IRON-MINERAL CORE

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Introduction

In this experiment, the ferritin contains an iron-mineral core [FeO(OH)]₈[FeO(H₂PO₄)]. The iron-mineral core is attached to the inside of the protein wall (as depicted in Scheme I of the main section of the tutorial). It is bound covalently to the carboxylate sidechains on the protein wall. The protein is capable of storing as many as 4500 iron atoms in its interior, giving a concentration equivalent to 0.25M Fe inside the protein. (Recall, at pH=7, [Fe³⁺] = 10¹⁸ M due to formation of insoluble hydroxides.)

Questions: In your experiment, what amount of iron did the ferritin sample contain? Was the sample saturated?

Molecular Model of the Ferritin Protein with Mineral Core

Figure 1: This is a molecular model of the ferritin protein with a representation of the iron-mineral core. The view looks down the 4-fold channel at the mineral core. The subunits that comprise the 4-fold channel are represented as ribbons and the ribbons have the same color code as the ribbons in Figure 1 in the main section of the tutorial. The rest of the subunits are shown in the stick representation. The iron-mineral core is depicted as rust-colored. This model shows only half of the ferritin shell. Recall that the mineral core is connected to the protein shell with covalent bonds to carboxylate residues. However, for a clearer view of the three-dimensionality, the mineral core is not shown to be connected to the protein shell in this picture.

Model Compound for Iron-mineral Core

Until recently, it was thought that all ferritin cores were microcrystalline and identical. However, ferritin cores from a variety of sources have now been studied using a variety of experimental techniques (i.e., x-ray absorption spectroscopy, Mossbauer spectroscopy, and high-resolution electron microscopy) and a number of variations in the degree of structural and magnetic ordering and level of hydration has been shown. One simplified model compound of the iron-mineral core that has been developed is [Fe₁₂O₂(OCH₃)₁₈(O₂CCH₃)₆(CH₃OH)_4.67].

Figure 1: This is a stick representation of the simplified model compound for the iron-mineral core [Fe12O2(OCH3)18(O2CCH3)6(CH3OH)4.67]. The reference for this model compound is: K.L. Taft, et. al., Science, 259, 1302 (1993). "A Mixed-Valent Polyiron Oxo Complex that Models the Biomineralization of the Ferritin Core." Note: The carbon atoms are green, the hydrogens are white, the iron atoms are magenta, and the oxygen atoms are red in the stick representation.

Questions concerning the mineral core

1. Please list the 3 major differences between the model compound and the actual mineral core.
2. Recall that the iron-mineral core is not freely floating within the protein shell. The core binds to the carboxylate sidechains on the protein wall. What type of bond is formed (covalent or ionic)?

Return To Iron-removal Process Section of Tutorial

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