Substrate specificity and functional aspects of violaxanthin-de-epoxidase,an enzyme of the xanthophyll cycle

The fast light-dependent xanthophyll transformations (<<xanthophyll-cycle>> ) in thylakoids of green plants are catalysed by two enzymes: the strictly pH controlled and ascorbate-dependent (violaxanthin)de-epoxidase (VDE) and the NADPH(2)- and O-2-dependent (zeaxanthin)-epoxidase. The substrate speci ficity of the VDE was studied by using structurally different epoxy-xanthop hylls. For this purpose the enzyme was isolated by a freeze-thaw treatment of thylakoid-vesicles of spinach at pH 7.5 and incubated with various epoxy -substrates in the presence of the cosubstrate ascorbate and a lipid factor (phosphatidylcholine) at pH 5.2. Under these conditions the K-m-value for the substrate violaxanthin (Vio) was 11.1 mu mol/L and for antheraxanthin ( Ant) 5.3 mu mol/L. Only the epoxy-oxygen at the 5,6 (5',6') position of xan thophylls was cleaved by the VDE, whereas ring-spanning epoxides at positio n 3,6 (3',6') were not accessible to the enzyme. Moreover, the structure an d chemical ligands of the second jonon ring were insignificant for the de-e poxidation of the 5,6-epoxy-groups of the first ring. Therefore, the epoxy- free (or also epoxy-containing) second jonon ring is not involved in the bi nding of the xanthophyll to the catalytic center and does not affect the en zyme reaction. However, due to a steric hindrance, any tested cis-configura tion in the polyene chain of the xanthophylls, as well as the 8-oxy group i n fucoxanthin, prevent the deepoxidation. The epoxy-xanthophylls available for the VDE are suggested to occur as rod-like, trans-configurated pigments within the lipid bilayer of thylakoids. When the mobile VDE is bound to th e lumenal side of the thylakoid at pH less than or equal to 6.5 (Hager and Holocher, 1994), the epoxy-xanthophylls, guided by lipids, invade a fold, c hannel or tube-like structure of the enzyme, which functions as the catalyt ic center for the de-epoxidation. Functional aspects of the Vio-de-epoxidat ion and of the xanthophyll-cycle are discussed.