Partitioning of free energy gain between the photoisomerized retinal and the protein in bacteriorhodopsin.

Photoisomerization of the all-trans-retinal of bacteriorhodopsin to 13-cis,15-anti initiates a sequence of thermal reactions in which relaxation of the polyene chain back to all-trans is coupled to various changes in the protein and the translocation of a proton across the membrane. We investigated the nature of this high-energy state in a genetically modified bacteriorhodopsin. When the electric charges of residues 85 and 96, the two aspartic acids critical for proton transport, are both changed to what they become after photoexcitation of the wild-type protein, i.e., neutral and anionic, respectively, the retinal assumes a thermally stable 13-cis,15-anti configuration. Thus, we have reversed cause and effect in the photocycle. It follows that when the 13-cis,15-anti isomeric state is produced by illumination, in the wild type it is unstable initially only because of conflicts with the retinal binding pocket. Later in the photocycle, the free energy gain is transferred from the chromophore to the protein. Before recovery of the initial state, it will come to be represented entirely by the free energy of the changed protonation states of aspartic acids 85 and 96.