Waldo G S, Wright E, Whang Z H, Briat J F, Theil E C, Sayers D E
Department of Physics and Biochemistry, North Carolina State University, Raleigh 27695, USA.
Plant Physiol. 1995 Nov;109(3):797-802. doi: 10.1104/pp.109.3.797.
Ferritin in plants is a nuclear-encoded, multisubunit protein found in plastids; an N-terminal transit peptide targets the protein to the plastid, but the site for formation of the ferritin Fe mineral is unknown. In biology, ferritin is required to concentrate Fe to levels needed by cells (approximately 10(-7) M), far above the solubility of the free ion (10(-18) M); the protein directs the reversible phase transition of the hydrated metal ion in solution to hydrated Fe-oxo mineral. Low phosphate characterizes the solid-phase Fe mineral in the center of ferritin of the cytosolic animal ferritin, but high phosphate is the hallmark of Fe mineral in prokaryotic ferritin and plant (pea [Pisum sativum L.] seed) ferritin. Earlier studies using x-ray absorption spectroscopy showed that high concentrations of phosphate present during ferritin mineralization in vivo altered the local structure of Fe in the ferritin mineral so that it mimicked the prokaryotic type, whether the protein was from animals or bacteria. The use of x-ray absorption spectroscopy to analyze the Fe environment in pea-seed ferritin now shows that the natural ferritin mineral in plants has an Fe-P interaction at 3.26A, similar to that of bacterial ferritin; phosphate also prevented formation of the longer Fe-Fe interactions at 3.5A found in animal ferritins or in pea-seed ferritin reconstituted without phosphate. Such results indicate that ferritin mineralization occurs in the plastid, where the phosphate content is higher; a corollary is the existence of a plastid Fe uptake system to allow the concentration of Fe in the ferritin mineral.
植物中的铁蛋白是一种核编码的多亚基蛋白，存在于质体中；一个N端转运肽将该蛋白靶向输送到质体，但铁蛋白铁矿物的形成位点尚不清楚。在生物学中，铁蛋白需要将铁浓缩到细胞所需的水平（约10⁻⁷ M），远高于游离离子的溶解度（10⁻¹⁸ M）；该蛋白引导溶液中水合金属离子向水合铁氧矿物的可逆相变。低磷酸盐是胞质动物铁蛋白中铁蛋白中心固相铁矿物的特征，但高磷酸盐是原核铁蛋白和植物（豌豆[Pisum sativum L.]种子）铁蛋白中铁矿物的标志。早期使用X射线吸收光谱的研究表明，体内铁蛋白矿化过程中存在的高浓度磷酸盐改变了铁蛋白矿物中铁的局部结构，使其模仿原核类型，无论该蛋白是来自动物还是细菌。现在使用X射线吸收光谱分析豌豆种子铁蛋白中的铁环境表明，植物中的天然铁蛋白矿物在3.26Å处存在铁-磷相互作用，类似于细菌铁蛋白；磷酸盐还阻止了在动物铁蛋白或无磷酸盐重构的豌豆种子铁蛋白中发现的3.5Å处更长的铁-铁相互作用的形成。这些结果表明，铁蛋白矿化发生在磷酸盐含量较高的质体中；一个必然的结果是存在一个质体铁摄取系统，以允许铁在铁蛋白矿物中浓缩。
