Abstract
Recombinant light-harvesting complex II (LHCII) proteins with modified carotenoid composition have been obtained by in vitro reconstitution of the Lhcb1 protein overexpressed in bacteria. The monomeric protein possesses three xanthophyll-binding sites. The L1 and L2 sites, localized by electron crystallography in the helix A/helix B cross, have the highest affinity for lutein, but also bind violaxanthin and zeaxanthin with lower affinity. The latter xanthophyll causes disruption of excitation energy transfer. The occupancy of at least one of these sites, probably L1, is essential for protein folding. Neoxanthin is bound to a distinct site (N1) that is highly selective for this species and whose occupancy is not essential for protein folding. Whereas xanthophylls in the L1 and L2 sites interact mainly with chlorophyll a, neoxanthin shows strong interaction with chlorophyll b, inducing the hyperchromic effect of the 652 nm absorption band. This observation explains the recent results of energy transfer from carotenoids to chlorophyll b obtained by femtosecond absorption spectroscopy. Whereas xanthophylls in the L1 and L2 sites are active in photoprotection through chlorophyll-triplet quenching, neoxanthin seems to act mainly in (1)O(2)(*) scavenging.
Highlights
Light energy for the photosynthesis of green plants is collected by an antenna system composed of many homologous proteins belonging to the Lhc multigene family [1]
LHCII1 is the most abundant light-harvesting complex in higher plants. The structure of this complex has been resolved at 3.4 Å by electron microscopy [2] and is formed by three hydrophobic transmembrane helices connected by hydrophilic loops and an amphipathic helix exposed to the luminal surface of the membrane
In the structural model of light-harvesting complex II (LHCII) [2], 2 xanthophyll molecules have been located in the center of the complex, forming an internal cross-brace interacting with helices A and B
Summary
Light energy for the photosynthesis of green plants is collected by an antenna system composed of many homologous proteins belonging to the Lhc multigene family [1]. In the structural model of LHCII [2], 2 xanthophyll molecules have been located in the center of the complex, forming an internal cross-brace interacting with helices A and B. These appear to be crucial for protein stabili-. Carotenoids have at least five different roles in photosynthesis: 1) light harvesting, 2) chlorophyll triplet quenching, 3) singlet oxygen scavenging, 4) excess energy dissipation, and 5) structure stabilization and assembly. Biochemical, spectroscopic, and functional characterization of these recombinant proteins provides evidence for distinct binding sites for lutein and neoxanthin and for strong interaction of carotenoid molecules with both chlorophylls a and b, affecting their spectroscopic properties
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