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 relaxa tion of the polyene chain back to all-trans is coupled to various chan ges in the protein and the translocation of a proton across the membra ne. We investigated the nature of this high-energy state in a genetica lly modified bacteriorhodopsin, When the electric charges of residues 85 and 96, the two aspartic acids critical for proton transport, are b oth 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 revers ed 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 bi nding pocket. Later in the photocycle, the free energy gain is transfe rred from the chromophore to the protein. Before recovery of the initi al state, it will come to be represented entirely by the free energy o f the changed protonation states of aspartic acids 85 and 96.