Visual pigments and bacteriorhodopsins formed from aromatic retinal analogs

Exploring the Retinal Binding Cavity of Archaerhodopsin-3 by Replacing the Retinal Chromophore With a Dimethyl Phenylated Derivative

Rhodopsins act as photoreceptors with their chromophore retinal (vitamin-A aldehyde) and they regulate light-dependent biological functions. Archaerhodopsin-3 (AR3) is an outward proton pump that has been widely utilized as a tool for optogenetics, a method for controlling cellular activity by light. To characterize the retinal binding cavity of AR3, we synthesized a dimethyl phenylated retinal derivative, (2E,4E,6E,8E)-9-(2,6-Dimethylphenyl)-3,7-dimethylnona-2,4,6,8-tetraenal (DMP-retinal). QM/MM calculations suggested that DMP-retinal can be incorporated into the opsin of AR3 (archaeopsin-3, AO3). Thus, we introduced DMP-retinal into AO3 to obtain the non-natural holoprotein (AO3-DMP) and compared some molecular properties with those of AO3 with the natural A1-retinal (AO3-A1) or AR3. Light-induced pH change measurements revealed that AO3-DMP maintained slow outward proton pumping. Noteworthy, AO3-DMP had several significant changes in its molecular properties compared with AO3-A1 as follows; 1) spectroscopic measurements revealed that the absorption maximum was shifted from 556 to 508 nm and QM/MM calculations showed that the blue-shift was due to the significant increase in the HOMO-LUMO energy gap of the chromophore with the contribution of some residues around the chromophore, 2) time-resolved spectroscopic measurements revealed the photocycling rate was significantly decreased, and 3) kinetical spectroscopic measurements revealed the sensitivity of the chromophore binding Schiff base to attack by hydroxylamine was significantly increased. The QM/MM calculations show that a cavity space is present at the aromatic ring moiety in the AO3-DMP structure whereas it is absent at the corresponding β-ionone ring moiety in the AO3-A1 structure. We discuss these alterations of the difference in interaction between the natural A1-retinal and the DMP-retinal with binding cavity residues.

Redshifted and Near-infrared Active Analog Pigments Based upon Archaerhodopsin-3

Archaerhodopsin‐3 (AR3) is a member of the microbial rhodopsin family of hepta‐helical transmembrane proteins, containing a covalently bound molecule of all‐trans retinal as a chromophore. It displays an absorbance band in the visible region of the solar spectrum (λmax 556 nm) and functions as a light‐driven proton pump in the archaeon Halorubrum sodomense. AR3 and its mutants are widely used in neuroscience as optogenetic neural silencers and in particular as fluorescent indicators of transmembrane potential. In this study, we investigated the effect of analogs of the native ligand all‐trans retinal A1 on the spectral properties and proton‐pumping activity of AR3 and its single mutant AR3 (F229S). While, surprisingly, the 3‐methoxyretinal A2 analog did not redshift the absorbance maximum of AR3, the analogs retinal A2 and 3‐methylamino‐16‐nor‐1,2,3,4‐didehydroretinal (MMAR) did generate active redshifted AR3 pigments. The MMAR analog pigments could even be activated by near‐infrared light. Furthermore, the MMAR pigments showed strongly enhanced fluorescence with an emission band in the near‐infrared peaking around 815 nm. We anticipate that the AR3 pigments generated in this study have widespread potential for near‐infrared exploitation as fluorescent voltage‐gated sensors in optogenetics and artificial leafs and as proton pumps in bioenergy‐based applications.

Modulation of spectral properties and pump activity of proteorhodopsins by retinal analogues

Microbial proteorhodopsins are light-driven proton pumps, using the vitamin A derivative retinal as chromophore. We show that retinal analogues can shift their absorbance band with preservation of functionality. This may provide attractive opportunities in biotechnology, optogenetics and as potential sensors.

Unique biphasic band shape of the visible circular dichroism of bacteriorhodopsin in purple membrane: Excitons, multiple transitions or protein heterogeneity?

OVER A DECADE AND A HALF AGO, WHEN THE FIRST VISIBLE MEMBRANE SUSPENSION CIRCULAR DICHROIC (CD) SPECTRUM OF THE PURPLE MEMBRANE (PM) WAS PRESENTED, TWO MECHANISMS WERE PROPOSED TO ACCOUNT FOR THE OBSERVED BIPHASIC SHAPED CD BAND: (a) excitonic interactions among the retinals of the sole protein bacteriorhodopsin (bR) in the crystalline structure of the PM, and (b) combination of CD bands with opposite rotational strengths due to a retinal-apoprotein heterogeneity of the bR molecules or due to two possible close-lying long-wavelength transitions of the retinal of the bR with opposite rotational strengths. Since that time, an impressive body of experimental and theoretical evidence has been accumulated, mostly consistent with an exciton model but many at serious odds with any heterogeneity or multiple transition model. Recently, a number of articles have appeared reporting analyses of new experimental observations which are proposed to cast serious doubts on the viability of the exciton model, and therefore, may revive the heterogeneity or multiple transition model as an explanation for the unique shape of the CD band of the PM. The intent of this article is to demonstrate that if all observations found in literature baring on this question are considered in toto and in a consistent manner, they can be interpreted without exception by excitons, and furthermore, that there is no plausible evidence available to warrant the revival of the heterogeneity or multiple transition model as an explanation for the unique shape of the biphasic CD band of the PM.