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Thermodynamics of Zn2+ Binding to Cys2His2 and Cys2HisCys Zinc Fingers and a Cys4 Transcription Factor Site
- Rich, Anne M., Bombarda, Elisa, Schenk, Austin D., Lee, Paul E., Cox, Elizabeth
H., Spuches, Anne M., Hudson, Lynn D., Kieffer, Bruno, Wilcox, Dean E.
- Journal of the American Chemical Society 2012 v.134 no.25 pp. 10405-10418
- calorimetry, circular dichroism spectroscopy, cysteine, enthalpy, entropy, glucocorticoid receptors, histidine, ligands, myelin sheath, nuclear magnetic resonance spectroscopy, pH, peptides, protons, titration, transcription (genetics), zinc, zinc finger motif
- The thermodynamics of Zn²⁺ binding to three peptides corresponding to naturally occurring Zn-binding sequences in transcription factors have been quantified with isothermal titration calorimetry (ITC). These peptides, the third zinc finger of Sp1 (Sp1-3), the second zinc finger of myelin transcription factor 1 (MyT1-2), and the second Zn-binding sequence of the DNA-binding domain of glucocorticoid receptor (GR-2), bind Zn²⁺ with Cys₂His₂, Cys₂HisCys, and Cys₄ coordination, respectively. Circular dichroism confirms that Sp1-3 and MyT1-2 have considerable and negligible Zn-stabilized secondary structure, respectively, and indicate only a small amount for GR-2. The pKₐ’s of the Sp1-3 cysteines and histidines were determined by NMR and used to estimate the number of protons displaced by Zn²⁺ at pH 7.4. ITC was also used to determine this number, and the two methods agree. Subtraction of buffer contributions to the calorimetric data reveals that all three peptides have a similar affinity for Zn²⁺, which has equal enthalpy and entropy components for Sp1-3 but is more enthalpically disfavored and entropically favored with increasing Cys ligands. The resulting enthalpy–entropy compensation originates from the Zn-Cys coordination, as subtraction of the cysteine deprotonation enthalpy results in a similar Zn²⁺-binding enthalpy for all three peptides, and the binding entropy tracks with the number of displaced protons. Metal and protein components of the binding enthalpy and entropy have been estimated. While dominated by Zn²⁺ coordination to the cysteines and histidines, other residues in the sequence affect the protein contributions that modulate the stability of these motifs.