X-ray Structure of the High-Salt Form of the Peridinin-Chlorophyll <i>a</i>-Protein from the Dinoflagellate <i>Amphidinium carterae</i>: Modulation of the Spectral Properties of Pigments by the Protein Environment

Light-harvesting complexes have evolved into very different structures but fulfill the same function, efficient harvesting of solar energy. In these complexes, pigments are fine-tuned and properly arranged to gather incoming photons. In the photosynthetic dinoflagellate <i>Amphidinium carterae</i>, two variants of the soluble light-harvesting complex PCP have been found [main form PCP (MFPCP) and high-salt PCP (HSPCP)], which show small variations in their pigment arrangement and tuning mechanisms. This feature makes them ideal models for studying pigment−protein interactions. Here we present the X-ray structure of the monomeric HSPCP determined at 2.1 Å resolution and compare it to the structure of trimeric MFPCP. Despite the high degree of structural similarity (rmsd C<sub>α</sub>−C<sub>α</sub> of 1.89 Å), the sequence variations lead to a changed overall pigment composition which includes the loss of two carotenoid molecules and a dramatic rearrangement of the chlorophyll phytol chains and of internal lipid molecules. On the basis of a detailed structural comparison, we favor a macrocycle geometry distortion of the chlorophylls rather than an electrostatic effect to explain energetic splitting of the chlorophyll <i>a</i> Q<sub>y</sub> bands [Ilagan, R. P. (2006) <i>Biochemistry 45</i>, 14052−14063]. Our analysis supports their assignment of peridinin 611* as the single blue-shifted peridinin in HSPCP but also highlights another electrostatic feature due to glutamate 202 which could add to the observed binding site asymmetry of the 611*/621* peridinin pair.