TCB Publications - Abstract

Juergen Koepke, Xiche Hu, Cornelia Muenke, Klaus Schulten, and Hartmut Michel. The crystal structure of the light harvesting complex II (B800-850) from Rhodospirillum molischianum. Structure, 4:581-597, 1996.

KOEP96 In photosynthesis light is absorbed by light-harvesting antenna complexes (LHs) and its energy is transferred to the photosynthetic reaction center. In purple photosynthetic bacteria and higher plants the LHs are integral membrane protein/pigment complexes. LH-II from the purple bacterium Rhodospirillum molischianum is an octamer of heterodimers, the later consisting of two polypeptides, the $\alpha$ and $\beta$-apoproteins, noncovalently binding three bacteriochlorophyll-a (BChl-a) molecules and at least one lycopene molecule as an additional chromophore. LH-II absorbs light and converts it into a BChl-a exciton, which is then transferred to the photosynthetic reaction center through the core light harvesting complex LH-I. The crystal structure of LH-II from Rhodospirillum molischianum has been determined by molecular replacement at 2.4 Åresolution using X-ray diffraction. The search model for molecular replacement was a computationally modelled octamer of $\alpha\beta$ heterodimer of a nonameric LH-II from Rps. acidophila. The crystal structure displays two concentric cylinders of membrane-spanning helical protein subunits with the $\alpha$-apoprotein at the inner and the $\beta$-apoprotein at the outer side. Sixteen BChl-a molecules absorbing maximally at 846 nm (B850), oriented perpendicular to the plane of the membrane and sandwiched between the helical apoproteins, form a ring of radius 23.0 Å. The other eight BChl-a molecules absorbing maximally at 800nm (B800) situated between the $\beta$-apoproteins and bound through their central Mg atoms to an aspartate ($\alpha$-Asp6), form a concentric ring of radius 28.8 Å. Eight membrane spanning lycopene pigments, held in place through aromatic side groups, stretch out between the B800 and B850 BChl-a's. The light-harvesting complexes from different bacteria assume various ring sizes. In LH-II of Rs. molischianum, the $Q_{y}$ transition dipole moments of neighboring B850 and B800 BChl-a's are nearly parallel to each other, i.e., are optimally aligned for Forster exciton transfer; Dexter energy transfer is possible through B850 BChl-a's are in van der Waals distance to a lycopene, such that singlet and triplet energy transfer between lycopene and the BChl-a's is optimal for light energy transfer in that it samples all spatial absorption and emission characteristics as well as places all oscillator strength into energetically low lying, thermally accessible exciton states.

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