title: Calcium ions are required for the enhanced thermal stability of the light-harvesting-reaction center core complex from thermophilic purple sulfur bacterium Thermochromatium tepidum.
abstract: Thermochromatium tepidum is a thermophilic purple sulfur photosynthetic bacterium collected from the Mammoth Hot Springs, Yellowstone National Park. A previous study showed that the light-harvesting-reaction center core complex (LH1-RC) purified from this bacterium is highly stable at room temperature (Suzuki, H., Hirano, Y., Kimura, Y., Takaichi, S., Kobayashi, M., Miki, K., and Wang, Z.-Y. (2007) Biochim. Biophys. Acta 1767, 1057-1063). In this work, we demonstrate that thermal stability of the Tch. tepidum LH1-RC is much higher than that of its mesophilic counterparts, and the enhanced thermal stability requires Ca2+ as a cofactor. Removal of the Ca2+ from Tch. tepidum LH1-RC resulted in a complex with the same degree of thermal stability as that of the LH1-RCs purified from mesophilic bacteria. The enhanced thermal stability can be restored by addition of Ca2+ to the Ca2+-depleted LH1-RC, and this process is fully reversible. Interchange of the thermal stability between the two forms is accompanied by a shift of the LH1 Qy transition between 915 nm for the native and 880 nm for the Ca2+-depleted LH1-RC. Differential scanning calorimetry measurements reveal that degradation temperature of the native LH1-RC is 15 degrees C higher and the enthalpy change is about 28% larger than the Ca2+-depleted LH1-RC. Substitution of the Ca2+ with other metal cations caused a decrease in thermal stability of an extent depending on the properties of the cations. These results indicate that Ca2+ ions play a dual role in stabilizing the structure of the pigment-membrane protein complex and in altering its spectroscopic properties, and hence provide insight into the adaptive strategy of this photosynthetic organism to survive in extreme environments using natural resources.
authors.affiliation: Faculty of Science, Ibaraki University, Mito 310-8512, Japan.
authors.forename: Yukihiro
authors.initials: Y
authors.lastname: Kimura
authors.affiliation: Faculty of Science, Ibaraki University, Mito 310-8512, Japan.
authors.forename: Long-Jiang
authors.initials: LJ
authors.lastname: Yu
authors.affiliation: Faculty of Science, Ibaraki University, Mito 310-8512, Japan.
authors.forename: Yu
authors.initials: Y
authors.lastname: Hirano
authors.affiliation: Faculty of Science, Ibaraki University, Mito 310-8512, Japan.
authors.forename: Hiroaki
authors.initials: H
authors.lastname: Suzuki
authors.affiliation: Faculty of Science, Ibaraki University, Mito 310-8512, Japan. Electronic address: wang@mx.ibaraki.ac.jp.
authors.forename: Zheng-Yu
authors.initials: ZY
authors.lastname: Wang
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology*; D002118:Calcium / Q000737:chemistry* / Q000378:metabolism; D002841:Chromatiaceae / Q000201:enzymology*; D003067:Coenzymes / Q000737:chemistry* / Q000378:metabolism; D004795:Enzyme Stability / Q000502:physiology; D006358:Hot Temperature; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry* / Q000378:metabolism
chemical_list: D003067:Coenzymes; D045342:Light-Harvesting Protein Complexes; D002118:Calcium