The molecular basis for the high photosensitivity of rhodopsin.
Based on structural information derived from the F NMR data of labeled rhodopsins, rhodopsin crystal structure, and excited-state properties of model polyenes, we propose a molecular mechanism that accounts specifically for the causes of the well-known enhanced photoreactivity of rhodopsin (increased rates and quantum yield of isomerization). It involves the key features of close proximity of C-187 to H-12 and chromophore bond lengthening upon light absorption. The resultant "sudden punch" to H-12 triggers dual processes of decay of the Franck-Condon-excited rhodopsin, a productive directed photoisomerization and a nonproductive decay returning to the ground state as two separate molecular pathways [based on real-time fluorescence results of Chosrowjan, H., Mataga, N., Shibata, Y., Tachibanaki, S., Kandori, H., Shichida, Y., Okada, T. & Kouyama, T. (1998) J. Am. Chem. Soc. 120, 9706-9707]. The two processes are controlled by the local protein structure: an empty space provided by the intradiscal loop connecting transmembrane helices 4 and 5 and a protein wall composed of amino acid units in transmembrane 3. Suggestions, involving retinal analogs and rhodopsin mutants, to improve the unusually high photosensitivity of rhodopsin are proposed.
Liu
Robert S H
RS
Department of Chemistry, University of Hawaii, Honolulu, HI 96822, USA. rliu@gold.chem.hawaii.edu
Colmenares
Leticia U
LU
D000818:Animals; D001436:Bacteriorhodopsins / Q000737:chemistry; D001665:Binding Sites; D002244:Carbon / Q000737:chemistry; D018360:Crystallography, X-Ray; D003545:Cysteine / Q000737:chemistry; D006859:Hydrogen / Q000737:chemistry; D007700:Kinetics; D008027:Light; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D008958:Models, Molecular; D011090:Polyenes / Q000737:chemistry; D012160:Retina / Q000378:metabolism; D012243:Rhodopsin / Q000737:chemistry*; D013237:Stereoisomerism
D011090:Polyenes; D001436:Bacteriorhodopsins; D002244:Carbon; D006859:Hydrogen; D012243:Rhodopsin; D003545:Cysteine