Deactivation and Proton Transfer in Light-induced Metarhodopsin II/Metarhodopsin III Conversion

Abstract

Vertebrate rhodopsin shares with other retinal proteins the 11-cis-retinal chromophore and the light-induced 11-cis/trans isomerization triggering its activation pathway. However, only in rhodopsin the retinylidene Schiff base bond to the apoprotein is eventually hydrolyzed, making a complex regeneration pathway necessary. Metabolic regeneration cannot be short-cut, and light absorption in the active metarhodopsin (Meta) II intermediate causes anti/syn isomerization around the retinylidene linkage rather than reversed trans/cis isomerization. A new deactivating pathway is thereby triggered, which ends in the Meta III "retinal storage" product. Using time-resolved Fourier transform infrared spectroscopy, we show that the identified steps of receptor activation, including Schiff base deprotonation, protein structural changes, and proton uptake by the apoprotein, are all reversed. However, Schiff base reprotonation is much faster than the activating deprotonation, whereas the protein structural changes are slower. The final proton release occurs with pK ≈ 4.5, similar to the pK of a free Glu residue and to the pK at which the isolated opsin apoprotein becomes active. A forced deprotonation, equivalent to the forced protonation in the activating pathway, which occurs against the unfavorable pH of the medium, is not observed. This explains properties of the final Meta III product, which displays much higher residual activity and is less stable than rhodopsin arising from regeneration with 11-cis-retinal. We propose that the anti/syn conversion can only induce a fast reorientation and distance change of the Schiff base but fails to build up the full set of dark ground state constraints, presumably involving the Glu 134 /Arg 135 cluster.The photoreceptor rhodopsin located in the retinal rods of the vertebrate eye contains the chromophore 11-cis-retinal bound by a protonated Schiff base to Lys 296 of the apoprotein (1). Light absorption triggers isomerization around the C 11 ϭC 12 double bond of the polyene chain of the chromophore (2-4), leading to the strained all-trans-form and storage of two thirds of the light energy in the chromophore-protein system (5-8). The receptor subsequently proceeds through a number of intermediates each characterized by its specific absorption spectrum in the UV-visible and mid infrared range. Related conformational changes of the binding pocket and of other, more remote parts of the apoprotein eventually lead to the active G-protein binding state, metarhodopsin II (Meta II).3 It is in equilibrium with its precursor metarhodopsin I (Meta I), depending on temperature and pH (9, 10) and on other factors such as lipids, protein environment, and pressure (11-15).The formation of the active species through the photointermediates has been described as a stepwise lowering of the stabilizing effect of the Schiff base counterion, which is a complex structure that comprises highly conserved Glu 181 and Glu 113 . In Meta I, the counterion appears to undergo a shift relative to the Schiff base, where...

Crystal structure of the ligand-free G-protein-coupled receptor opsin

“…However, the TM6 movement is not enough for keeping the active conformation because the protonation of Glu-134 appears to be a prerequisite for coupling to the G protein [53,76,86,94,131,135,171,172,173,174]. …”

The Branched Photocycle of the Slow-Cycling Channelrhodopsin-2 Mutant C128T

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New Insights into Light-Induced Deactivation of Active Rhodopsin by SVD and Global Analysis of Time-Resolved UV/Vis- and FTIR-Data

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