Determination of the stoichiometry and strength of binding of xanthophyllsto the photosystem II light harvesting complexes

Xanthophylls have a crucial role in the structure and function of the light harvesting complexes of photosystem II (LHCII) in plants. The binding of x anthophylls to LHCII has been investigated, particularly with respect to th e xanthophyll cycle carotenoids violaxanthin and zeaxanthin, It was found t hat most of the violaxanthin pool was loosely bound to the major complex an d could be removed by mild detergent treatment. Gentle solubilization of ph otosystem II particles and thylakoids allowed the isolation of complexes, i ncluding a newly described oligomeric preparation, enriched in trimers, tha t retained all of the in vivo violaxanthin pool, It was estimated that each LHCII monomer can bind at least one violaxanthin, The extent to which diff erent pigments can be removed from LHCII indicated that the relative streng th of binding was chlorophyll b > neoxanthin > chlorophyll a > lutein > zea xanthin > violaxanthin. The xanthophyll binding sites are of two types: int ernal sites binding lutein and peripheral sites binding neoxanthin and viol axanthin, In CP29, a minor LHCII, both a lutein site and the neoxanthin sit e can be occupied by violaxanthin. Upon activation of the violaxanthin de-e poxidase, the highest de-epoxidation state was found for the main LHCII com ponent and the lowest for CP29, suggesting that only violaxanthin loosely b ound to LHCII is available for de-epoxidation.