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Fujisawa T, Unno M. Near-Infrared Excited Raman Optical Activity as a Tool to Uncover Active Sites of Photoreceptor Proteins. J Phys Chem B 2024; 128:2228-2235. [PMID: 38441478 DOI: 10.1021/acs.jpcb.4c00094] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 03/16/2024]
Abstract
Raman optical activity (ROA) is a chiral sensitive technique to measure the difference in Raman scattering intensity between right and left circularly polarized light. The method has been applied to the study of biological molecules such as proteins, and it is now recognized as a powerful tool for investigating biomolecular structures. We have expanded the capability of this chiroptical technique to colored molecules, such as photoreceptor proteins, by using a near-infrared excitation. A photoreceptor protein contains a light-absorbing chromophore as an active site, and the precise determination of its structure is vital for comprehending the protein's function at the atomic level. In a photoreceptor protein, the protein environment can distort an achiral chromophore into a chiral conformation. ROA spectroscopy offers detailed structural information about the chromophore under physiological conditions. Here we explore recent progress in near-infrared ROA spectroscopy and its application to biological systems.
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Affiliation(s)
- Tomotsumi Fujisawa
- Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan
| | - Masashi Unno
- Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan
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Fujisawa T, Kinoue K, Seike R, Kikukawa T, Unno M. Configurational Changes of Retinal Schiff Base during Membrane Na + Transport by a Sodium Pumping Rhodopsin. J Phys Chem Lett 2024; 15:1993-1998. [PMID: 38349321 DOI: 10.1021/acs.jpclett.3c03435] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/23/2024]
Abstract
Microbial rhodopsins are photoreceptors containing the retinal Schiff base chromophore and are ubiquitous among microorganisms. The Schiff base configuration of the chromophore, 15-anti (C═N trans) or 15-syn (C═N cis), is structurally important for their functions, such as membrane ion transport, because this configuration dictates the orientation of the positively charged NH group that interacts with substrate ions. The 15-anti/syn configuration is thus essential for elucidating the ion-transport mechanisms in microbial rhodopsins. Here, we identified the Schiff base configuration during the photoreaction of a sodium pumping rhodopsin from Indibacter alkaliphilus using Raman spectroscopy. We found that the unique configurational change from the 13-cis, 15-anti to all-trans, 15-syn form occurs between the photointermediates termed O1 and O2, which accomplish the Na+ uptake and release, respectively. This isomerization is considered to give rise to the highly irreversible O1 → O2 step that is crucial for unidirectional Na+ transport.
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Affiliation(s)
- Tomotsumi Fujisawa
- Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan
| | - Kouta Kinoue
- Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan
| | - Ryouhei Seike
- Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan
| | - Takashi Kikukawa
- Faculty of Advanced Life Science, Hokkaido University, Sapporo 060-0810, Japan
| | - Masashi Unno
- Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan
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Fujisawa T, Shingae T, Ren J, Haraguchi S, Hanamoto T, Hoff WD, Unno M. Spectroscopic Validation of Crystallographic Structures of a Protein Active Site by Chiroptical Spectroscopy. J Phys Chem Lett 2023; 14:9304-9309. [PMID: 37816034 DOI: 10.1021/acs.jpclett.3c01954] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/12/2023]
Abstract
Out-of-plane distortions of a cofactor molecule in a protein active site are functionally important, and in photoreceptors, it has been proposed that they are crucial for spectral tuning and energy storage in photocycle intermediates. However, these subtle structural features are often beyond the grasp of structural biology. This issue is strikingly exemplified by photoactive yellow protein: its 14 independently determined crystal structures exhibit considerable differences in the dihedral angles defining the chromophore geometry, even though most of these are at excellent resolution. Here we developed a strategy to verify cofactor distortions in crystal structures by using quantum chemical calculations and chiroptical spectroscopy, particularly Raman optical activity and electronic circular dichroism spectroscopies. Based on this approach, we identify seven crystal structures with the chromophore geometries inconsistent with the experimentally observed data. The strategy implemented here promises to be widely applicable to uncovering cofactor distortions at active sites and to studies of reaction intermediates.
