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Morimoto M, Hirao H, Kondo M, Dewa T, Kimura Y, Wang-Otomo ZY, Asakawa H, Saga Y. Atomic force microscopic analysis of the light-harvesting complex 2 from purple photosynthetic bacterium Thermochromatium tepidum. PHOTOSYNTHESIS RESEARCH 2023:10.1007/s11120-023-01010-4. [PMID: 36930432 DOI: 10.1007/s11120-023-01010-4] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/22/2022] [Accepted: 02/22/2023] [Indexed: 06/18/2023]
Abstract
Structural information on the circular arrangements of repeating pigment-polypeptide subunits in antenna proteins of purple photosynthetic bacteria is a clue to a better understanding of molecular mechanisms for the ring-structure formation and efficient light harvesting of such antennas. Here, we have analyzed the ring structure of light-harvesting complex 2 (LH2) from the thermophilic purple bacterium Thermochromatium tepidum (tepidum-LH2) by atomic force microscopy. The circular arrangement of the tepidum-LH2 subunits was successfully visualized in a lipid bilayer. The average top-to-top distance of the ring structure, which is correlated with the ring size, was 4.8 ± 0.3 nm. This value was close to the top-to-top distance of the octameric LH2 from Phaeospirillum molischianum (molischianum-LH2) by the previous analysis. Gaussian distribution of the angles of the segments consisting of neighboring subunits in the ring structures of tepidum-LH2 yielded a median of 44°, which corresponds to the angle for the octameric circular arrangement (45°). These results indicate that tepidum-LH2 has a ring structure consisting of eight repeating subunits. The coincidence of an octameric ring structure of tepidum-LH2 with that of molischianum-LH2 is consistent with the homology of amino acid sequences of the polypeptides between tepidum-LH2 and molischianum-LH2.
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Affiliation(s)
- Masayuki Morimoto
- Nanomaterials Research Institute (NanoMaRi), Kanazawa University, Kanazawa, 920-1192, Japan
- Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa, 920-1192, Japan
| | - Haruna Hirao
- Faculty of Science and Engineering, Kindai University, Higashi-Osaka, Osaka, 577-8502, Japan
| | - Masaharu Kondo
- Graduate School of Engineering, Nagoya Institute of Technology, Nagoya, 466-8555, Japan
| | - Takehisa Dewa
- Graduate School of Engineering, Nagoya Institute of Technology, Nagoya, 466-8555, Japan
| | - Yukihiro Kimura
- Graduate School of Agricultural Science, Kobe University, Kobe, 657-8501, Japan
| | | | - Hitoshi Asakawa
- Nanomaterials Research Institute (NanoMaRi), Kanazawa University, Kanazawa, 920-1192, Japan.
- Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa, 920-1192, Japan.
- Nano Life Science Institute (WPI-NanoLSI), Kanazawa University, Kanazawa, 920-1192, Japan.
| | - Yoshitaka Saga
- Faculty of Science and Engineering, Kindai University, Higashi-Osaka, Osaka, 577-8502, Japan.
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Timpmann K, Kangur L, Freiberg A. Hysteretic Pressure Dependence of Ca 2+ Binding in LH1 Bacterial Membrane Chromoproteins. J Phys Chem B 2023; 127:456-464. [PMID: 36608327 DOI: 10.1021/acs.jpcb.2c05938] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Abstract
Much of the thermodynamic parameter values that support life are set by the properties of proteins. While the denaturing effects of pressure and temperature on proteins are well documented, their precise structural nature is rarely revealed. This work investigates the destabilization of multiple Ca2+ binding sites in the cyclic LH1 light-harvesting membrane chromoprotein complexes from two Ca-containing sulfur purple bacteria by hydrostatic high-pressure perturbation spectroscopy. The native (Ca-saturated) and denatured (Ca-depleted) phases of these complexes are well distinguishable by much-shifted bacteriochlorophyll a exciton absorption bands serving as innate optical probes in this study. The pressure-induced denaturation of the complexes related to the failure of the protein Ca-binding pockets and the concomitant breakage of hydrogen bonds between the pigment chromophores and protein environment were found cooperative, involving all or most of the Ca2+ binding sites, but irreversible. The strong hysteresis observed in the spectral and kinetic characteristics of phase transitions along the compression and decompression pathways implies asymmetry in the relevant free energy landscapes and activation free energy distributions. A phase transition pressure equal to about 1.9 kbar was evaluated for the complexes from Thiorhodovibrio strain 970 from the pressure dependence of biphasic kinetics observed in the minutes to 100 h time range.
