title: Metal-binding thermodynamics of the histidine-rich sequence from the metal-transport protein IRT1 of Arabidopsis thaliana.
abstract: 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.
authors.affiliation: Departments of Chemistry and Biological Sciences, Dartmouth College, Hanover, New Hampshire 03755, USA.
authors.forename: Nicholas E
authors.initials: NE
authors.lastname: Grossoehme
authors.forename: Shreeram
authors.initials: S
authors.lastname: Akilesh
authors.forename: Mary Lou
authors.initials: ML
authors.lastname: Guerinot
authors.forename: Dean E
authors.initials: DE
authors.lastname: Wilcox
mesh_terms: D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000737:chemistry* / Q000378:metabolism*; D001692:Biological Transport; D027682:Cation Transport Proteins / Q000737:chemistry* / Q000378:metabolism*; D002413:Cations, Divalent / Q000378:metabolism*; D006639:Histidine; D007700:Kinetics; D008670:Metals / Q000378:metabolism*; D010446:Peptide Fragments / Q000737:chemistry; D013816:Thermodynamics
chemical_list: D029681:Arabidopsis Proteins; D027682:Cation Transport Proteins; D002413:Cations, Divalent; C099651:IRT1 protein, Arabidopsis; D008670:Metals; D010446:Peptide Fragments; D006639:Histidine