Calcium ions are required for the enhanced thermal stability of the light-harvesting-reaction center core complex from thermophilic purple sulfur bacterium Thermochromatium tepidum.
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.
Kimura
Yukihiro
Y
Faculty of Science, Ibaraki University, Mito 310-8512, Japan.
Yu
Long-Jiang
LJ
Faculty of Science, Ibaraki University, Mito 310-8512, Japan.
Hirano
Yu
Y
Faculty of Science, Ibaraki University, Mito 310-8512, Japan.
Suzuki
Hiroaki
H
Faculty of Science, Ibaraki University, Mito 310-8512, Japan.
Wang
Zheng-Yu
ZY
Faculty of Science, Ibaraki University, Mito 310-8512, Japan. Electronic address: wang@mx.ibaraki.ac.jp.
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
D003067:Coenzymes; D045342:Light-Harvesting Protein Complexes; D002118:Calcium