light-harvesting complex of photosystem II (LHC-II)
Spinacia oleracea (Spinach)
Light harvesting is the first step in photosynthesis. In green plants, the function of harvesting solar energy is performed by a series of light-harvesting complexes in the thylakoid membrane of chloroplasts. The major light-harvesting complex of photosystem II (LHC-II) is the most abundant membrane protein in chloroplasts. LHC-II serves as the principal solar energy collector in the photosynthesis of green plants and probably also functions in photoprotection under high-light conditions. LHC-II exisits as a trimer under physiological conditions and every monomer binds chlorophyll a (Chla) and chlorophyll b (Chlb) molecules, carotenoids and a phospholipid.
Here the first X-ray structure of LHC-II in an icosahedral proteoliposome assembly is shown at atomic resolution. The 14 chlorophylls in each monomer are clearly distinguished as eight Chla and six Chlb molecules. The chlorophylls are vertically distributed into two layers within the membrane, each layer close to the stromal or lumenal surface. In the trimeric LHC-II, all chlorophylls from the stromal layer are arranged into two circular rings, an inner and outer ring. Chlorophylls at the inner ring located in the core region of a trimer are thought to have an important role in intermonomeric energy transfer, whereas chlorophylls at the outer ring arranged in a mosaic pattern would absorb incident light energy from all directions. The chlorophylls are found to be arranged for efficient light harvesting. Four carotenoid-binding sites per monomer have been observed. It is suggested that the xanthopyhll-cycle carotenoid at the monomer-monomer interface may be involved in quenching excessive energy, judging from the arrangement and orientation of the xanthopyhll-cycle carotenoid.
Protein Data Bank (PDB)
Liu, Z. Yan, H. Wang, K. Kuang, T. Zhang, J. Gui, L. An, X. Chang, W.; "Crystal structure of spinach major light-harvesting complex at 2.72 A resolution"; Nature; (2004) 428:287-292 PubMed:15029188.
author: Aki Nagata