Metabolic reconstructions identify plant 3-methylglutaconyl-CoA hydratase that is crucial for branched-chain amino acid catabolism in mitochondria
Metabolic reconstructions identify plant 3-methylglutaconyl-CoA hydratase that is crucial for branched-chain amino acid catabolism in mitochondria
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DOI：
10.1111/tpj.13955
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发表时间：
2018-07-01
期刊：
影响因子：
7.2
通讯作者：
Basset, Gilles J.
中科院分区：
文献类型：
--
作者：
Latimer, Scott;Li, Yubing;Basset, Gilles J.
支链氨基酸（BCAAs）亮氨酸、异亮氨酸和缬氨酸是哺乳动物必需营养素。在植物中，当碳水化合物受到限制时，支链氨基酸可以作为替代能源，支链氨基酸的分解代谢为呼吸链提供电子，并为三羧酸循环提供中间体。然而，BCAAs降解途径的实际结构尚不清楚。本研究通过拟南芥和水稻的基因网络建模和植物-原核生物比较基因组学检测了3-甲基谷氨酰基辅酶a水合酶（4.2.1.18）的候选基因，这是植物中缺失的亮氨酸分解代谢酶之一。对来自各种精子植物的这些候选蛋白进行比对，发现它们在细菌中缺乏非同源的n端延伸，绿色荧光蛋白融合实验表明，基因At4g16800的产物拟南芥蛋白是针对线粒体的。重组At4g16800催化3-羟甲基戊二酰辅酶a脱水生成3-甲基戊二酰辅酶a，并表现出与其原核同源物相似的动力学特征。当at4g16800基因敲除植株遭受黑暗诱导的碳饥饿时，其莲座叶片的衰老速度比对照植株更快，这种表型与游离和总亮氨酸、异亮氨酸和缬氨酸积累的显著增加相一致。at4g16800突变体的种子显示出类似的游离BCAAs积累。这些数据表明，3-甲基谷氨酰基辅酶a水合酶不仅参与亮氨酸的降解，而且还参与异亮氨酸和缬氨酸的降解。此外，有证据表明，与锥虫科观察到的情况不同，亮氨酸分解代谢并不有助于萜类前体甲羟戊酸的形成。
The proteinogenic branched-chain amino acids (BCAAs) leucine, isoleucine and valine are essential nutrients for mammals. In plants, BCAAs double as alternative energy sources when carbohydrates become limiting, the catabolism of BCAAs providing electrons to the respiratory chain and intermediates to the tricarboxylic acid cycle. Yet, the actual architecture of the degradation pathways of BCAAs is not well understood. In this study, gene network modeling in Arabidopsis and rice, and plant-prokaryote comparative genomics detected candidates for 3-methylglutaconyl-CoA hydratase (4.2.1.18), one of the missing plant enzymes of leucine catabolism. Alignments of these protein candidates sampled from various spermatophytes revealed non-homologous N-terminal extensions that are lacking in their bacterial counterparts, and green fluorescent protein-fusion experiments demonstrated that the Arabidopsis protein, product of gene At4g16800, is targeted to mitochondria. Recombinant At4g16800 catalyzed the dehydration of 3-hydroxymethylglutaryl-CoA into 3-methylglutaconyl-CoA, and displayed kinetic features similar to those of its prokaryotic homolog. When at4g16800 knockout plants were subjected to dark-induced carbon starvation, their rosette leaves displayed accelerated senescence as compared with control plants, and this phenotype was paralleled by a marked increase in the accumulation of free and total leucine, isoleucine and valine. The seeds of the at4g16800 mutant showed a similar accumulation of free BCAAs. These data suggest that 3-methylglutaconyl-CoA hydratase is not solely involved in the degradation of leucine, but is also a significant contributor to that of isoleucine and valine. Furthermore, evidence is shown that unlike the situation observed in Trypanosomatidae, leucine catabolism does not contribute to the formation of the terpenoid precursor mevalonate.