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 [in Japanese]
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Thermophilic purple sulfur bacterium,Thermochromatium tepidum, can grow attemperatures up to 58 °C and exhibits anunusual Qy absorption at 915 nm for the corelight-harvesting complex (LH1), about 35 nmred-shift from those of its mesophiliccounterparts. We demonstrate in this study,using a highly purified LH1-reaction centercomplex, that the LH1 Qy transition is stronglydependent on metal cations and Ca2+ isinvolved in the unusual red-shift. Removal ofthe Ca2+ resulted in formation of a species withthe LH1 Qy absorption at 880 nm, and additionof the Ca2+ to the 880-nm species recovered thenative 915-nm form. Interchange between thetwo forms is fully reversible. Based onspectroscopic and isothermal titrationcalorimetry analyses, the Ca2+-binding to theLH1 complex was estimated to occur in astoichiometric ratio of Ca2+/αβ -subunit = 1:1and the binding constant was in 105 M-1 order ofmagnitude, which is comparable with those forEF-hand Ca2+-binding proteins. Despite thehigh affinity, conformational changes in theLH1 complex upon Ca2+-binding were smalland occurred slowly with a typical timeconstant of about 6 minutes. Replacement ofthe Ca2+ with other metal cations caused blueshiftsof the Qy bands depending on theproperty of the cations, indicating that thebinding site is highly selective. Based on theamino acid sequences of the LH1 complex,possible Ca2+-binding sites are proposed, whichconsists of several acidic amino acid residuesnear the membrane interfaces of the Cterminalregion of the α-polypeptide and the Nterminalregion of the β-polypeptide.
- The Journal of Biological Chemistry
The Journal of Biological Chemistry (283), 13867-13873, 2008-05-16
American Society for Biochemistry and Molecular Biology