Specific Ca²⁺-binding motif in the LH1 complex from photosynthetic bacterium Thermochromatium tepidum as revealed by optical spectroscopy and structural modeling

Native and Ca²⁺-depleted light-harvesting-reaction center core complexes (LH1-RC) from the photosynthetic bacterium Thermochromatium (Tch.) tepidum exhibit maximal LH1-Q_y absorption at 915 and 889 nm, respectively. To understand the structural origins of the spectral variation, we performed spectroscopic and structure modeling investigations. For the 889 nm form of LH1-RC, bacteriochlorophyll a (BChl a) in the native form was found by means of near-infrared Fourier-transform Raman spectroscopy, a higher degree of macrocycle distortion and a stronger hydrogen bond with the β-Trp _-8 residue. SWISS-MODEL structure modeling suggests the presence of a specific coordination motif of Ca²⁺ at the C-terminus of the α-subunit of LH1, while MODELLER reveals the tilt of α- and β-polypeptides with reference to the structural template, as well as a change in the concentric orientation of BChl a molecules, both of which may be connected to the long-wavelength LH1-Q_y absorption of the 915 nm form. The carotenoid spirilloxanthin shows a twisted all-trans configuration in both forms of LH1 as evidenced by the resonance Raman spectroscopic results. With regard to the thermal stability, the 915 nm form was shown by the use of temperature-dependent fluorescence spectroscopy to be approximately 20 K more stable than the 889 nm form, which may be ascribed to the specific Ca ²⁺-binding motif of LH1.