Metal-Binding Thermodynamics of the Histidine-Rich Sequence from the Metal-Transport Protein IRT1 of Arabidopsis thaliana

The widespread ZIP family of transmembrane metal-transporting proteins is characterized by a large intracellular loop that contains a histidine-rich sequence whose biological role is unknown. To provide a chemical basis for this role, we prepared and studied a peptide corresponding to this sequence from the first iron-regulated transporter (IRT1) of Arabidopsis thaliana, which transports Fe²⁺ as well as Mn²⁺, Co²⁺, Zn²⁺, and Cd²⁺. Isothermal titration calorimetry (ITC) measurements, which required novel experiments and data analysis, and supporting spectroscopic methods were used to quantify IRT1's metal-binding affinity and associated thermodynamics. The peptide, PHGHGHGHGP, binds metal ions with 1:1 stoichiometry and stabilities that are consistent with the Irving−Williams series. Comparison of the metal-binding thermodynamics of the peptide with those of trien provides new insight about enthalpic and entropic contributions to the stability of the metal−peptide complex. Although Fe²⁺ and other IRT1-transported metal ions do not bind very tightly, this His-rich sequence has a very high entropy-driven affinity for Fe³⁺, which may have biological significance.