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. 2013 Aug 7;110(34):13821–13826. doi: 10.1073/pnas.1304235110

Fig. 2.

Fig. 2.

Possible Fe-siderophore uptake systems in Gram-positive bacteria. (A) No siderophore is bound to the SBP. In the situation when Fe-siderophore is bound to the SBP, the siderophore is imported in cytoplasm. (B) On the other hand, when apo-siderophore is bound to the SBP, two Fe-siderophore uptake mechanisms, the Gram-positive siderophore-shuttle mechanism (C) or the displacement mechanism (D), are possible. (C) In the Gram-positive siderophore-shuttle mechanism, iron is transferred from an Fe-siderophore to the apo-siderophore:SBP complex and the new Fe-siderophore is then imported into the cytoplasm. It is not clear whether the resulting apo-siderophore remains bound to the SBP or not after iron-exchange has occurred. (D) In the displacement mechanism, the apo-siderophore is released from the SBP, and the Fe-siderophore occupies the binding pocket. No iron exchange occurs, and the original Fe-siderophore is transported to the cytoplasm.