Johnson Ethan T, Schmidt-Dannert Claudia
Department of Biochemistry, Molecular Biology and Biophysics, University of Minnesota, St. Paul, Minnesota 55108.
Department of Biochemistry, Molecular Biology and Biophysics, University of Minnesota, St. Paul, Minnesota 55108.
J Biol Chem. 2008 Oct 10;283(41):27776-27784. doi: 10.1074/jbc.M804486200. Epub 2008 Aug 8.
Green bacteria synthesize several types of (bacterio)chlorophylls for the assembly of functional photosynthetic reaction centers and antenna complexes. A distinctive feature of green bacteria compared with other photosynthetic microbes is that their genomes contain multiple homologs of the large subunit (BchH) of the magnesium chelatase which is a three-subunit enzyme complex (BchH, BchD, and BchI) that inserts magnesium into protoporphyrin IX as the first committed step of (bacterio)chlorophyll biosynthesis. There is speculation that the additional BchH homologs may regulate the biosynthesis of each type of chlorophyll, although the biochemical properties of the different magnesium chelatase complexes from a single species of green bacteria have not yet been compared. In this study, we investigated the activities of all three chelatase complexes from the green sulfur bacterium Chlorobaculum tepidum and interactions with the next enzyme in the pathway, magnesium protoporphyrin IX methyltransferase (BchM). Although all three chelatase complexes insert magnesium into protoporphyrin IX, the activities range by a factor of 10(5). Further, there are differences in the interactions between the BchH homologs and BchM; two of the subunits increase the methyltransferase activity by 30-60%, and the third decreases it by 30%. Expression of the chelatase complexes alone and together with BchM in Escherichia coli overproducing protoporphyrin IX suggests that the chelatase is the rate-limiting enzyme. We observed that BchM uses protoporphyrin IX without bound metal as a substrate. Our results conflict with expectations generated by previous gene inactivation studies and suggest a complex regulation of chlorophyll biosynthesis in green bacteria.
绿色细菌合成几种类型的（细菌）叶绿素，用于组装功能性光合反应中心和天线复合体。与其他光合微生物相比，绿色细菌的一个显著特征是其基因组包含镁螯合酶大亚基（BchH）的多个同源物，镁螯合酶是一种三聚体酶复合体（BchH、BchD和BchI），它将镁插入原卟啉IX中，这是（细菌）叶绿素生物合成的第一个关键步骤。有人推测，额外的BchH同源物可能调节每种叶绿素的生物合成，尽管来自单一绿色细菌物种的不同镁螯合酶复合体的生化特性尚未进行比较。在本研究中，我们研究了绿色硫细菌嗜热绿菌中所有三种螯合酶复合体的活性，以及它们与该途径中下一个酶——镁原卟啉IX甲基转移酶（BchM）的相互作用。尽管所有三种螯合酶复合体都将镁插入原卟啉IX中，但活性范围相差10^5倍。此外，BchH同源物与BchM之间的相互作用存在差异；其中两个亚基可使甲基转移酶活性提高30 - 60%，而第三个亚基则使其降低30%。在过量产生原卟啉IX的大肠杆菌中单独表达螯合酶复合体以及与BchM一起表达表明，螯合酶是限速酶。我们观察到BchM使用未结合金属的原卟啉IX作为底物。我们的结果与先前基因失活研究所产生的预期结果相矛盾，并表明绿色细菌中叶绿素生物合成存在复杂的调控。
