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

Abstract

Thermophilic purple sulfur bacterium, Thermochromatium tepidum, can grow at temperatures up to 58 degrees C and exhibits an unusual Qy absorption at 915 nm for the core light-harvesting complex (LH1), an approximately 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+/alphabeta-subunit = 1:1 and the binding constant was in 10(5) 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 approximately 6 min. Replacement of the Ca2+ with other metal cations caused blue shifts 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 that consist of several acidic amino acid residues near the membrane interfaces of the C-terminal region of the alpha-polypeptide and the N-terminal region of the beta-polypeptide.

Puf operon sequences and inferred structures of light-harvesting complexes of three closely related Chromatiaceae exhibiting different absorption characteristics

“…tepidum that also exhibits a redshifted Q y absorption maximum at 915 nm (Madigan 2003). The formation of a Ca 2+ -binding site in this region of the polypeptide is thought to be responsible for the absorption shift in this bacterium (Kimura et al 2008). Although pufA 1 and pufA 2 of strain 970 display this deletion as well, the Q y redshift cannot be attributed solely to this region, because the same sequence pattern was also determined for the two Trv.…”

Examination of the putative Ca2+-binding site in the light-harvesting complex 1 of thermophilic purple sulfur bacterium Thermochromatium tepidum

“…Wang (&) Faculty of Science, Ibaraki University, Mito 310-8512, Japan e-mail: wang@mx.ibaraki.ac.jp occur in a stoichiometric ratio of Ca 2? /ab-subunit = 1:1 and the binding constant was in 10 5 M -1 order of magnitude (Kimura et al 2008). Despite the high affinity, conformational changes of the LH1 complex upon Ca 2?…”

“…In a series of investigations on the Tch. tepidum LH1 complex (Wang et al 2003;Suzuki et al 2007;Kimura et al 2008;2009), we found that the LH1 is highly stable at room temperature when copurified with reaction center (RC) in a LH1-RC complex form, and calcium ions are involved in both the enhanced thermal stability and the large red shift of the LH1 Q y transition. Removal of the Ca 2?…”

“…Two possible Ca 2? -binding sites were proposed on the basis of the amino acid sequences of the LH1 complex (Kimura et al 2008). One consists of several acidic amino acid residues near the membrane surface of the C-terminal region of the a-polypeptide and another is in the N-terminal region of the b-polypeptide as shown in Fig.…”

Spectral properties of LH2 exhibit very similar even when heterologously express LH2 with <i>β</i>-subunit fusion protein in <i>Rhodobacter</i> <i>sphaeroides</i>

“…Similarly, the LH1 αTrp43 was proposed to be located near the putative binding site for BChl molecule [19]. Dissimilar to the purple non-sulfur bacteria, calcium ions were demonstrated to be involved in the unusual red shift of the LH1 Q y transition of the core complex in Thermochromatium tepidum [20]. Hydrogen bonds were intensively studied in the binding of BChl molecules to RC [21].…”