Violaxanthin De-epoxidase and the Xanthophyll Cycle

Sammanfattning: The xanthophyll cycle is a mechanism for short term adaptation of plants to varying light intensities. It involves a reversible conversion of violaxanthin to zeaxanthin, via the intermediate antheraxanthin, upon high light intensities. This conversion is catalyzed by violaxanthin de-epoxidase (VDE) and the product after de-epoxidation, zeaxanthin, is involved in non-photochemical quenching of excess light energy absorbed by the plant. VDE is a 43 kDa protein, soluble in the thylakoid lumen at high pH. At low lumenal pH, VDE undergoes a conformational change, promoting the active enzyme to bind to the thylakoid membrane. There, zeaxanthin is formed from violaxanthin and excess energy can be dissipated as harmless heat energy, through non-photochemical quenching. Besides violaxanthin, VDE also requires ascorbic acid as a substrate. Ascorbic acid is oxidized when violaxanthin is reduced. The presence of a lipid that can form an inverted hexagonal phase is required for binding of VDE to occur. Regulatory divalent cations were found to inhibit VDE activity. VDE has three distinct regions; a cystein-rich N terminus and a charged C terminus, that both form mainly a-helical secondary structures, and a lipochalin region in between with multiple b-strands in a barrel structure. Four histidine residues, within the lipocalin region, are highly concerved. Chemical modifications and point mutations of the histidines showed that they are important for enzymatic activity. The 3D structure for VDE is not known, but the conformational change of the enzyme has been seen as a difference in accessibility of disulfide bridges for reduction at pH 5 and 7. Binding studies of VDE to the thylakoid membrane showed that the conserved histidine residues are involved in the conformational change. Protonation of the histidine sidegroup around pH 6 leads to binding of the enzyme to the thylakoid membrane, within a narrow range and with a cooperativity around 4, with respect to protons, for spinach VDE with 4 histidines. Wheat VDE, with 3 histidine residues has a cooperativity around 3. Mutant forms of VDE, with less than four histidine residues, showed a lower cooperativity for membrane binding than the wildtype.