SummaryThe biosynthesis of the tocotrienol and tocopherol forms of vitamin E is initiated by prenylation of homogentisate. Geranylgeranyl diphosphate (GGDP) is the prenyl donor for tocotrienol synthesis, whereas phytyl diphosphate (PDP) is the prenyl donor for tocopherol synthesis. We have previously shown that tocotrienol synthesis is initiated in monocot seeds by homogentisate geranylgeranyl transferase (HGGT). This enzyme is related to homogentisate phytyltransferase (HPT), which catalyzes the prenylation step in tocopherol synthesis. Here we show that monocot HGGT is localized in the plastid and expressed primarily in seed endosperm. Despite the close structural relationship of monocot HGGT and HPT, these enzymes were found to have distinct substrate specificities. Barley (Hordeum vulgare cv. Morex) HGGT expressed in insect cells was six times more active with GGDP than with PDP, whereas the Arabidopsis HPT was nine times more active with PDP than with GGDP. However, only small differences were detected in the apparent Km values of barley HGGT for GGDP and PDP. Consistent with its in vitro substrate properties, barley HGGT generated a mixture of tocotrienols and tocopherols when expressed in the vitamin E‐null vte2‐1 mutant lacking a functional HPT. Relative levels of tocotrienols and tocopherols produced in vte2‐1 differed between organs and growth stages, reflective of the composition of plastidic pools of GGDP and PDP. In addition, HGGT was able to functionally substitute for HPT to rescue vte2‐1‐associated phenotypes, including reduced seed viability and increased fatty acid oxidation of seed lipids. Overall, we show that monocot HGGT is biochemically distinct from HPT, but can replace HPT in important vitamin E‐related physiological processes.
摘要维生素 E 的生育三烯酚和生育酚形式的生物合成是通过同戊烯二酸的前炔化作用开始的。香叶基二磷酸甘油酯(GGDP)是生育三烯酚合成的前酰供体,而生育酚合成的前酰供体是植烯醇二磷酸酯(PDP)。我们之前已经证明,在单子叶植物种子中,生育三烯酚的合成是由同源戊二酸龙葵酰转移酶(HGGT)启动的。这种酶与催化生育酚合成中的前酰化步骤的同源戊二酸植物酰基转移酶(HPT)有关。在这里,我们发现单子叶植物的 HGGT 定位于质体,主要在种子胚乳中表达。尽管单子叶植物 HGGT 和 HPT 的结构关系密切,但我们发现这些酶具有不同的底物特异性。在昆虫细胞中表达的大麦(Hordeum vulgare cv. Morex)HGGT 对 GGDP 的活性是对 PDP 的活性的六倍,而拟南芥 HPT 对 PDP 的活性是对 GGDP 的活性的九倍。然而,大麦 HGGT 对 GGDP 和 PDP 的表观 Km 值仅有微小差异。与其体外底物特性相一致的是,大麦 HGGT 在缺乏功能性 HPT 的维生素 E 缺失 vte2-1 突变体中表达时,会产生生育三烯酚和生育酚的混合物。在不同器官和生长阶段,vte2-1 产生的生育三烯酚和生育酚的相对水平不同,这反映了质体 GGDP 和 PDP 池的组成。此外,HGGT 能够在功能上替代 HPT 来挽救 vte2-1 相关的表型,包括种子活力降低和种子脂质的脂肪酸氧化增加。总之,我们的研究表明,单子叶植物 HGGT 在生物化学上不同于 HPT,但在重要的维生素 E 相关生理过程中可以替代 HPT。