LHCII

Ruban, A. V., D. Ress, P. A. A., and P. Horton (1992). Mechanism of pH-dependent dissipation of absorbed excitation energy by photosynthetic membranes. II The relationship between LHCII aggregation and qE in isolated thylakoids. Biochim Biophys Acta 1102 39-44. 

Structure of the Photosystem II Antenna Xanthophylls in LHCII Structure.117... 

Identification of the Chlorophyll Excitation Quencher in Aggregated LHCII.124... 

Neoxanthin Distortion upon Aggregation and Crystallization of LHCII... 

The structure of the major trimeric LHCII complex has been recently obtained at 2.72 A (Figure 7.3) (Liu et al., 2004). It was revealed that each 25kDa protein monomer contains three transmembrane and three amphiphilic a-helixes. In addition, each monomer binds 14 chlorophyll (8 Chi a and 6 Chi b) and 4 xanthophyll molecules 1 neoxanthin, 2 luteins, and 1 violaxanthin. The first three xanthophylls are situated close to the integral helixes and are tightly bound to some amino acids by hydrogen bonds to hydroxyl oxygen atoms and van der Waals interactions to chlorophylls, and hydrophobic amino acids such as tryptophan and phenylalanine. 

FIGURE 7.3 Structure of PSII membranes, macrocomplexes and LHCII antenna. Left from the top electron microscopy of grana stacks, PSII macrocomplexes, LHCII trimers, and LHCII oligomers. Right from the top Atomic structure of LHCII monomer (I and II are side and top views). Bottom part displays LHCII... 

The identification of xanthophylls in vivo is a complex task and should be approached gradually with the increasing complexity of the sample. In the case of the antenna xanthophylls, the simplest sample is the isolated LHCII complex. Even here four xanthophylls are present, each having at least three major absorption transitions, 0-0, 0-1, and 0-2 (Figure 7.4). Heterogeneity in the xanthophyll environment and overlap with the chlorophyll absorption add additional complexity to the identification task. No single spectroscopic method seems suitable to resolve the overlapping spectra. However, the combination of two spectroscopic techniques, low-temperature absorption and resonance Raman spectroscopy, has proved to be fruitful (Ruban et al., 2001 Robert et al., 2004). 

Resonance Raman spectra of all four LHCII xanthophylls reveal differences in the v, frequencies, which normally depends upon the conjugation number (Heyde et al 1971 Rimai et al 1973). In addition, the neoxanthin transition is further upshifted reflecting the m-conformation. The v, region of this xanthophyll possesses additional bands at 1120,1132, and 1203 cm-1 characteristic for the 9-cis configuration (Hu et al., 1997). The v3 band frequency also differs in these xanthophylls. Finally, v4 is small and featureless in all isolated pigments. 

IDENTIFICATION OF XANTHOPHYLLS ASSOCIATED WITH THE TRANSMEMBRANE HELIXES OF LHCII ANTENNA COMPLEX ... 

Neoxanthin and the two lutein molecules have close associations with three transmembrane helixes, A, B, and C, forming three chlorophyll-xanthophyll-protein domains (Figure 7.5). Considering the structure of LHCII complex in terms of domains is useful for understanding how the antenna system works, and the functions of the different xanthophylls. Biochemical evidence suggests that these xanthophylls have a much stronger affinity of binding to LHCII in comparison to violaxanthin... 

FIGURE 7.5 Structural domains of LHCII xanthophylls. Aromatic amino acids tyrosine in the neoxanthin domain and tryptophan and phenylalanine in the violaxanthin domain are labeled as Y, W, and F, respectively. 

FIGURE 7.7 (a) Structure of the LHCII trimer showing lutein 2 from the monomer 1 (monl) interacting with... 

FIGURE 7.8 v4 resonance Raman spectra of all four LHCII xanthophylls. 

The same bands were resolved in the resonance Raman spectra for the PSII membranes (Ruban et al., unpublished). Therefore, this method, for example, can be used to assess whether the LHCII trimers are intact in vivo at various physiological conditions. 

Various spectroscopic approaches applied to the 510 nm transition indicate an unusual environment for the redshifted lutein (Figures 7.5 and 7.7a). Interaction with the Chi a603 could force lutein 2 molecule to adopt a twisted configuration. In addition, strong interaction with a number of aromatic residues, in particular tryptophan and phenylalanine, which possess relatively large surface areas, could further promote this distortion. It is reasonable to assume that the energy required to produce this distortion comes from the forces involved in the stabilization of LHCII trimers. 

A close analysis of the trimers order in the crystal revealed that the exposed part of neoxanthin molecule is completely free from interactions with any protein or pigment components (Pascal et al., 2005). In addition, an examination of the neoxanthin configuration, taken from the structure of LHCII, points toward strong distortion of the d.v-end of the molecule (Figure 7.9). This fact suggests that the twist most likely occurs within the protein interior, implying that some movement in the LHCII monomer must take place during the transition into dissipative state. Apparently, this movement affects not only lutein 1, as previously discussed, but also neoxanthin. 

It has been important to determine if the neoxanthin distortion signature could be detected during the nonphotochemical quenching in vivo. Resonance Raman measurements on leaves and chlo-roplasts of various Arabidopsis mutants have revealed a small increase in the 950 cm 1 region. The relationship between the amplitude of this transition and the amount of NPQ suggests that the LHCII aggregation may be the sole cause of the protective chlorophyll fluorescence quenching in vivo (Ruban et al., 2007). 

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