Enzymes and Mechanisms for Violaxanthin-zeaxanthin Conversion

Abstract

The xanthophyll cycle is of great importance in relation to light stress. Particularly, interest has been focused on the possible photoprotective role of zeaxanthin. In higher plants under light stress, zeaxanthin is formed from violaxanthin in a reaction catalyzed by violaxanthin de-epoxidase (VDE). The reverse reaction is catalyzed by zeaxanthin epoxidase (ZE) under low light or in darkness. VDE has been purified from spinach and lettuce as a 43-kDa protein. The gene has been cloned and sequenced from several species, and a few mutants have been isolated. The gene is nuclear encoded and the transit peptide is characteristic for targeting to the thylakoid lumen. The activity of VDE is affected by factors such as a pH-dependent binding to the thylakoid membrane, concentration of ascorbic acid, temperature and availability of violaxanthin in relation to amount, type and distribution of pigment-protein complexes in the membrane. The information about ZE is more limited. The enzyme has not yet been isolated but its gene has been cloned and sequenced and a number of mutants have been isolated. The role of the xanthophyll cycle in the dissipation of excess light energy will be discussed particularly in relation to the recent progress in studies on various mutants. The possible role of the xanthophyll cycle in other processes, such as protection against oxidative stress of lipids, regulation of membrane fluidity, participation in blue light responses, and regulation of abscisic acid synthesis will also be presented.