Xanthophyll - Xanthophyll Cycle

Xanthophyll Cycle

The xanthophyll cycle involves the enzymatic removal of epoxy groups from xanthophylls (e.g. violaxanthin, antheraxanthin, diadinoxanthin) to create so-called de-epoxidised xanthophylls (e.g. diatoxanthin, zeaxanthin). These enzymatic cycles were found to play a key role in stimulating energy dissipation within light-harvesting antenna proteins by non-photochemical quenching- a mechanism to reduce the amount of energy that reaches the photosynthetic reaction centers. Non-photochemical quenching is one of the main ways of protecting against photoinhibition. In higher plants there are three carotenoid pigments that are active in the xanthophyll cycle: violaxanthin, antheraxanthin and zeaxanthin. During light stress violaxanthin is converted to zeaxanthin via the intermediate antheraxanthin, which plays a direct photoprotective role acting as a lipid-protective anti-oxidant and by stimulating non-photochemical quenching within light-harvesting proteins. This conversion of violaxanthin to zeaxanthin is done by the enzyme violaxanthin de-epoxidase, while the reverse reaction is performed by zeaxanthin epoxidase

In diatoms and dinoflagellates the xanthophyll cycle consists of the pigment diadinoxanthin, which is transformed into diatoxanthin (diatoms) or dinoxanthin (dinoflagellates), at high light.

Wright et al. (Feb 2011) found that, "The increase in zeaxanthin appears to surpass the decrease in violaxanthin in spinach" and commented that the discrepancy could be explained by "a synthesis of zeaxanthin from beta-carotene", however they noted further study is required to explore this hypothesis.