Commentary and Perspective
Introduction of Session 1, “Photochemistry of retinal proteins”
2023 Volume 20 Issue Supplemental Article ID: e201021
Details
2023 Volume 20 Issue Supplemental Article ID: e201021
Retinal proteins contain a vitamin-A aldehyde retinal as a chromophore in the middle of their transmembrane domain via protonated Schiff base linkage [1]. The chromophore retinal absorbs light at different wavelengths (λmax: 350–700 nm) due to difference in the electronic energy gap between its ground- and excited states, and then the photo-absorption triggers isomerization of the retinal at a specific double bond, resulting in biological functions such as vision, and iontransportation [1]. Thus, light absorption and photoisomerization are one of the key steps for the retinal proteins.
The first speaker of this session named “photochemistry of retinal proteins” was Dr. James H. Geiger from Michigan State University. Dr, Geiger gave a talk entitled ‘the retinoid binding protein family, a powerful template for protein design and discovery’. In short, he created new proteins with a wide variety of properties, including near infra-red emission, photoswitching over an adjustable range of wavelengths, excited state proton transfer leading to large apparent Stokes shift [2].
The second speaker was Dr. Masashi Unno from Saga University, who discussed “Retinal conformations in microbial rhodopsins probed by Raman optical activity”. Using Raman optical activity (ROA), he provided the first experimental evidence that the twist direction of the retinal chromophore indeed determines a sign of the ROA spectrum [3]. Furthermore, he disrupted a hydrogen bond responsible for the distortion of the retinal in a sodium-pumping rhodopsin and showed that the sign of the ROA signals of this non-functional mutant is flipped. The results document the ability of ROA to elucidate the chromophore conformation in retinal proteins.
The third speaker was Dr. Josef Wachtveitl from Goethe University, who discussed “The inward proton pump Xenorhodopsin: A kinetic view on the variable vectoriality of proton pumping”. He employed an inward proton pump rhodopsin from Nanosalina (NsXeR) and performed time-resolved spectroscopy from the picosecond to second time scale. The results provided a detailed description of the molecular switch that regulates the accessibility of the proton-binding to the retinal Schiff base during the pumping process. The blue-light illumination of NsXeR in the M state showed a potential-dependent vectoriality of the photocurrent, supporting the idea of a variable accessibility for the reprotonation of the retinal Schiff base [4].
The fourth speaker was Dr. Igor Schapiro from the Hebrew University of Jerusalem, who discussed “Ingight into the control of retinal photoisomerization by the protein environment”. He presented studies on the photoisomerization mechanism of the retinal and origin of the bond selectivity in the isomerization by hybrid quantum mechanics/molecular mechanics simulations to understand which amino acid sidechains control it both in animal and microbial rhodopsins [5].
The last speaker was Mr. Masahiro Sugiura from Nagoya Institute of Technology, who discussed “Characteristic photochemical properties of near-infrared light-absorbing enzuymerhodopsin (NeoR)”. In general, the retinal chromophore isomerizes from all-trans to 13-cis and from 11-cis to all-trans in microbial and animal rhodopsins, respectively. In contrast, he showed that a near-infrared light-absorbing enzymerhodopsin from Obelidium mucronatum (OmNeoR) contains the all-trans form in the dark, but it isomerizes into the 7-cis form upon illumination [6].
