|MadSci Network: Biochemistry|
Human transferrin binds iron with a dissociation constant of approximately 1022 M-1. Ferric iron binds to transferrin *only* together with an anion (usually carbonate); without this cofactor, iron binding to transferrin is nearly null. It is suspected that the binding of carbonate takes place prior to binding of iron. Binding of iron(III) to transferrin is so incredibly tight, that a special mechanism exists for its release ; this is accomplished through acidification (which presumably protonates the counterion carbonate), followed by reduction of Fe(III) into Fe(II). There are biological reasons for why iron-transferrin complexes are so tight; I assume you have already studied the literature covering these reasons.
My guess as to why the tyrosines are conserved -- phenolic oxygen is a particularly nice chelant for iron(III). A multitude of synthetic iron chelators has been discovered, and phenols or phenol-like moieties are often their key components. Since there is only one natural amino acid with a phenolic oxygen, tyrosine becomes the only choice if extremely tight chelation of Fe(III) is necessary.
Hopefully this helps.
Artem G. Evdokimov
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