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 positively charged (Arg82), a negatively charged (Asp85 or Asp212), or neutral hydrogen bonding (Asp115 or Tyr185) amino acid residue known to be functionally important within the retinal cavity is replaced by a neutral, non-hydrogen bonding one. Only the replacements of the charged residues 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 isomerization catalysis and selectivity in wt bR by charged residues is discussed in terms of the known protein structure, the valence-bond wave functions of the ground and excited state of the retinal, and the electrostatic stabilization interactions within the retinal cavity.