Xanthophyll Cycle in the Light of Thylakoid Membrane Lipids - Membrane Packing, Curvature Elastic Stress and Enzyme Binding
Sammanfattning: The xanthophyll cycle involves the light-dependent and reversible conversion of violaxanthin to zeaxanthin. The enzyme, violaxanthin de-epoxidase (VDE) catalyses the conversion of violaxanthin to zeaxanthin, via the intermediate antheraxanthin. VDE is a membrane-hosted enzyme during its activity. Low lumenal pH, violaxanthin, ascorbate and monogalactosyldiacylglycerol are required for activity. The formed zeaxanthin is involved in the protection of the photosynthetic apparatus from overexcitation. Apart from the energy-dissipating role, the xanthophyll cycle has been associated with a modulation of the physical properties of the thylakoid membrane.
During photosynthetic activity, the lumenal pH drops and VDE undergoes a conformational change causing the active enzyme to bind to the lumenal side of the thylakoid membrane. Binding studies of VDE to the thylakoid membranes showed that the conserved histidine residues in the lipocalin region are involved in the conformational change of the enzyme.
By altering the ratio between the lamellar and non-lamellar structures in the thylakoid membrane, the activity of VDE was influenced. When the thylakoids were treated with linolenic acid which increases non-lamellar prone structures in the membrane, enhanced zeaxanthin formation was seen. A model is proposed involving membrane curvature stress, thylakoid membrane packing and the xanthophyll cycle. According to this model the different xanthophyll pigments have different preferences for different membrane structures and can themselves induce differentially curved membrane regions. Zeaxanthin with its increased molecular hydrophobic length affects lipid packing, and brings about a release of curvature stress leading to a less favoured lipid environment for VDE.
Xanthophylls, based on their rigid molecular structure, have been suggested to modify membrane structure, more specifically to decrease membrane fluidity. We have shown that accumulation of zeaxanthin in the isolated thylakoid membrane resulted in an increased rigidity of the membrane measured as a red-shifted fluorescence emission spectrum using laurdan, as a probe.
These results further support the hypothesis that xanthophyll cycle pigments are located in the lipid phase of the thylakoid membrane and affect membrane physical properties.
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