Photoreactions of Metarhodopsin III

Abstract

Meta III is an inactive intermediate thermally formed following light activation of the visual pigment rhodopsin. It is produced from the Meta I/Meta II photoproduct equilibrium of rhodopsin by a thermal isomerization of the protonated Schiff base C=N bond of Meta I, and its chromophore configuration is therefore all-trans 15-syn. In contrast to the dark state of rhodopsin, which catalyzes exclusively the cis to trans isomerization of the C11=C12 bond of its 11-cis 15-anti chromophore, Meta III does not acquire this photoreaction specificity. Instead, it allows for light-dependent syn to anti isomerization of the C15=N bond of the protonated Schiff base, yielding Meta II, and for trans to cis isomerizations of C11=C12 and C9=C10 of the retinal polyene, as shown by FTIR spectroscopy. The 11-cis and 9-cis 15-syn isomers produced by the latter two reactions are not stable, decaying on the time scale of few seconds to dark state rhodopsin and isorhodopsin by thermal C15=N isomerization, as indicated by time-resolved FTIR methods. Flash photolysis of Meta III produces therefore Meta II, dark state rhodopsin, and isorhodopsin. Under continuous illumination, the latter two (or its unstable precursors) are converted as well to Meta II by presumably two different mechanisms.

Transition of Rhodopsin into the Active Metarhodopsin II State Opens a New Light-induced Pathway Linked to Schiff Base Isomerization

“…Light causes syn/anti isomerization and subsequent Schiff base deprotonation. Dependent on the illumination conditions, trans/cis and subsequent syn/anti isomerization of the Schiff base was also observed (36). Pathway 2b can be reverted: Meta II reacts thermally to Meta III, via Meta I (37).…”

Arrestin can act as a regulator of rhodopsin photochemistry

“…Green light can convert P500 back to Meta II (Bartl et al, 2001). The conformation of the chromophore and the protonation state of the Schiff-base (blue proton) is illustrated for each photoproduct, including the isomerization around the Schiff base that occurs during Meta III formation (Vogel et al, 2003(Vogel et al, , 2004a(Vogel et al, , 2004bZimmermann et al, 2004). Although it has been shown that P500 most likely binds an all-trans retinal (Bartl et al, 2001), the exact nature of this photoproduct has not been determined.…”

Activity Switches of Rhodopsin†

“…As noted above, Meta III containing retinal in the all-trans-15syn configuration is a light-sensitive pigment (17,20,22). Its photochemistry is rather complex, as light absorption triggers two different pathways which occur in parallel, as observed by time-resolved FTIR and flash photolysis experiments (55,56). On the one hand, when Meta III is activated by a short, green laser flash, one observes Schiff base syn ⁄ anti isomerization to all-trans-15-anti retinal, thus forming an early photointermediate which relaxes thermally through several intermediates into the Meta I ⁄ Meta II equilibrium.…”

“…As noted above, Meta III containing retinal in the all‐ trans ‐15‐ syn configuration is a light‐sensitive pigment (17,20,22). Its photochemistry is rather complex, as light absorption triggers two different pathways which occur in parallel, as observed by time‐resolved FTIR and flash photolysis experiments (55,56).…”

“…At low temperatures trans/cis isomerization is not observed anymore in the binding pocket of Meta III. Instead, syn/anti isomerization of the Schiff base becomes the predominant photoreaction (56). In analogy to rhodopsin activation, this allowed us to trap early photointermediates of Meta III at 80 and 173 K (21), which we term M‐Batho and M‐Lumi, respectively.…”