FIGURE 2.

Reaction sequences and enzymes of the violaxanthin (A) and diadinoxanthin (B) cycle. The violaxanthin cycle is present in higher plants and green and brown algae, the diadinoxanthin cycle is found in diatoms, haptophytes and dinophytes. Figure 2 also shows the cofactor requirements of the enzymes catalyzing the de-epoxidation reaction (violaxanthin de-epoxidase, VDE and diadinoxanthin de-epoxidase, DDE) and the epoxidation reaction (zeaxanthin epoxidase, ZEP and diatoxanthin epoxidase, DEP), respectively. Representation of the cofactors in bold or normal type indicates whether high or low concentrations of the respective cofactors are needed for high enzyme activity. The establishment of a proton gradient inhibits diatoxanthin epoxidation (high ΔpH control) and is thus presented in bold type whereas zeaxanthin epoxidation is unaffected by the presence of the transmembrane ΔpH (ΔpH control depicted in normal type). The pH value of the thylakoid lumen which leads to VDE and DDE activation (possibly by VDE or DDE dimerization) and membrane binding is also indicated for the two xanthophyll cycles.