L. Song et al., PROTEIN CATALYSIS OF THE RETINAL SUBPICOSECOND PHOTOISOMERIZATION IN THE PRIMARY PROCESS OF BACTERIORHODOPSIN PHOTOSYNTHESIS, Science, 261(5123), 1993, pp. 891-894
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.