant structures, the a,b-pairs shown in the tutorial above, are associated into cylindrical structures composed of 8 or 9 pairs (see images in lecture 20). The interactions between the pairs can be explored in the structure recently deposited in the Brookhaven Protein Data Bank as pdb file 1kzu.pdb, which contains three such pairs.

Click here to load the structure.

The protein is shown in cartoon format to highlight the helical segments, with sidechains in wireframe. The protein is colored by chain so that the a,b-pairs are red (H) and yellow (G); green (E) and green-blue (D); blue (B) and dark blue (A). The formyl methionine group at the N-terminus is colored in CPK. The bacteriochlorophyll (BChl) of B800 is shown as dark green, the B850 pair of BChls are shown as cyan and purple, carotenoids are orange. In the native structure, three similar units are arranged in a cylindrical configuration containing 9 a,b-pairs, with interactions between pairs at the interfaces similar to those between the pairs shown here. Note that protein-protein contacts between the subunits are limited to the domains at the aqueous interface. In the lipid phase of the membrane there are no direct contacts, but a number of buried waters near the interface on the B800 side of the membrane form H-bonds which may have a bridging role.

Show water oxygens.

Click here to load the structure of the complete complex. The complex is color coded by chain so that each a,b-pair is in almost the same color, and the color changes from blue through green and yellow to red around the ring. Pigments are shown as wireframe models, with BChl in purple, and the main carotenoid in orange.

Wireframe model.

McDermott, G, Prince, S.M., Freer, A.A., Hawthornthwaite-Lawless, A.M., Papiz, M.Z., Cogdell, R.J. and Isaacs, N.W. (1995) Crystal structure of an integral membrane light-harvesting complex from photosynthetic bacteria. Nature 374, 517-521