FIG. 1. Schematic representation of the photosynthetic apparatus in the intracytoplasmic membrane of purple bacteria. The RC (red) is surrounded by the light-harvesting complex I (LH-I, green) to form the LH-I-RC complex, which is surrounded by multiple light-harvesting complexes LH-II (green), forming altogether the PSU. Photons are absorbed by the light-harvesting complexes and excitation is transferred to the RC initiating a charge (electron-hole) separation. The RC binds quinone QB, reduces it to hydroquinone QBH2, and releases the latter. QBH2 is oxidized by the bci complex, which uses the exothermic reaction to pump protons across the membrane; electrons are shuttled back to the RC by the cytochrome c2 complex (blue) from the ubiquinone-cytochrome bc1 complex (yellow). The electron transfer across the membrane produces a large proton gradient that drives the synthesis of ATP from ADP by the ATPase (orange). Electron flow is represented in blue, proton flow in red, and quinone flow, likely confined to the intramembrane space, in black.
chromophores in the photosynthetic membrane and opens a door to the study of excitation transfer in the PSU based on a priori principles.
LH-II. The structure of LH-II from Rs. molischianum had been determined to 2.4 Å resolution (19) and is shown in Fig. 3a. The complex is an octameric aggregate of αβ-heterodimers; the latter contains a pair of short peptides (α- and β-apoproteins) noncovalently binding three BChl a molecules and one lycopene (a specific type of carotenoid). Presumably, there exists a second lycopene for each αβ-heterodimer. The electron density map indeed contains a stretch of assignable density, but the stretch is not long enough to positively resolve the entire lycopene (19). Two concentric cylinders of α-helices, with the α-apoproteins inside and the β-apoproteins outside, form a scaffold for BChls and lycopenes. Fig. 3b depicts the 24 BChl molecules and 8 lycopene molecules in LH-II with all other components stripped away. Sixteen B850 BChl molecules form a continuous overlapping ring of 23 Å radius (based on central Mg atoms of BChls) with each BChl oriented perpendicular to the membrane plane. The Mg–Mg distance
FIG. 2. Energy levels of the electronic excitations in the PSU of BChl a containing purple bacteria. The diagram illustrates a funneling of excitation energy toward the photosynthetic RC. The dashed lines indicate (vertical) intracomplex excitation transfer, and the solid lines (diagonal) indicate intercomplex excitation transfer. LH-I exists in all purple bacteria; LH-II exists in most species; LH-III arises in certain species only.
FIG. 3. The octameric LH-II complex from Rs. molischianum (19). (a) The α-helical segments are represented as cylinders with the α-apoproteins (inside) in blue and the β-apoprotein (outside) in magenta. The BChl molecules are in green with phytyl tails truncated for clarity. The lycopenes are in yellow, (b) Arrangement of chromophores with BChls represented as squares, and with carotenoids (lycopenes) in a licorice representation. Bars connected with the BChls represent the Qy transition dipole moments as defined by the vector connecting the N atom of pyrrol I and the N atom of pyrrol III (22). Representative distances between central Mg atoms of B800 BChl and B850 BChl are given in Å. The B850 BChls bound to the α-apoprotein and the β-apoprotein are denoted as BS50a and B850b, respectively; BChl B850a′ is bound to the (left) neighboring heterodimer.
between neighboring B850a and B850b BChls is 9.2 Å (within an αβ-heterodimer) and between B850a′ and B850b is 8.9 Å (between heterodimers). Eight B800 BChls, forming another ring of 28 Å radius, are arranged with their tetrapyrrol rings nearly parallel to the membrane plane and exhibit a Mg–Mg distance of 22 Å between neighboring BChls, i.e., the BChls are coupled only weakly. The ligation sites for the B850 BChls are α-His-34 and β-His-35, and the B800 BChls ligate to α-Asp-6. Eight lycopene molecules span the transmembrane region: each makes contact with B800 BChl and the B850a BChl.
It is remarkable that LH-II results from the self-aggregation of a large number of identical, noncovalently bonded transmembrane helices, BChls, and carotenoids. With its simple, symmetric architecture, LH-II constitutes an ideal model system for studying aggregate formation and adhesive interactions of proteins. Mechanical models reveal perfect self-complementarity of the αβ-heterodimers that interlock with each other to form a circular aggregate (23).