In photosynthesis, the light-harvesting antennae are very efficient at absorbing solar energy and transferring the excitation energy toward the reaction center, the place where the input energy is utilized to trigger the charge separation of a special pair of chlorophyll molecules. Light- harvesting complexes are associated at the periphery of Photosystem II of higher plants and consist of four homologous members, namely LHC-II, CP29, CP26, and CP24 (for structure of the photosystem and its supercomplexes see also Chapters 4 and 6). Among them, the LHC-II is the most abundant in chloroplasts and is known as the major light-harvesting complex II; the other three have been termed minor light-harvesting complexes II, or chlorophyll-binding proteins (CPs- , because of their low abundance. The LHC-II exists as a homo- or hetero-trimer formed by the products of three highly homologous nuclear genes, Lhcb1, Lhcb2, and Lhcb3, in unequal stoichiometries. The molecular weight of the apoprotein varies from 24 000 to 29 000 daltons. According to early biochemical data, each LHC - II monomer binds 13-15 Chl a and Chl b molecules , 3-4 carotenoids [2, 3], and 1 tightly bound phospholipid
LHC-II has been found to be a multifunctional complex in the thylakoid membrane. Firstly, it serves as efficient light-harvesting machinery, powering the pho-tosynthetic reactions. Under high-light conditions, when the input energy becomes excessive, LHC - II is able to switch from the efficient light-harvesting state to a dissipative state and safely dissipate the excess energy as heat. Through this act, it provides a photoprotective mechanism for the plants [5-7]. LHC-II also has a role in regulating the distribution of excitation energy to photosystems II and I through the reversible phosphorylation of the its N-terminus . In addition, the stacking of thylakoid membranes in chloroplast grana is largely related to the close interactions between LHC-IIs in two adjacent layers of the membrane .
In 1994, the structure of pea LHC-II was determined by electron crystallography at 3.4A resolution, parallel to the membrane plane, and at approximately 4.9 A
perpendicular to this plane . Some characteristic features of the LHC-II structure, including three long transmembrane a -helices (helix A, B, C) and a short amphiphilic helix (helix D) at the luminal surface, were revealed for the first time in this model. Twelve chlorophylls and two carotenoids have been positioned with rough coordinates and orientations. Using the electron crystallographic model, the chlorophyll identities and orientations in LHC- II have been intensively studied through site-directed mutagenesis, in vitro reconstitution, and spectroscopic studies [11-13]. Some significant differences in the chlorophyll assignment stem from different research groups being present during the investigation process . For a better understanding of the basic functional mechanism of LHC-II, it is very important to solve the complete structure of LHC-II at a resolution that allows for an unambiguous determination of the orientation of the transition dipole moments of chlorophylls as well as their identities (Chl a or Chl b).
Toward this end, the X-ray crystallographic structure of spinach LHC-II has been determined at 2.72À resolution . With a high-quality dataset ofstructure factor amplitude and an accurate phase set, the electron densities for the oxygen atom of the C7 - formyl group of Chl b, as well as the side chains and head groups of each individual chlorophyll molecules, were observed for the first time, leading to the final precise assignment of chlorophyll identities and orientations. Because of this, many new structural features of LHC-II have been unraveled at atomic detail. About one year after the reporting of the assignments, the X-ray structure of pea LHC-II was solved at 2.5À resolution from a different crystal form, by the Kuhlbrandt group . Superposition of the two structures yielded extremely small root mean square deviation of a-carbon atoms at 0.35 À, indicating the high degree of similarity between the two species.
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