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Molecular mechanisms of xenosiderophore and heme acquisitions using periplasmic binding protein (PBP) dependent ATP-binding cassette transporters to scavenge the essential nutrient iron are elusive yet in  Vibrio cholerae . Our current study delineates the structures, dynamics and ligand binding properties of two Type III PBPs of  V. cholerae, Vc FhuD and  Vc HutB. Through crystal structures and fluorescence quenching studies we demonstrate unique features of  Vc FhuD to bind both hydroxamate and catecholate type xenosiderophores. Like  E. coli  FhuD,  Vc FhuD binds ferrichrome and ferri-desferal using conserved Tryptophans and R102. However, unlike  Ec FhuD, slightly basic ligand binding pocket of  Vc FhuD could favour ferri-enterobactin binding with plausible participation of R203, along with R102, like it happens in catecholate binding PBPs. Structural studies coupled with spectrophotometric and native PAGE analysis indicated parallel binding of two heme molecules to  Vc HutB in a pH dependent manner, while mutational analysis established the relative importance of Y65 and H164 in heme binding. MD simulation studies exhibited an unforeseen inter-lobe swinging motion in Type III PBPs, magnitude of which is inversely related to the packing of the linker helix with its neighboring helices. Small inter-lobe movement in  Vc FhuD or dramatic twisting in  Vc HutB is found to influence ligand binding.

Structure and dynamics of Type III periplasmic proteins VcFhuD and VcHutB reveal molecular basis of their distinctive ligand binding properties