FhuD protein

Koster, W. and Braun, V. (1990). Iron(III)hydroxamate transport binding of ferrichrome to the periplasmic FhuD protein, J. Biol. Chem., 265, 21 407-21 410. 

In the ferrichrome transport system (Figure 3.4), FhuD is the periplasmic ferrichrome binding protein, FhuB is the intrinsic cytoplasmic membrane protein, which probably evolved by fusion of two genes, and FhuC is the cytoplasmic... 

Figure 11. Crystal structure of the hydroxamate binding protein FhuD from Escherichia coli complexed with gallichrome. (Reproduced by permission of H. J. Vogel and T. E. Clarke)...

Rohrbach, M. R., Braun, V. and Koster W. (1995). Ferrichrome transport in Escherichia coli K-12 altered substrate specificity of mutated periplasmic FhuD and interaction of FhuD with the integral membrane protein FhuB, J. Bacteriol., Ill, 7186-7193. 

FhuD delivers the ferric-siderophore complex to the FhuBC complex in the cytoplasmic membrane. FhuB is an intrinsic cytoplasmic membrane protein through which the iron complex can pass, driven by energy supplied by ATP hydrolysis catalyzed by the ATPase FhuC. Ferrous iron may be released from the hydroxamate via reduction by the reductase FhuF, which is loosely associated with the cytoplasmic membrane and, like FhuD, appears to have a lower specificity than FhuA and is active with coprogen and ferrichrome. ... 

After Fe + siderophores have docked at the binding pocket of the outer-membrane receptors of E. colt, translocation into the periplasmic space is mediated by the TonB complex. Once released into the periplasm, siderophores are rapidly bound by the specific periplasmic binding proteins FhuD (hydroxamate siderophores), FepB (enterobactin), and FecB (ferric dicitrate).FhuD, for example, exhibits a broad substrate specificity for a variety of hydroxamate siderophores including ferrichrome, coprogen, aerobactin, ferrioxamine B, shizokinen, rhodotorulic acid, and the antibiotic albomycin. The dissociation constants of FhuD with these siderophores range from 0.3 to 5.4 X-ray structures of FhuD... 

Figure 2 Siderophore-mediated iron-uptake systems in E. coli. Siderophore-iron complexes bind to transporter proteins located in the outer membrane (also known as OM), a barrier that is characteristic of Gram-negative bacteria. The region between the outer and inner is known as the periplasmic space. Specific carrier proteins such as FhuD transport iron fi om the outer membrane to the inner or cytosolic membrane (also known as CM). The TonB/ExbB/ExbD complex spans the inner and outer membranes and interacts with FepA, as shown, as well as all of the outer membrane receptors. The linkage that the TonB/ExbB/ExbD complex provides between the inner or cytosolic membrane to the outer membrane is thought to allow transmission of sufficient energy from the cytosol to drive siderophore-iron uptake across the outer membrane...

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