Towards a structural description of the mechanism of state transitions in photosynthesis

John F. Allen
School of Biological and Chemical Sciences
Queen Mary, University of London
Mile End Road
E1 4NS

In  chloroplasts, subcellular organelles of green plants and algae, photosynthetic pigments are mostly bound to apoproteins to form membrane-intrinsic light-harvesting complexes, most notably the chloroplast light-harvesting complex II (LHC II). The family of complexes to which LHC II belongs is thought to account for approximately 50 % of the chlorophyll molecules in nature and therefore to be the primary light-absorbing pigment-protein complex sustaining life on Earth. LHC II was the second protein (after bacteriorhodopsin) for which important structural information was provided by electron crystallography, that is, by collection of electron diffraction data from 2-D crystals at liquid helium temperatures, using a modified high-resolution electron microscope. However, the maximum resolution of this structure (from pea leaf LHC II, 4.7 Å in only one plane of projection), leaves doubt about key features of the complex, including whole surface-exposed loops and the identity and location of the chorophyll molecules (even chorophylls a and b cannot be distinguished). A more complete structure, from X-ray diffraction of icosahedral, 3-D crystals (from spinach), is at 2.7 Å resolution [1]. The new X-ray structure reveals 8 Chl a and 6 Chl b molecules per monomer (the complex is trimeric) and a wealth of detail concerning chlorophyll ligands and subunit interactions. However, neither structure reveals any information on the site of phosphorylation, which regulates the essential function of redistribution of excitation energy between the two reaction centres (see below). This site is at the N-terminus (Thr 5 in pea; Thr 3 in spinach) where even the 3-D crystals seem to be disordered (to Ser 14). Furthermore, the structural effect of phosphosphorylation is certainly global [2] but completely unresolved; no crystal structure exists for the phosphorylated form of LHC II. Prospects will be reviewed for a complete structural description of state transitions in oxygenic photosynthesis [3] 

1. Liu, Z., Yan, H. et al. (2004). Crystal structure of spinach major light-harvesting complex at 2.72 Å resolution. Nature 428, 287-92.

2. Nilsson, A., Stys, D., Drakenberg, T., Spangfort, M. D., Forsén, S. and Allen, J. F. (1997) J. Biol. Chem. 272, 18350-18357

3. Allen, J.F. (2003). Botany. State transitions--a question of balance. Science 299, 1530-1532

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