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Affiliation(s)
- Kõu Timpmann
- Institute of Physics, University of Tartu, W. Ostwald Str. 1, 50411 Tartu, Estonia
| | - Liina Kangur
- Institute of Physics, University of Tartu, W. Ostwald Str. 1, 50411 Tartu, Estonia
| | - Arvi Freiberg
- Institute of Physics, University of Tartu, W. Ostwald Str. 1, 50411 Tartu, Estonia.,Estonian Academy of Sciences, Kohtu 6, 10130 Tallinn, Estonia
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Timpmann K, Linnanto JM, Yadav D, Kangur L, Freiberg A. Hydrostatic High-Pressure-Induced Denaturation of LH2 Membrane Proteins. J Phys Chem B 2021; 125:9979-9989. [PMID: 34460261 DOI: 10.1021/acs.jpcb.1c05789] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The denaturation of globular proteins by high pressure is frequently associated with the release of internal voids and/or the exposure of the hydrophobic protein interior to a polar aqueous solvent. Similar evidence with respect to membrane proteins is not available. Here, we investigate the impact of hydrostatic pressures reaching 12 kbar on light-harvesting 2 integral membrane complexes of purple photosynthetic bacteria using two types of innate chromophores in separate strategic locations: bacteriochlorophyll-a in the hydrophobic interior and tryptophan at both protein-solvent interfacial gateways to internal voids. The complexes from mutant Rhodobacter sphaeroides with low resilience against pressure were considered in parallel with the naturally robust complexes of Thermochromatium tepidum. In the former case, a firm correlation was established between the abrupt blue shift of the bacteriochlorophyll-a exciton absorption, a known indicator of the breakage of tertiary structure pigment-protein hydrogen bonds, and the quenching of tryptophan fluorescence, a supposed result of further protein solvation. No such effects were observed in the reference complex. While these data may be naively taken as supporting evidence of the governing role of hydration, the analysis of atomistic model structures of the complexes confirmed the critical part of the structure in the pressure-induced denaturation of the membrane proteins studied.
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Affiliation(s)
- Kõu Timpmann
- Institute of Physics, University of Tartu, W. Ostwald Str. 1, Tartu 50411, Estonia
| | - Juha Matti Linnanto
- Institute of Physics, University of Tartu, W. Ostwald Str. 1, Tartu 50411, Estonia
| | - Dheerendra Yadav
- Institute of Physics, University of Tartu, W. Ostwald Str. 1, Tartu 50411, Estonia
| | - Liina Kangur
- Institute of Physics, University of Tartu, W. Ostwald Str. 1, Tartu 50411, Estonia
| | - Arvi Freiberg
- Institute of Physics, University of Tartu, W. Ostwald Str. 1, Tartu 50411, Estonia.,Estonian Academy of Sciences, Kohtu Str. 6, Tallinn 10130, Estonia
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Timpmann K, Rätsep M, Kangur L, Lehtmets A, Wang-Otomo ZY, Freiberg A. Exciton Origin of Color-Tuning in Ca 2+-Binding Photosynthetic Bacteria. Int J Mol Sci 2021; 22:ijms22147338. [PMID: 34298960 PMCID: PMC8303132 DOI: 10.3390/ijms22147338] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/22/2021] [Revised: 07/02/2021] [Accepted: 07/06/2021] [Indexed: 11/16/2022] Open
Abstract
Flexible color adaptation to available ecological niches is vital for the photosynthetic organisms to thrive. Hence, most purple bacteria living in the shade of green plants and algae apply bacteriochlorophyll a pigments to harvest near infra-red light around 850–875 nm. Exceptions are some Ca2+-containing species fit to utilize much redder quanta. The physical basis of such anomalous absorbance shift equivalent to ~5.5 kT at ambient temperature remains unsettled so far. Here, by applying several sophisticated spectroscopic techniques, we show that the Ca2+ ions bound to the structure of LH1 core light-harvesting pigment–protein complex significantly increase the couplings between the bacteriochlorophyll pigments. We thus establish the Ca-facilitated enhancement of exciton couplings as the main mechanism of the record spectral red-shift. The changes in specific interactions such as pigment–protein hydrogen bonding, although present, turned out to be secondary in this regard. Apart from solving the two-decade-old conundrum, these results complement the list of physical principles applicable for efficient spectral tuning of photo-sensitive molecular nano-systems, native or synthetic.
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Affiliation(s)
- Kõu Timpmann
- Institute of Physics, University of Tartu, W. Ostwald Str. 1, 50411 Tartu, Estonia; (K.T.); (M.R.); (L.K.); (A.L.)
| | - Margus Rätsep
- Institute of Physics, University of Tartu, W. Ostwald Str. 1, 50411 Tartu, Estonia; (K.T.); (M.R.); (L.K.); (A.L.)
| | - Liina Kangur
- Institute of Physics, University of Tartu, W. Ostwald Str. 1, 50411 Tartu, Estonia; (K.T.); (M.R.); (L.K.); (A.L.)
| | - Alexandra Lehtmets
- Institute of Physics, University of Tartu, W. Ostwald Str. 1, 50411 Tartu, Estonia; (K.T.); (M.R.); (L.K.); (A.L.)
| | | | - Arvi Freiberg
- Institute of Physics, University of Tartu, W. Ostwald Str. 1, 50411 Tartu, Estonia; (K.T.); (M.R.); (L.K.); (A.L.)
- Correspondence:
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