Electronic Properties and Thermal Stability of Soluble Redox Proteins from a Thermophilic Purple Sulfur Photosynthetic Bacterium, <i>Thermochromatium tepidum</i>

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  • Masayuki Kobayashi
    Department of Chemical Science and Engineering, Ariake National College of Technology
  • Takayuki Saito
    Department of Industrial Chemistry, Hachinohe National College of Technology
  • Kiyomichi Takahashi
    Department of Biomolecular Engineering, Graduate School of Engineering, Tohoku University
  • Zheng-Yu Wang
    Department of Biomolecular Engineering, Graduate School of Engineering, Tohoku University
  • Tsunenori Nozawa
    Department of Biomolecular Engineering, Graduate School of Engineering, Tohoku University

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<jats:title>Abstract</jats:title> <jats:p>The electronic properties of soluble redox proteins from a thermophilic purple sulfur photosynthetic bacterium Thermochromatium (T.) tepidum have been studied in terms of electronic absorption, magnetic CD, midpoint redox potential, and electron-transfer behavior. T. tepidum possesses one high potential iron–sulfur protein (HiPIP), one cytochrome c’, and two low potential cytochrome c-552’s. The two low potential cytochrome c-552’s can be separated further into a higher and a lower potential species. The HiPIP has a midpoint redox potential of +340 mV at pH 7. From electron-transfer ability shown by flash experiments, the HiPIP was confirmed to be the possible electron-transfer protein to the reaction center under physiological conditions. The HiPIP is thermally stable up to 60 °C, which is much higher than that of the mesophilic photosynthetic bacterium, Allochromatium (A.) vinosum. Cytochrome c’ has a high-spin protoheme with a redox midpoint potential of +100 mV at pH 7, similar to those of many other photosynthetic bacteria. The molecular weight was determined to be 14147 Da by MALDI-TOF/MS experiments. The higher potential cytochrome c-552’s have hemes c’s with midpoint redox potentials of −15 and +85 mV, and the lower one of −15 mV. The molecular weights for the higher potential cytochrome c-552 and the lower potential c-552 were found to be 20702 and 47757 Da, respectively. The higher potential cytochrome c-552 has an attached flavoprotein with a molecular mass of 42000 Da. The lower potential cytochrome c-552 has been found for the first time in T. tepidum.</jats:p>

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