Mugineic Acid Derivatives as Molecular Probes for the Mechanistic Elucidation of Iron Acquisition in Barley
Mugineic Acid Derivatives as Molecular Probes for the Mechanistic Elucidation of Iron Acquisition in Barley
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DOI：
10.1002/anie.201004853
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发表时间：
2010-01-01
影响因子：
16.6
通讯作者：
Tanino, Keiji
中科院分区：
文献类型：
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作者：
Namba, Kosuke;Kobayashi, Kaori;Tanino, Keiji
铁是植物的必需元素，在呼吸作用、光合作用和固氮作用等多种过程中发挥着重要作用。这种元素对动物来说也是必不可少的，因为动物通常从食用植物中摄取这种元素。[1]因此，植物从土壤中吸收铁对所有生物都至关重要。然而，尽管地球表面的铁含量很高，[2]大多数植物在碱性环境中很难吸收铁，因为其三价盐（Fe 3+）的水溶性很差。[3]为了克服这个问题，禾本科植物已经开发出一种基于植物铁载体作为螯合剂的分泌来溶解Fe 3+并通过选择性转运蛋白吸收所得铁络合物的特定策略。[4]Mugineic acid（MA，1;方案1）首先被确定为大麦中的植物铁载体，[5，6]并且MA的类似物已经从各种禾本科物种和栽培品种中分离出来。[7]MA及其类似物均与Fe III形成水溶性1：1络合物。在之前的研究中，我们鉴定了一个在大麦中特异性编码FeIII· MA转运蛋白（HvYS 1）的基因;[8]该基因属于YSL家族。[9]HvYS 1的定位和底物特异性表明它是FeIII· MA复合物在大麦根中的特异性转运蛋白。[8]我们进一步揭示了第六个外膜环决定了HvYS 1的FeIII-植物铁载体特异性。[10]因此，更详细的机制，包括阐明转运蛋白的3D结构模式，Fe III复合物的识别机制，以及复合物在植物体内的命运，已成为我们下一步的重点。为了推动这些功能的研究，我们必须建立有效的制备方法MA衍生物被用作分子探针。引入功能标记的MA骨架迄今为止是不成功的，主要是因为所有以前制备的标记产品失去了它们的能力，形成FeIII复合物的结构修饰的结果。我们现在已经建立了MA（1）和2 '-脱氧mugineic酸（DMA，2）的有效短步骤合成，[11]这是水稻，小麦和玉米的植物铁载体[12]，具有类似的铁（III）运输功能。将合成的DMA（2）的活性与MA（1）及其非对映体2 ′-表-mugineic acid（2 ′-epi-MA，3）（以类似的方式合成）的活性进行比较，清楚地表明这三种植物铁载体表现出相同水平的铁转运能力。[11]这一结果为2 '-羟基可用于标记mugineic acid类似物进行功能研究提供了线索。因此，我们在MA（1）的2 ′-羟基上引入了不同的标记基团，并研究了所得探针的铁转运活性，由于未保护的MA（1）或2 ′-epi-MA（3）的多功能极性结构，通过在2 ′-羟基上选择性地引入任何取代基来制备标记探针是不有利的，尽管1和3可以容易地制备。[11因此，我们尝试合成具有游离2 '-羟基作为标记前体的受保护的MA。我们用Cbz保护的2-羟基-L-烯丙基甘氨酸叔丁酯4开始合成（方案2）。[11]答4：1的非对映异构体混合物（有利于具有羟基的非天然α构型的非对映异构体），如通过Cbz-保护的L-烯丙基甘氨酸叔丁酯的烯丙基氧化获得的。我们知道
Iron, an essential element for plants, plays versatile and significant roles in a variety of processes, including respiration, photosynthesis, and nitrogen fixation. The element is also indispensable for animals, for whom the source is usually uptake from dietary plants.[1] Thus, iron uptake from the soil by plants is crucial for all living creatures. However, despite the high levels of iron on the surface of the earth,[2] most plants have difficulty absorbing iron in alkaline environments owing to the poor water solubility of its trivalent (Fe3+) salts.[3] To overcome this problem, graminaceous plants have developed a specific strategy based on the secretion of phytosiderophores as chelators to solubilize Fe3+ and the uptake of the resulting iron complexes through selective transporters.[4] Mugineic acid (MA, 1; Scheme 1) was first identified as a phytosiderophore in barley,[5, 6] and analogues of MA have since been isolated from various graminaceous species and cultivars.[7] MA and its analogues all form water-soluble 1: 1 complexes with FeIII. In a previous study, we identified a gene that specifically encodes an FeIII· MA transporter (HvYS1) in barley;[8] the gene belongs to the YSL family.[9] The localization and substrate specificity of HvYS1 indicate that it is a specific transporter for the FeIII· MA complex in barley roots.[8] We further revealed that the sixth outer-membrane loop determines the FeIII–phytosiderophore specificity of HvYS1.[10] Therefore, more detailed mechanisms, including the elucidation of the 3D structural pattern of the transporter, the recognition mechanism of FeIII complexes, and the fate of the complexes inside the plants, have become our next focus. To drive these functional studies forward, we had to establish efficient preparative methods for MA derivatives to be utilized as molecular probes. The introduction of functionalities for labeling of the MA skeleton had so far been unsuccessful, mainly because all previously prepared labeled products lost their ability to form FeIII complexes as a result of the structural modifications. We have now established an efficient short-step synthesis of MA (1) and 2’-deoxymugineic acid (DMA, 2),[11] which is a phytosiderophore for rice, wheat, and maize [12] with a similar iron (III)-transport function. Comparison of the activity of synthetic DMA (2) with the activities of MA (1) and its diastereomer 2’-epi-mugineic acid (2’-epi-MA, 3), which was synthesized in a similar manner, clearly showed that these three phytosiderophores exhibit the same level of iron-transport ability.[11] This result provided the clue that the 2’-hydroxy group could be suitable for the labeling of mugineic acid analogues for their functional study. Thus, we introduced various labeling groups at the 2’-hydroxy group of MA (1) and investigated the iron-transport activities of the resulting probes.Because of the multifunctional polar structures of unprotected MA (1) or 2’-epi-MA (3), the preparation of labeled probes by the selective introduction of any substituent at the 2’-hydroxy group is by no means advantageous, even though 1 and 3 can be readily prepared.[11, 13] We therefore attempted to synthesize protected MAs with a free 2’-hydroxy group as a labeling precursor. We began the synthesis with Cbz-protected 2-hydroxy-L-allylglycine tert-butyl ester 4 (Scheme 2).[11] A 4: 1 diastereomeric mixture (in favor of the diastereomer with the nonnatural α configuration of the hydroxy group) was used as obtained by allylic oxidation of Cbz-protected L-allylglycine tert-butyl ester. We knew that