Szilágyi Anna, Sommarin Marianne, Akerlund Hans-Erik
Department of Biochemistry, Center for Chemistry and Chemical Engineering, Lund University, P.O.B. 124, SE-221 00 Lund, Sweden.
Biochim Biophys Acta. 2007 Sep;1768(9):2310-8. doi: 10.1016/j.bbamem.2007.06.001. Epub 2007 Jun 12.
Zeaxanthin, an important component in protection against overexcitation in higher plants, is formed from violaxanthin by the enzyme violaxanthin de-epoxidase. We have investigated factors that may control the maximal degree of conversion in the violaxanthin cycle. The conversion of violaxanthin to zeaxanthin in isolated spinach thylakoids was followed at different temperatures and in the presence of lipid packing modifiers. The maximum degree of conversion was found to be 35%, 70% and 80% at 4 degrees C, 25 degrees C and 37 degrees C respectively. In the presence of membrane modifying agents, known to promote non-lamellar structures (H(II)), such as linolenic acid the conversion increased, and the maximal level of violaxanthin de-epoxidation obtained was close to 100%. In contrast, substances promoting lamellar phases (L(alpha)), such as alpha-tocopherol and 8-cetylether (C(16)EO(8)), only 55% and 35% of the violaxanthin was converted at 25 degrees C, respectively. The results are interpreted in light of the lipid composition of the thylakoid membrane, and we propose a model where a negative curvature elastic stress in the thylakoid lipid bilayer is required for violaxanthin de-epoxidase activity. In this model zeaxanthin with its longer hydrophobic stretch is proposed to promote lamellar arrangements of the membrane. As a result, zeaxanthin relieves the curvature elastic stress, which in turn leads to inactivation of violaxanthin de-epoxidase.
玉米黄质是高等植物中防止过度激发的重要成分，它由紫黄质通过紫黄质脱环氧化酶形成。我们研究了可能控制紫黄质循环中最大转化率的因素。在不同温度下以及在脂质堆积调节剂存在的情况下，跟踪了分离的菠菜类囊体中紫黄质向玉米黄质的转化。发现在4℃、25℃和37℃时，最大转化率分别为35%、70%和80%。在已知能促进非片层结构（H(II)）的膜修饰剂存在下，如亚麻酸，转化率增加，获得的紫黄质脱环氧化的最大水平接近100%。相反，促进片层相（L(α)）的物质，如α-生育酚和8-十六烷基醚（C(16)EO(8)），在25℃时分别只有55%和35%的紫黄质发生了转化。根据类囊体膜的脂质组成对结果进行了解释，我们提出了一个模型，其中类囊体脂质双层中的负曲率弹性应力是紫黄质脱环氧化酶活性所必需的。在这个模型中，具有较长疏水链的玉米黄质被认为可以促进膜的片层排列。结果，玉米黄质减轻了曲率弹性应力，这反过来又导致紫黄质脱环氧化酶失活。
