THE CRYSTAL-STRUCTURE OF THE LIGHT-HARVESTING COMPLEX-II (B800-850) FROM RHODOSPIRILLUM-MOLISCHIANUM

Background: The light-harvesting complexes II (LH-2s) are integral mem brane proteins that form ring-like structures, oligomers of alpha beta -heterodimers, in the photosynthetic membranes of purple bacteria. The y contain a large number of chromophores organized optimally for light absorption and rapid light energy migration. Recently, the structure of the nonameric LH-2 of Rhodopseudomonas acidophila has been determin ed; we report here the crystal structure of the octameric LH-2 from Rh odospirillum molischianum. The unveiling of similarities and differenc es in the architecture of these proteins may provide valuable insight into the efficient energy transfer mechanisms of bacterial photosynthe sis. Results: The crystal structure of LH-2 from Rs, molischianum has been determined by molecular replacement at 2.4 Angstrom resolution us ing X-ray diffraction, The crystal structure displays two concentric c ylinders of sixteen membrane-spanning helical subunits, containing two rings of bacteriochlorophyll-a (BChl-a) molecules. One ring comprises sixteen B850 BChl-as perpendicular to the membrane plane and the othe r eight B800 BChl-as that are nearly parallel to the membrane plane; e ight membrane-spanning lycopenes (the major carotenoid in this complex ) stretch out between the B800 and B850 BChl-as. The B800 BChl-as exhi bit a different ligation from that of Rps. acidophila (aspartate is th e Mg ligand as opposed to formyl-methionine in Rps. acidophila). Concl usions: The light-harvesting complexes from different bacteria assume various ring sizes. In LH-2 of Rs. molischianum, the Q(y) transition d ipole moments of neighboring B850 and B800 BChl-as are nearly parallel to each other, that is, they are optimally aligned for Forster excito n transfer. Dexter energy transfer between these chlorophylls is also possible through interactions mediated by lycopenes and B850 BChl-a ph ytyl tails; the B800 BChl-a and one of the two B850 BChl-as associated with each heterodimeric unit are in van der Waals distance to a lycop ene, such that singlet and triplet energy transfer between lycopene an d the BChl-as can occur by the Dexter mechanism. The ring structure of the B850 BChl-as is optimal for light energy transfer in that it samp les all spatial absorption and emission characteristics and places all oscillator strength into energetically low lying, thermally accessibl e exciton states.