title: Molecular bases for the selection of the chromophore of animal rhodopsins.
abstract: The functions of microbial and animal rhodopsins are triggered by the isomerization of their all-trans and 11-cis retinal chromophores, respectively. To lay the molecular basis driving the evolutionary transition from the all-trans to the 11-cis chromophore, multiconfigurational quantum chemistry is used to compare the isomerization mechanisms of the sensory rhodopsin from the cyanobacterium Anabaena PCC 7120 (ASR) and of the bovine rhodopsin (Rh). It is found that, despite their evolutionary distance, these eubacterial and vertebrate rhodopsins start to isomerize via distinct implementations of the same bicycle-pedal mechanism originally proposed by Warshel [Warshel A (1976) Nature 260:678-683]. However, by following the electronic structure changes of ASR (featuring the all-trans chromophore) during the isomerization, we find that ASR enters a region of degeneracy between the first and second excited states not found in Rh (featuring the 11-cis chromophore). We show that such degeneracy is modulated by the preorganized structure of the chromophore and by the position of the reactive double bond. It is argued that the optimization of the electronic properties of the chromophore, which affects the photoisomerization efficiency and the thermal isomerization barrier, provided a key factor for the emergence of the striking amino acid sequence divergence observed between the microbial and animal rhodopsins.
authors.affiliation: Department of Chemistry, Bowling Green State University, Bowling Green, OH 43403;
authors.forename: Hoi Ling
authors.initials: HL
authors.lastname: Luk
authors.affiliation: Dipartimento di Biotecnologie, Chimica e Farmacia, Università di Siena, Siena I-53100, Italy.
authors.forename: Federico
authors.initials: F
authors.lastname: Melaccio
authors.affiliation: Dipartimento di Biotecnologie, Chimica e Farmacia, Università di Siena, Siena I-53100, Italy.
authors.forename: Silvia
authors.initials: S
authors.lastname: Rinaldi
authors.affiliation: Department of Chemistry, Bowling Green State University, Bowling Green, OH 43403;
authors.forename: Samer
authors.initials: S
authors.lastname: Gozem
authors.affiliation: Department of Chemistry, Bowling Green State University, Bowling Green, OH 43403; Dipartimento di Biotecnologie, Chimica e Farmacia, Università di Siena, Siena I-53100, Italy molivuc@bgsu.edu.
authors.forename: Massimo
authors.initials: M
authors.lastname: Olivucci
mesh_terms: D017033:Anabaena / Q000378:metabolism*; D000818:Animals; D002417:Cattle; D007536:Isomerism; D008958:Models, Molecular; D012243:Rhodopsin / Q000737:chemistry* / Q000378:metabolism*
chemical_list: D012243:Rhodopsin
keywords: computational photobiology; excited states; retinal chromophore; rhodopsin; ultrafast isomerization