Comparative analyses of three-dimensional models of bacterial reaction centers

Despite the fact that the three-dimensional structure of an integral membrane protein was first determined 20 years ago, structures have been solved for very few membrane proteins. The reaction center is an exception with many mutant and modified structures available from 3 different bacterial species. In order to relate these structures to the function of the reaction center, an accurate assessment of the reliability of the structural models is required. Here we describe the quality of the structures of the bacterial reaction center based upon different criteria, such as evaluation of the geometry of the models and comparison of different models. Overall, the structures are found to be most accurate in the membrane-embedded regions with the periplasmic and cytoplasmic exposed regions having more disorder and differences among the structural models. In general, the cofactors and the surrounding protein regions are among the most accurately determined regions of the protein, except for the secondary quinone and its binding pocket that shows a large variation among structures. The limited accuracy of the secondary quinone is due to its partial occupancy as a consequence of its functional role and to the presence of surface features, including lipids and detergent molecules.