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

Yukihiro Kimura, Yu Hirano, Long Jiang Yu, Hiroaki Suzuki, Masayuki Kobayashi, Zheng Yu Wang

Research output: Contribution to journalArticlepeer-review

48 Citations (Scopus)

Abstract

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), an ∼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 ∼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 α-polypeptide and the N-terminal region of the β-polypeptide.

Original languageEnglish
Pages (from-to)13867-13873
Number of pages7
JournalJournal of Biological Chemistry
Volume283
Issue number20
DOIs
Publication statusPublished - May 16 2008

ASJC Scopus subject areas

  • Biochemistry
  • Molecular Biology
  • Cell Biology

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