The 2.4 Å cryo-EM structure of a heptameric light-harvesting 2 complex reveals two carotenoid energy transfer pathways

We report the 2.4 angstrom ngstrom resolution structure of the light-harvesting 2 (LH2) complex from Marichromatium (Mch.) purpuratum determined by cryogenic electron microscopy. The structure contains a heptameric ring that is unique among all known LH2 structures, explaining the unusual spectroscopic properties of this bacterial antenna complex. We identify two sets of distinct carotenoids in the structure and describe a network of energy transfer pathways from the carotenoids to bacteriochlorophyll a molecules. The geometry imposed by the heptameric ring controls the resonant coupling of the long-wavelength energy absorption band. Together, these details reveal key aspects of the assembly and oligomeric form of purple bacterial LH2 complexes that were previously inaccessible by any technique.

Automated summary

The study reports the structure of the LH2 complex from Marichromatium purpuratum with a resolution of 2.4 angstroms determined by cryogenic electron microscopy. It reveals a heptameric ring unique among all known LH2 structures, controlling the resonant coupling of the long-wavelength energy absorption band, and describes distinct carotenoids and energy transfer pathways within the complex. These details provide key insights into the assembly and oligomeric form of purple bacterial LH2 complexes that were previously inaccessible by any technique.