Relevance of the diastereotopic ligation of magnesium atoms of chlorophylls in the major light-harvesting complex II (LHC II) of green plants

The recent high-resolution crystal structure of LHC II [Liu et al. (2004) Nature 428: 287-292] makes possible an unprecedented insight into the stereochemical features of how chlorophylls (Chl)s are bound. The diastereotopic ligation generates four structurally different pigment types, two Chl a and two Chl b, which are distinguished not only by the groups in the 7-position (methyl in Chl a and formyl in Chl b) but also by the face of the tetrapyrrole to which the fifth magnesium ligand is bound. Within a LHC II monomer, out of the eight Chl a six have a 'normal' alpha-coordination and two are beta-coordinated while out of the six Chl b only one has the 'special' beta- coordination. In Photosystem I where a more meaningful statistical analysis could be made, out of 96 Chl a only 14 are beta-coordinated, again indicating a preference for the 'normal' alpha-coordination [Balaban et al. (2002) Biochim Biophys Acta Bioenerget 1556: 197-207; Oba and Tamiaki (2002a) Photosynth Res 74: 1-10]. Astonishingly, all the special beta-Chls are part of the stromal ring of Chls within the LHC II trimers and occupy key positions for the excitation energy transfer. Sequential energy traps are engineered with one hetero- and three homo-dimers. A careful pairing of carotenoids with the special beta-Chls, which could quench their triplet states efficiently, implies a functional relevance of this diastereotopic ligation.