Xanthophylls as modulators of membrane protein function.

This review discusses the structural aspect of the role of photosynthetic antenna xanthophylls. It argues that xanthophyll hydrophobicity/polarity could explain the reason for xanthophyll variety and help to understand their recently emerging function--control of membrane organization and the work of membrane proteins. The structure of a xanthophyll molecule is discussed in relation to other amphiphilic compounds like lipids, detergents, etc. Xanthophyll composition of membrane proteins, the role of their variety in protein function are discussed using as an example for the major light harvesting antenna complex of photosystem II, LHCII, from higher plants. A new empirical parameter, hydrophobicity parameter (H-parameter), has been introduced as an effective measure of the hydrophobicity of the xanthophyll complement of LHCII from different xanthophyll biosynthesis mutants of Arabidopsis. Photosystem II quantum efficiency was found to correlate well with the H-parameter of LHCII xanthophylls. PSII down-regulation by non-photochemical chlorophyll fluorescence quenching, NPQ, had optimum corresponding to the wild-type xanthophyll composition, where lutein occupies intrinsic sites, L1 and L2. Xanthophyll polarity/hydrophobicity alteration by the activity of the xanthophyll cycle explains the allosteric character of NPQ regulation, memory of illumination history and the hysteretic nature of the relationship between the triggering factor, ΔpH, and the energy dissipation process.