The photophysical mechanism of NPQ involves a change of the pigment configurations, creating an check details energy dissipation pathway via one of the pigments. The exact mechanism is under much debate and
several models have been proposed, based on intra- or intermolecular conformational changes and/or cofactor exchange (Holzwarth et al. 2009; Ruban et al. 2007; Ahn et al. 2008; Standfuss et al. 2005; Holt et al. 2005). In vitro, fluorescence quenching occurs upon aggregation of the LHCII complexes, with spectroscopic signatures similar to the (Wawrzyniak et al. 2008) state in leaves and chloroplasts, suggesting that they underlie very similar photophysical mechanisms. In particular, Resonance Raman shows a twist of the neoxanthin (Neo) carotenoid upon quenching in vivo as well as in vitro (Ruban et al. 2007), demonstrating that conformational changes indeed occur. For the major light-harvesting complex II from plants (LHCII), conformational switching was observed without self-aggregation of LHCII proteins entrapped in gels (Ilioaia et al. 2008) and of LHCII trimer complexes studied by single-molecule Trichostatin A price fluorescence microscopy (Kruger et al. 2010). This ASK inhibitor suggests that the individual antenna complexes have a built-in capacity to
switch between different functional conformational states, triggered by the protein local environment that can shift the dynamic equilibrium between the light-harvesting and the NPQ states. A shift of a dynamic equilibrium has been observed before with MAS NMR, e.g. for 7-helix membrane proteins Interleukin-2 receptor in relation to signal transduction, and NMR is a
good method to analyze the relation between structure and the triggering of function for such processes (Ratnala et al. 2007; Etzkorn et al. 2007). Despite the availability of two high-resolution LHCII crystal structures (Standfuss et al. 2005; Liu et al. 2004), the more subtle conformational dynamics related to NPQ remain to be resolved. In the LH2 NMR model it was shown that by using the X-ray structure of LH2, the NMR data could predict different aspects of conformational strain in the form of localized electronic perturbations, on the level of (1) the protein backbone, (2) the selective pigment-coordinating sites, and (3) the protein-bound chromophores. Recently, the first NMR experiments were performed on the LHCII trimer complexes of the green alga Chlamydomonas reinhardtii, which have a high degree of homology with the LHCII complexes of higher plants (Pandit et al. 2011b). The dispersion of the NMR signals is good, and possible conformational changes will be observable already in uniformly isotope-labeled samples. The NMR samples can be prepared in aggregated or detergent-solubilized conditions, modulating the photophysical state of the LHCII in vitro.
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