Structural and biochemical characterization of Gun4 suggests a mechanism for its role in chlorophyll biosynthesis
Structural and biochemical characterization of Gun4 suggests a mechanism for its role in chlorophyll biosynthesis
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DOI：
10.1021/bi050240x
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发表时间：
2005-05-31
期刊：
影响因子：
2.9
通讯作者：
Hunter, CN
中科院分区：
文献类型：
--
作者：
Davison, PA;Schubert, HL;Hunter, CN
Gun4参与了叶绿体和细胞核之间的发育信号通路，涉及到叶绿素生物合成途径中的第一个专用中间体--镁原卟啉IX(MgPIX)。在这里，我们给出了长热链球菌Gun4在1.5埃的晶体结构，描述了Gun4与底物和产物卟啉分子的结合亲和力，并确定了蛋白质上可能的(Mg)P-IX结合部位。动力学分析表明，Gun4极大地提高了卟啉底物向金属卟啉产物的转化效率，并降低了在低浓度下活性所需的阈值镁离子浓度。因此，Gun4在生理上有意义的镁离子浓度控制着镁螯合酶，并可能在体内起到分子开关的作用，因此在没有它的情况下，镁螯合酶是不活跃的。这一机制可能允许Gun4调节镁原卟啉水平，既用于叶绿素的生物合成，也用于向细胞核发出信号。
Gun4 has been implicated in a developmental signaling pathway between the chloroplast and the nucleus involving magnesium protoporphyrin IX (MgPIX), the first dedicated intermediate in the chlorophyll biosynthetic pathway. Here we present the crystal structure of Thermosynechococcus elongatus Gun4 at 1.5 angstrom, describe the binding affinities of Gun4 for substrate and product porphyrin molecules, and identify a likely (Mg)P-IX binding site on the protein. Kinetic analyses show that Gun4 dramatically increases the efficiency of transformation of porphyrin substrate to metalloporphyrin product and that it also reduces the threshold Mg2+ concentration required for activity at low porphyrin concentration. Gun4 therefore controls magnesium chelatase at physiologically significant Mg2+ concentrations and likely acts as a molecular switch in vivo so that in its absence magnesium chelatase is inactive. This mechanism could allow Gun4 to mediate magnesium protoporphyrin levels both for chlorophyll biosynthesis and for signaling to the nucleus.