PROTEIN CATALYSIS OF THE RETINAL SUBPICOSECOND PHOTOISOMERIZATION IN THE PRIMARY PROCESS OF BACTERIORHODOPSIN PHOTOSYNTHESIS

The rate of retinal photoisomerization in wild-type bacteriorhodopsin (wt bR) is compared with that in a number of mutants in which a positi vely charged (Arg82), a negatively charged (Asp85 or Asp212), or neutr al hydrogen bonding (Asp115 or Tyr185) amino acid residue known to be functionally important within the retinal cavity is replaced by a neut ral, non-hydrogen bonding one. Only the replacements of the charged re sidues reduced the photoisomerization rate of the 13-cis and all-trans isomers present in these mutants by factors of approximately 1/4 and approximately 1/20, respectively. Retinal photo- and thermal isomeriza tion catalysis and selectivity in wt bR by charged residues is discuss ed in terms of the known protein structure, the valence-bond wave func tions of the ground and excited state of the retinal, and the electros tatic stabilization interactions within the retinal cavity.