Roper J M, Smith A G
Department of Plant Sciences, University of Cambridge, England, UK.
Eur J Biochem. 1997 May 15;246(1):32-7. doi: 10.1111/j.1432-1033.1997.t01-1-00032.x.
Within the chloroplast of higher plants, a crucial branchpoint of the tetrapyrrole synthesis pathway is the chelation of either Fe2+ to make haem, or Mg2+ for chlorophyll, catalysed by ferrochelatase or magnesium chelatase, respectively. One model that has been proposed for the control of this branchpoint, based on biochemical studies, is that the two enzymes are spatially separated within the chloroplast, ferrochelatase being exclusively in the thylakoids, while magnesium chelatase is associated with the envelope [Matringe, M., Camadro, J.-M., Joyard, J. & Douce, R. (1994) J. Biol. Chem. 269, 15010-15015]. We have used a sensitive molecular method to investigate this possibility. Radiolabelled precursor proteins for ferrochelatase from Arabidopsis have been imported into isolated chloroplasts. Their distribution in the different subchloroplastic fractions have then been determined, and compared with that for light-harvesting chlorophyll protein, which is exclusively thylakoidal, and the envelope-located phosphate translocator. Clear evidence for the specific association of ferrochelatase protein with both thylakoid and envelope membranes has been obtained, thus suggesting strongly that the control of the branchpoint cannot be by spatial separation of the two chelatases.
在高等植物的叶绿体中，四吡咯合成途径的一个关键分支点是分别由亚铁螯合酶或镁螯合酶催化，将Fe2+螯合形成血红素，或将Mg2+螯合形成叶绿素。基于生化研究提出的一种控制该分支点的模型是，这两种酶在叶绿体中在空间上是分开的，亚铁螯合酶仅存在于类囊体中，而镁螯合酶与包膜相关[Matringe, M., Camadro, J.-M., Joyard, J. & Douce, R. (1994) J. Biol. Chem. 269, 15010 - 15015]。我们使用了一种灵敏的分子方法来研究这种可能性。来自拟南芥的亚铁螯合酶的放射性标记前体蛋白已被导入分离的叶绿体中。然后确定了它们在不同亚叶绿体组分中的分布，并与仅存在于类囊体中的捕光叶绿素蛋白和位于包膜的磷酸转运体的分布进行了比较。已经获得了亚铁螯合酶蛋白与类囊体膜和包膜膜特异性结合的明确证据，因此强烈表明分支点的控制不可能是通过两种螯合酶的空间分离。
