Cunningham F X, Pogson B, Sun Z, McDonald K A, DellaPenna D, Gantt E
Department of Plant Biology, University of Maryland, College Park 20742, USA.
Plant Cell. 1996 Sep;8(9):1613-26. doi: 10.1105/tpc.8.9.1613.
Carotenoids with cyclic end groups are essential components of the photosynthetic membranes in all plants, algae, and cyanobacteria. These lipid-soluble compounds protect against photooxidation, harvest light for photosynthesis, and dissipate excess light energy absorbed by the antenna pigments. The cyclization of lycopene (psi, psi-carotene) is a key branch point in the pathway of carotenoid biosynthesis. Two types of cyclic end groups are found in higher plant carotenoids: the beta and epsilon rings. Carotenoids with two beta rings are ubiquitous, and those with one beta and one epsilon ring are common; however, carotenoids with two epsilon rings are rare. We have identified and sequenced cDNAs that encode the enzymes catalyzing the formation of these two rings in Arabidopsis. These beta and epsilon cyclases are encoded by related, single-copy genes, and both enzymes use the linear, symmetrical lycopene as a substrate. However, the epsilon cyclase adds only one ring, forming the monocyclic delta-carotene (epsilon, psi-carotene), whereas the beta cyclase introduces a ring at both ends of lycopene to form the bicyclic beta-carotene (beta, beta-carotene). When combined, the beta and epsilon cyclases convert lycopene to alpha-carotene (beta, epsilon-carotene), a carotenoid with one beta and one epsilon ring. The inability of the epsilon cyclase to catalyze the introduction of a second epsilon ring reveals the mechanism by which production and proportions of beta,beta- and beta, epsilon-carotenoids may be controlled and adjusted in plants and algae, while avoiding the formation of the inappropriate epsilon,epsilon-carotenoids.
带有环状端基的类胡萝卜素是所有植物、藻类和蓝细菌光合膜的重要组成部分。这些脂溶性化合物可防止光氧化，为光合作用收集光能，并消散天线色素吸收的多余光能。番茄红素（ψ，ψ-胡萝卜素）的环化是类胡萝卜素生物合成途径中的一个关键分支点。在高等植物类胡萝卜素中发现了两种类型的环状端基：β环和ε环。具有两个β环的类胡萝卜素普遍存在，而具有一个β环和一个ε环的类胡萝卜素也很常见；然而，具有两个ε环的类胡萝卜素却很罕见。我们已经鉴定并测序了拟南芥中编码催化这两种环形成的酶的cDNA。这些β环化酶和ε环化酶由相关的单拷贝基因编码，并且这两种酶都使用线性对称的番茄红素作为底物。然而，ε环化酶只添加一个环，形成单环δ-胡萝卜素（ε，ψ-胡萝卜素），而β环化酶在番茄红素的两端都引入一个环，形成双环β-胡萝卜素（β，β-胡萝卜素）。当β环化酶和ε环化酶结合时，它们将番茄红素转化为α-胡萝卜素（β，ε-胡萝卜素），一种具有一个β环和一个ε环的类胡萝卜素。ε环化酶无法催化第二个ε环的引入，这揭示了在植物和藻类中控制和调节β，β-胡萝卜素和β，ε-胡萝卜素的产生和比例的机制，同时避免形成不合适的ε，ε-胡萝卜素。
