ABSTRACT: An abundance of x-ray-derived structural data exists for the photosynthetic reaction center of the purple bacteria Rhodobacter sphaeroides; yet the preferred binding site of the secondary electron acceptor ubiquinone (QB) and how its binding influences the rate of electron transfer in the system are still uncertain. Current hypotheses propose a gated electron transfer; but the identification of a gate has eluded discovery.[Stowell, M. H. B., McPhillips, T. M., Rees, D. C., Soltis, S. M., Abresch, E., and Feher, G. (1997) Science 276, 812-816; Graige, M. S., Feher, G., and Okamura, M. Y. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 11679-11684] Suggested possibilities for the gate include the migration of QB from an inactive binding site to an electron-transfer active site, a change in orientation for QB to allow that migration, or a conformational change in the surrounding protein that either allows QB to migrate or otherwise allows electron-transfer to occur. In molecular dynamics simulations, we find that the His L190-neighboring residue Glh (Glu) L212 can compete with QB for the hydrogen bond with Nd of His L190. In fact, this glutamic acid residue’s role could be described as the conformational gate for ubiquinone binding/migration and possibly even for electron transfer.
 

  Funded by grants from Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy, Contract No. DE-FG03-97ER14806 and DE-FG03-01ER15164, and the Oklahoma Center for the Advancement of Science and Technology, OCAST Award No. H97-091; Supercomputing allocation award no. MCA96-N019 from the NSF/National Computational Science Alliance at the NCSA center at University of Illinois, Urbana-Champaign.