Rhodopsin with 11-cis-locked chromophore is capable of forming an active state photoproduct.
The visual pigment rhodopsin is characterized by an 11-cis retinal chromophore bound to Lys-296 via a protonated Schiff base. Following light absorption the C(11)=C(12) double bond isomerizes to trans configuration and triggers protein conformational alterations. These alterations lead to the formation of an active intermediate (Meta II), which binds and activates the visual G protein, transducin. We have examined by UV-visible and Fourier transform IR spectroscopy the photochemistry of a rhodopsin analogue with an 11-cis-locked chromophore, where cis to trans isomerization around the C(11)=C(12) double bond is prevented by a 6-member ring structure (Rh(6.10)). Despite this lock, the pigment was found capable of forming an active photoproduct with a characteristic protein conformation similar to that of native Meta II. This intermediate is further characterized by a protonated Schiff base and protonated Glu-113, as well as by its ability to bind a transducin-derived peptide previously shown to interact efficiently with native Meta II. The yield of this active photointermediate is pH-dependent and decreases with increasing pH. This study shows that with the C(11)=C(12) double bond being locked, isomerization around the C(9)=C(10) or the C(13)=C(14) double bonds may well lead to an activation of the receptor. Additionally, prolonged illumination at pH 7.5 produces a new photoproduct absorbing at 385 nm, which, however, does not exhibit the characteristic active protein conformation.
Fan
Guibao
G
Department of Organic Chemistry, Weizmann Institute of Science, Rehovot 76100, Israel.
Siebert
Friedrich
F
Sheves
Mordechai
M
Vogel
Reiner
R
D006863:Hydrogen-Ion Concentration; D010777:Photochemistry; D011487:Protein Conformation; D012172:Retinaldehyde / Q000138:chemical synthesis / Q000737:chemistry*; D012243:Rhodopsin / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet; D017550:Spectroscopy, Fourier Transform Infrared
D012243:Rhodopsin; D012172:Retinaldehyde