Calcium Ions Are Involved in the Unusual Red-Shift of the Light-Harvesting 1 Qy Transition of the Core Complex in Thermophilic Purple Sulfur Bacterium Thermochromatium tepidum

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論文(Article)
Thermophilic purple sulfur bacterium, Thermochromatium tepidum, can grow at temperatures up to 58 °C and exhibits an unusual Qy absorption at 915 nm for the core light-harvesting complex (LH1), about 35 nm red-shift from those of its mesophilic counterparts. We demonstrate in this study, using a highly purified LH1-reaction center complex, that the LH1 Qy transition is strongly dependent on metal cations and Ca2+ is involved in the unusual red-shift. Removal of the Ca2+ resulted in formation of a species with the LH1 Qy absorption at 880 nm, and addition of the Ca2+ to the 880-nm species recovered the native 915-nm form. Interchange between the two forms is fully reversible. Based on spectroscopic and isothermal titration calorimetry analyses, the Ca2+-binding to the LH1 complex was estimated to occur in a stoichiometric ratio of Ca2+/αβ -subunit = 1:1 and the binding constant was in 105 M-1 order of magnitude, which is comparable with those for EF-hand Ca2+-binding proteins. Despite the high affinity, conformational changes in the LH1 complex upon Ca2+-binding were small and occurred slowly with a typical time constant of about 6 minutes. Replacement of the Ca2+ with other metal cations caused blueshifts of the Qy bands depending on the property of the cations, indicating that the binding site is highly selective. Based on the amino acid sequences of the LH1 complex, possible Ca2+-binding sites are proposed, which consists of several acidic amino acid residues near the membrane interfaces of the Cterminal region of the α-polypeptide and the Nterminal region of the β-polypeptide.