While microbes often transport iron with the help of siderophores (cf. iron uptake), higher organisms transport this important element in the blood using special proteins (transferrins). Due to their extremely efficient binding of iron (stronger than bacterial siderophores), some transferrins have been observed to have antibacterial properties (iron deficiency!). For example, this is the reason for the antibiotic properties of lactoferrin, a transferrin found in breast milk.
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The iron is coordinated by four amino acid residues (1 x histidine, 1 x aspartic acid and 2 x tyrosine) and a bidentate HCO3- ion, the position of which is stabilized by interaction with a threonine and an arginine side chain of the protein backbone.
Coordination of the Fe3+ ion in one of the domains