posted on 2021-10-26, 20:07authored byPu Qian, David J. K. Swainsbury, Tristan I. Croll, Pablo Castro-Hartmann, Giorgio Divitini, Kasim Sader, C. Neil Hunter
Light-harvesting 2 (LH2) antenna
complexes augment the collection
of solar energy in many phototrophic bacteria. Despite its frequent
role as a model for such complexes, there has been no three-dimensional
(3D) structure available for the LH2 from the purple phototroph Rhodobacter sphaeroides. We used cryo-electron microscopy
(cryo-EM) to determine the 2.1 Å resolution structure of this
LH2 antenna, which is a cylindrical assembly of nine αβ
heterodimer subunits, each of which binds three bacteriochlorophyll a (BChl) molecules and one carotenoid. The high resolution
of this structure reveals all of the interpigment and pigment–protein
interactions that promote the assembly and energy-transfer properties
of this complex. Near the cytoplasmic face of the complex there is
a ring of nine BChls, which absorb maximally at 800 nm and are designated
as B800; each B800 is coordinated by the N-terminal carboxymethionine
of LH2-α, part of a network of interactions with nearby residues
on both LH2-α and LH2-β and with the carotenoid. Nine
carotenoids, which are spheroidene in the strain we analyzed, snake
through the complex, traversing the membrane and interacting with
a ring of 18 BChls situated toward the periplasmic side of the complex.
Hydrogen bonds with C-terminal aromatic residues modify the absorption
of these pigments, which are red-shifted to 850 nm. Overlaps between
the macrocycles of the B850 BChls ensure rapid transfer of excitation
energy around this ring of pigments, which act as the donors of energy
to neighboring LH2 and reaction center light-harvesting 1 (RC–LH1)
complexes.