Several features of Bacteriorhodopsin are shown:

1. Overall structure, lipid binding, and roles of amino acids.

PDB file 1brr

PubMed

2. Continued

3. Conformational changes in the M state

onPDB file 1cwq

PubMed

 

The photocycle for bacteriorhodopsin starts at the top, where a photon of light is absorbed. On a rapid timescale, the protein transitions from the ground state to states K, L and M, and loses a proton to the outside of the membrane. Now on a millisecond time scale, the protein continues to states N and O, and it picks up a proton from the inside of the membrane.

br cycle

Figure is from the following paper:

 

Protein conformational changes in the bacteriorhodopsin photocycle: comparison of findings from electron and X-ray crystallographic analyses.
Hirai T, Subramaniam S.
PLoS One. 2009 Jun 2;4(6):e5769

Recommended for a full discussion on the detection of various conformational states.

4. Continued: Changes in water channels in the M state