Fukushima Tatsuya, Allred Benjamin E, Raymond Kenneth N
Department of Chemistry, University of California , Berkeley, California 94720-1460, United States.
ACS Chem Biol. 2014 Sep 19;9(9):2092-100. doi: 10.1021/cb500319n. Epub 2014 Jul 29.
Iron is an essential element for all organisms, and microorganisms produce small molecule iron-chelators, siderophores, to efficiently acquire Fe(III). Gram-positive bacteria possess lipoprotein siderophore-binding proteins (SBPs) on the membrane. Some of the SBPs bind both apo-siderophores (iron-free) and Fe-siderophore (iron-chelated) and only import Fe-siderophores. When the SBP initially binds an apo-siderophore, the SBP uses the Gram-positive siderophore-shuttle mechanism (the SBPs exchange Fe(III) from a Fe-siderophore to the apo-siderophore bound to the protein) and/or displacement mechanism (the apo-siderophore bound to the SBP is released and a Fe-siderophore is then bound to the protein) to import the Fe-siderophore. Previously, we reported that the Bacillus cereus SBP, YxeB, exchanges Fe(III) from a ferrioxamine B (FO) to a desferrioxamine B (DFO) bound to YxeB using the siderophore-shuttle mechanism although the iron exchange was indirectly elucidated. Synthetic Cr-DFO (inert metal FO analog) and Ga-DFO (nonreducible FO analog) are bound to YxeB and imported via YxeB and the corresponding permeases and ATPase. YxeB exchanges Fe(III) from FO and Ga(III) from Ga-DFO to DFO bound to the protein, indicating that the metal-exchange occurs without metal reduction. YxeB also binds DFO derivatives including acetylated DFO (apo-siderophore) and acetylated FO (AcFO, Fe-siderophore). The iron from AcFO is transferred to DFO when bound to YxeB, giving direct evidence of iron exchange. Moreover, YxeB also uses the displacement mechanism when ferrichrome (Fch) is added to the DFO:YxeB complex. Uptake by the displacement mechanism is a minor pathway compared to the shuttle mechanism.
铁是所有生物体必需的元素，微生物会产生小分子铁螯合剂（铁载体）以有效获取Fe(III)。革兰氏阳性菌在细胞膜上拥有脂蛋白铁载体结合蛋白（SBP）。一些SBP既能结合脱铁铁载体（无铁）又能结合铁-铁载体（铁螯合），且只导入铁-铁载体。当SBP最初结合脱铁铁载体时，SBP利用革兰氏阳性菌铁载体穿梭机制（SBP将Fe(III)从铁-铁载体交换到与蛋白质结合的脱铁铁载体上）和/或置换机制（与SBP结合的脱铁铁载体被释放，然后铁-铁载体与蛋白质结合）来导入铁-铁载体。此前，我们报道过蜡样芽孢杆菌的SBP，即YxeB，利用铁载体穿梭机制将Fe(III)从铁胺B（FO）交换到与YxeB结合的去铁胺B（DFO）上，尽管铁交换是间接阐明的。合成的Cr-DFO（惰性金属FO类似物）和Ga-DFO（不可还原FO类似物）与YxeB结合并通过YxeB以及相应的通透酶和ATP酶导入。YxeB将FO中的Fe(III)和Ga-DFO中的Ga(III)交换到与蛋白质结合的DFO上，表明金属交换在无金属还原的情况下发生。YxeB还能结合DFO衍生物，包括乙酰化DFO（脱铁铁载体）和乙酰化FO（AcFO，铁-铁载体）。当与YxeB结合时，AcFO中的铁会转移到DFO上，这为铁交换提供了直接证据。此外，当向DFO:YxeB复合物中添加铁色素（Fch）时，YxeB也会使用置换机制。与穿梭机制相比，通过置换机制的摄取是一条次要途径。
