title: Solvation and the hidden thermodynamics of a zinc finger probed by nonstandard repair of a protein crevice.
abstract: The classical Zn finger contains a phenylalanine at the crux of its three architectural elements: a beta-hairpin, an alpha-helix, and a Zn(2+)-binding site. Surprisingly, phenylalanine is not required for high-affinity Zn2+ binding, but instead contributes to the specification of a precise DNA-binding surface. Substitution of phenylalanine by leucine leads to a floppy but native-like structure whose Zn affinity is maintained by marked entropy-enthalpy compensation (DeltaDeltaH -8.3 kcal/mol and -TDeltaDeltaS 7.7 kcal/mol). Phenylalanine and leucine differ in shape, size, and aromaticity. To distinguish which features correlate with dynamic stability, we have investigated a nonstandard finger containing cyclohexanylalanine at this site. The structure of the nonstandard finger is similar to that of the native domain. The cyclohexanyl ring assumes a chair conformation, and conformational fluctuations characteristic of the leucine variant are damped. Although the nonstandard finger exhibits a lower affinity for Zn2+ than does the native domain (DeltaDeltaG -1.2 kcal/mol), leucine-associated perturbations in enthalpy and entropy are almost completely attenuated (DeltaDeltaH -0.7 kcal/mol and -TDeltaDeltaS -0.5 kcal/mol). Strikingly, global changes in entropy (as inferred from calorimetry) are in each case opposite in sign from changes in configurational entropy (as inferred from NMR). This seeming paradox suggests that enthalpy-entropy compensation is dominated by solvent reorganization rather than nominal molecular properties. Together, these results demonstrate that dynamic and thermodynamic perturbations correlate with formation or repair of a solvated packing defect rather than type of physical interaction (aromatic or aliphatic) within the core.
authors.affiliation: Department of Biochemistry, Case Western Reserve University, Cleveland, Ohio 44106-4935, USA.
authors.forename: Marcel J
authors.initials: MJ
authors.lastname: Lachenmann
authors.forename: John E
authors.initials: JE
authors.lastname: Ladbury
authors.forename: Xiuqi
authors.initials: X
authors.lastname: Qian
authors.forename: Kun
authors.initials: K
authors.lastname: Huang
authors.forename: Rupinder
authors.initials: R
authors.lastname: Singh
authors.forename: Michael A
authors.initials: MA
authors.lastname: Weiss
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D006801:Humans; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010649:Phenylalanine / Q000737:chemistry; D011506:Proteins / Q000737:chemistry; D012997:Solvents / Q000737:chemistry*; D013816:Thermodynamics; D016335:Zinc Fingers
chemical_list: D011506:Proteins; D012997:Solvents; D010649:Phenylalanine