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Affiliation(s)
- Tomotsumi Fujisawa
- Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan
| | - Takahito Shingae
- Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan
| | - Jie Ren
- Department of Microbiology and Molecular Genetics, Oklahoma State University, Stillwater, Oklahoma 74078, United States
| | - Shojiro Haraguchi
- Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan
| | - Takeshi Hanamoto
- Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan
| | - Wouter D Hoff
- Department of Microbiology and Molecular Genetics, Oklahoma State University, Stillwater, Oklahoma 74078, United States
| | - Masashi Unno
- Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan
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Vaccaro PH, Xu Y. Virtual Issue on Chiroptical Spectroscopy. J Phys Chem A 2023; 127:7677-7681. [PMID: 37732338 DOI: 10.1021/acs.jpca.3c05566] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 09/22/2023]
Affiliation(s)
- Patrick H Vaccaro
- Department of Chemistry, Yale University, New Haven, Connecticut 06520-8107, United States
| | - Yunjie Xu
- Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
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Nishikawa K, Kuroiwa R, Tamogami J, Unno M, Fujisawa T. Raman Optical Activity of Retinal Chromophore in Sensory Rhodopsin II. J Phys Chem B 2023; 127:7244-7250. [PMID: 37556781 DOI: 10.1021/acs.jpcb.3c02391] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 08/11/2023]
Abstract
Raman optical activity (ROA) spectroscopy was used to study the conformation of the retinal chromophore in sensory rhodopsin II (SRII), which is a blue-green light sensor of microbes. The ROA spectrum consisted of the negative vibrational bands of the chromophore, whose relative intensities are similar to those of the parent Raman spectrum. This spectral feature was explained by the left-handed helical twist of the retinal chromophore on the basis of quantum chemical calculations. On the other hand, we found that the chromophore conformation based on the crystal structures of SRII has a right-handed helical twist, which does not agree with the observation. This specific result suggests that the consistency with chiro-optical properties can be a key criterion for the accurate prediction and/or evaluation of chromophore conformation in retinal-binding proteins.
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Affiliation(s)
- Kouhei Nishikawa
- Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan
| | - Ryosuke Kuroiwa
- Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan
| | - Jun Tamogami
- College of Pharmaceutical Sciences, Matsuyama University, Matsuyama, Ehime 790-8578, Japan
| | - Masashi Unno
- Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan
| | - Tomotsumi Fujisawa
- Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan
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Ohya M, Kikukawa T, Matsuo J, Tsukamoto T, Nagaura R, Fujisawa T, Unno M. Structure and Heterogeneity of Retinal Chromophore in Chloride Pump Rhodopsins Revealed by Raman Optical Activity. J Phys Chem B 2023. [PMID: 37201188 DOI: 10.1021/acs.jpcb.3c01801] [Citation(s) in RCA: 4] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/20/2023]
Abstract
Chloride transport by microbial rhodopsins is actively being researched to understand how light energy is converted to drive ion pumping across cell membranes. Chloride pumps have been identified in archaea and eubacteria, and there are similarities and differences in the active site structures between these groups. Thus, it has not been clarified whether a common mechanism underlies the ion pump processes for all chloride-pumping rhodopsins. Here, we applied Raman optical activity (ROA) spectroscopy to two chloride pumps, Nonlabens marinus rhodopsin-3 (NM-R3) and halorhodopsin from the cyanobacterium Mastigocladopsis repens (MrHR). ROA is a vibrational spectroscopy that provides chiral sensitivity, and the sign of ROA signals can reveal twisting of cofactor molecules within proteins. Our ROA analysis revealed that the retinal Schiff base NH group orients toward the C helix and forms a direct hydrogen bond with a nearby chloride ion in NM-R3. In contrast, MrHR is suggested to contain two retinal conformations twisted in opposite directions; one conformation has a hydrogen bond with a chloride ion like NM-R3, while the other forms a hydrogen bond with a water molecule anchored by a G helix residue. These results suggest a general pump mechanism in which the chloride ion is "dragged" by the flipping Schiff base NH group upon photoisomerization.
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Affiliation(s)
- Masaiku Ohya
- Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan
| | - Takashi Kikukawa
- Faculty of Advanced Life Science, Hokkaido University, Sapporo 060-0810, Japan
| | - Junpei Matsuo
- Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan
| | - Takashi Tsukamoto
- Faculty of Advanced Life Science, Hokkaido University, Sapporo 060-0810, Japan
| | - Ryota Nagaura
- Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan
| | - Tomotsumi Fujisawa
- Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan
| | - Masashi Unno
- Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan
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