Pathogenic strains of
Escherichia coli
and
Salmonella enterica
modify the tricatecholic siderophore enterobactin (Ent) by glucosylation of three aryl carbon atoms, a process controlled by the
iroA
locus [Hantke, K., Nicholson, G., Rabsch, W. & Winkelmann, G. (2003)
Proc. Natl. Acad. Sci. USA
100, 3677–3682]. Here, we report the purification of the IroB protein and its characterization as the Ent
C
-glucosyltransferase. IroB transfers glucosyl groups from uridine-5′-diphosphoglucose to C5 of one, two, or three of the 2,3-dihydroxybenzoyl units of Ent to yield monoglucosyl-
C
-Ent (MGE), diglucosyl-
C
-Ent (DGE), and triglucosyl-
C
-Ent (TGE). DGE, also known as salmochelin S4, and macrolactone-opened derivatives have been isolated from the culture broths of
S. enterica
and uropathogenic
E. coli
[Bister, B., Bischoff, D., Nicholson, G. J., Valdebenito, M., Schneider, K., Winkelmann, G., Hantke, K. & Sussmuth, R. D. (2004)
Biometals
17, 471–481], but MGE and TGE have not been reported previously. IroB has a
k
cat
of ≈10 min
-1
for the first C-glucosylation and is distributive, with sequential conversion and buildup of MGE and then DGE. The C5 to C1′ regio-selectivity of the 2,3-dihydroxybenzoyl-glucose linkage at all three rings of TGE suggests a C5 carbanion,
para
to the C2 phenolate oxygen, as the carbon nucleophile in this novel enzymatic
C
-glucosylation.
致病菌株的大肠杆菌和沙门氏菌通过对三个芳香碳原子进行葡萄糖基化的方式改变三羟基三酚类铁载体内肠菌素(Ent),这一过程由iroA基因位点控制。在这里,我们报告了IroB蛋白的纯化及其作为Ent C-葡萄糖基转移酶的特性。IroB将葡萄糖基从尿苷-5'-二磷酸葡萄糖转移至Ent的2,3-二羟基苯甲酰单元的C5,可产生单葡萄糖基-C-Ent(MGE)、双葡萄糖基-C-Ent(DGE)和三葡萄糖基-C-Ent(TGE)。DGE,也称为沙门氏菌素S4,和大环内酯开放衍生物已从沙门氏菌和尿路致病性大肠杆菌的培养基中分离出来,但MGE和TGE以前尚未报道。IroB对第一次C-葡萄糖基化的kcat约为10分钟^-1,是分布式的,随着MGE和DGE的顺序转化和积累。TGE三个环上2,3-二羟基苯甲酰-葡萄糖连接的C5到C1'区域选择性表明C5碳负离子,即C2酚氧的对位,是这种新型酶促C-葡萄糖基化的碳亲核试剂。