Interactions of the beta-ionone ring with the protein in the visual pigment rhodopsin control the activation mechanism. An FTIR and fluorescence study on artificial vertebrate rhodopsins.
The photoreactions of rhodopsin regenerated with three 9-cis retinal analogs, modified at or in the vicinity of the beta-ionone ring (namely 5,6-epoxy, 7,8-diH, diethyl-acyclic) have been investigated by UV-vis and FTIR difference spectroscopy. In parallel, the ability to catalyze the GDP-->GTP exchange of G-protein (transducin) has been monitored by time-dependent fluorescence spectroscopy. The first photoproduct obtained with all three pigments at liquid nitrogen temperature is a blue-shifted intermediate (BSI), followed by a lumi-like intermediate at 170 K. For the 5,6-epoxy-ISO and 7,8-diH-ISO pigment we obtain two further intermediates similar to the META-I and META-II states of native RHO. For the diethyl-acyclic-ISO pigment only one further intermediate can be stabilized at 280 K. As compared to META-II the respective photoproduct exhibits striking differences. The latter two pigments have also been investigated in the solubilized lipid-free state (detergent: dodecyl maltoside) at 280 K. For the 5,6-epoxy-ISO pigment, the UV-vis, FTIR, and activation data agree with the formation of a META-II-like photoproduct (81% activation). Less META-II formation is observed for the 7,8-dihydro-ISO pigment in membranes (65% activation), but full formation in detergent (100% activation). Neither the membrane-bound nor the solubilized diethyl-acyclic-ISO pigment forms a META-II-like intermediate (18% and 0% activation, respectively). Therefore, we conclude that the substitution of the beta-ionone ring by two ethyl groups abolishes steric interactions with the protein, which are essential for META-II formation.(ABSTRACT TRUNCATED AT 250 WORDS)
Jäger
F
F
Institut für Biophysik und Strahlenbiologie, Albert-Ludwigs-Universität Freiburg, FRG.
Jäger
S
S
Krütle
O
O
Friedman
N
N
Sheves
M
M
Hofmann
K P
KP
Siebert
F
F
D000818:Animals; D003080:Cold Temperature; D019204:GTP-Binding Proteins / Q000737:chemistry; D045792:Norisoprenoids; D012243:Rhodopsin / Q000737:chemistry*; D013050:Spectrometry, Fluorescence; D013056:Spectrophotometry, Ultraviolet; D017550:Spectroscopy, Fourier Transform Infrared; D013729:Terpenes / Q000737:chemistry*
D045792:Norisoprenoids; D013729:Terpenes; D012243:Rhodopsin; C008157:beta-ionone; D019204:GTP-Binding Proteins