Wichard Thomas, Bellenger Jean-Philippe, Loison Aurélie, Kraepiel Anne M L
Department of Geosciences, Princeton Environmental Institute, Guyot Hall, Princeton University, Princeton, New Jersey 08544, USA.
Environ Sci Technol. 2008 Apr 1;42(7):2408-13. doi: 10.1021/es702651f.
Molybdenum (Mo) and tungsten (W), which have similar chemistry, are present at roughly the same concentration in the earth's continental crust, and both are present in oxic systems as oxoanions, molybdate and tungstate. Molybdenum is a cofactor in the molybdenum-nitrogenase enzyme and is thus an important micronutrient for N2-fixing bacteria such as Azotobacter vinelandii (A. vinelandii). Tungsten is known to be toxic to N2-fixing bacteria, partly by substituting for Mo in nitrogenase. We showthatthe catechol siderophores produced by A. vinelandii, in addition to being essential for iron acquisition, modulate the relative uptake of Mo and W. These catechol siderophores (particularly protochelin), whose concentrations in the growth medium increase sharply at high W, complex all the tungstate along with molybdate and some of the iron. The molybdenum-catechol complex is taken up much more rapidly than the W complex, allowing A. vinelandii to satisfy its Mo requirement and avoid W toxicity. Mutants deficient in the production of catechol siderophores are more sensitive to tungstate and have higher cellular W quotas than the wild type. The binding of metals by excreted catechol siderophores allows A. vinelandii to discriminate in its uptake of essential metals, such as Fe and Mo, over that of toxic metals, such as W, and to sustain high growth rates under adverse environmental conditions.
钼（Mo）和钨（W）化学性质相似，在地壳中的浓度大致相同，并且在有氧体系中均以含氧阴离子形式存在，即钼酸盐和钨酸盐。钼是钼固氮酶的辅因子，因此对于诸如棕色固氮菌（A. vinelandii）这样的固氮细菌而言是一种重要的微量营养素。已知钨对固氮细菌有毒，部分原因是它会在固氮酶中取代钼。我们发现，棕色固氮菌产生的儿茶酚型铁载体，除了对获取铁至关重要外，还能调节钼和钨的相对摄取。这些儿茶酚型铁载体（特别是原螯合铁），其在生长培养基中的浓度在高钨条件下会急剧增加，它们会与所有的钨酸盐、钼酸盐以及部分铁结合形成复合物。钼 - 儿茶酚复合物的摄取速度比钨复合物快得多，这使得棕色固氮菌能够满足其对钼的需求并避免钨中毒。缺乏儿茶酚型铁载体产生能力的突变体对钨酸盐更敏感，并且细胞内的钨含量比野生型更高。分泌的儿茶酚型铁载体与金属的结合，使得棕色固氮菌能够在摄取必需金属（如铁和钼）时优先于有毒金属（如钨），并在不利的环境条件下维持高生长速率。
