Metal cations modulate the bacteriochlorophyll-protein interaction in the light-harvesting 1 core complex from Thermochromatium tepidum

The light-harvesting 1 reaction center (LH1-RC) complex from Thermochromatium (Tch.) tepidum exhibits unusual Q(y) absorption by LH1 bacteriochlorophyll-a (BChl-a) molecules at 915 nm, and the transition energy is finely modulated by the binding of metal cations to the LH1 polypeptides. Here, we demonstrate the metal-dependent interactions between BChl-a and the polypeptides within the intact LH1-RC complexes by near-infrared Raman spectroscopy. The wild-type LH1-RC (B915) exhibited Raman bands for the C3-acetyl and C13-keto C=0 stretching modes at 1637 and 1675 cm(-1), respectively. The corresponding bands appeared at 1643 and 1673 cm(-1) when Ca2+ was biosynthetically replaced with Sr2+ (B888) or at 1647 and 1669 cm(-1) the mesophilic counterpart, Allochromatium vinosum. These results indicate the significant difference in the BChl-polypeptide interactions between B915 and B888 and between 8915 and the mesophilic counterpart. The removal of the original metal cations from B915 and B888 resulted in marked band shifts of the C3-acetyl/C13-carbonyl nu C=0 modes to similar to 1645/similar to 1670 cm(-1), supporting a model in which the metal cations are involved in the fine-tuning of the hydrogen bonding between the BChl-a and LH1-polypeptides. Interestingly, the interaction modes were almost identical between the Ca2+-depleted B915 and Sr2+-depleted B888 and between B915 and Ca2+-substituted B888, despite the significant differences in their LH1 Q(y) peak positions and the denaturing temperatures, as revealed by differential scanning calorimetry. These results suggest that not only the BChl-polypeptide interactions but some structural origin may be involved in the unusual Q(y) red-shift and the enhanced thermal stability of the LH1-RC complexes from Tch. tepidum. (C) 2012 Elsevier B.V. All rights reserved.