ATR-FTIR Detection of Metal-Sensitive Structural Changes in the Light-Harvesting 1 Reaction Center Complex from the Thermophilic Purple Sulfur Bacterium Thermochromatium tepidum

Thermochromatium tepidum grows at the highest temperature among purple bacteria, and the light-harvesting 1 reaction center (LIU-RC) complex enhances the thermal stability by utilizing Ca2+, although the molecular mechanism has yet to be resolved. Here, we applied perfusion-induced attenuated total reflection (ATR) Fourier transform infrared (FTIR) spectroscopy to highly purified LH1-RC complexes from Tch. tepidum and detected for the first time metal-sensitive fine structural changes involved in the enhanced thermal stability of this complex. The Tch. tepidum LH1-RC complex exhibited Sr2/Ca2+ ATR FTIR difference bands that reflect 1800 1760 1660 1560 changes in the polypeptide backbones and amino acid residues upon the replacement of native Ca2+ with Sr2+. The difference bands also appeared in the following Ca27Sr(2+) difference spectra with almost identical intensities but inverse signs, demonstrating that the structural changes induced by the metal exchange are fully reversible. In contrast, these ATR FTIR signals were faintly detected in the mesophilic counterpart Allochromatium vinosum. A comparative analysis using LH1 complexes lacking the RCs strongly indicated that the metal-sensitive bands originate from polypeptide backbones and amino acid residues near the putative Ca2+-binding site at the C-terminal region of the Tch. tepidum LH1 complexes. Structural changes induced by Sr2+ and Ba2+ substitutions were essentially identical. However, Cd2+ substitution exhibited unique structural modifications, which may be responsible for the severely deteriorated thermal stability of Cd2+-substituted complexes. Possible assignments for the present ATR FTIR signals and their relation with the molecular mechanism of enhancing the thermal stability of Tch. tepidum LH1-RC proteins are discussed on the basis of the recent structural information on the Ca2+- binding site.