CzrA, the Staphylococcus aureus zinc-responsive transcriptional repressor, is a paradigm of dynamically driven allostery: Zn(II) binding inhibits DNA binding with little structural change, through a redistribution of entropy. Calorimetry shows that Zn(II) binding carries a large favorable solvent entropy that cannot be predicted from crystal structures and is strongly perturbed by mutations that alter residue-specific internal dynamics, yet the molecular identity of this solvent contribution is unresolved. The task is to obtain an atomistic, site-resolved description: where in the CzrA homodimer solvation shell the waters that contribute most to the solvent entropy change reside, what their site-specific dynamical properties (e.g., residence lifetimes, long-lived versus exchangeable) are, and how residue-level internal dynamics couple to local hydration. A 2023 follow-up by the same group provided a first partial localization (distributed desolvation predominantly at the dimer interface, linked to Zn quenching of a solvated apo-closed conformation) but explicitly could not determine residence times or connect the spectroscopic information quantitatively to thermodynamics.

Background

Allostery is the regulation phenomenon in which binding of a ligand at one site on a protein influences the binding of another ligand at a distant site. CzrA (chromosomal zinc-resistant operon repressor) is a homodimeric zinc-sensing transcriptional repressor from Staphylococcus aureus and a paradigm member of the ArsR (arsenic repressor) superfamily of metalloregulators. Each subunit carries a winged helix-turn-helix DNA-binding domain; Zn(II) binds at two interfacial sites bridging the C-terminal α5\alpha5 helices and allosterically inhibits DNA binding, thereby derepressing zinc-efflux genes. Isothermal titration calorimetry (ITC), which measures the heat released upon titrating Zn(II) into protein at several temperatures, yields the binding enthalpy, its temperature derivative ΔCp\Delta C_p (the change in heat capacity), and hence the total binding entropy. The change in conformational entropy (the entropy associated with the distribution of protein internal conformations and motions) is estimated separately from NMR relaxation measurements of methyl-bearing side chains, which report order parameters that quantify the amplitudes of picosecond-to-nanosecond side-chain motions. CzrA exhibits dynamically driven allostery: Zn(II) binding causes no major structural rearrangement, and the allosteric coupling is instead mediated by a redistribution of conformational entropy over the dimer (Capdevila et al., PNAS 2017). The 2018 JACS study by Capdevila, Edmonds, Campanello, Wu, Gonzalez-Gutierrez and Giedroc dissected the remaining entropy: the total entropy of Zn(II) binding far exceeds the conformational contribution, implying a large favorable solvent entropy contribution from reorganization of the hydration shell (the layer of water molecules surrounding the protein). Because structure-based estimates from changes in accessible surface area fail for CzrA (the apo and Zn-bound crystal structures are nearly identical), the authors introduced an operational estimate, −TΔSSOLV=−T[ΔCp,SOLVln⁡(T/TS)]-T\Delta S_{\mathrm{SOLV}} = -T[\Delta C_{p,\mathrm{SOLV}} \ln(T/T_S)] with TS=395T_S = 395 K, valid to the extent that non-solvation contributions to ΔCp\Delta C_p are small and a single TST_S applies across protein variants. Two observations motivate the open question. First, allosterically impaired mutants show strongly perturbed solvent entropy that cannot be predicted from their structures, so the entropy-relevant waters have no visible structural marker. Second, methods for site-specific water dynamics in protein hydration shells had just become available, including water-protein nuclear Overhauser effects (wNOE, the dipolar magnetization transfer between protein protons and nearby water protons, and its rotating-frame counterpart wROE, whose ratio reports on the rotational mobility and lifetime of bound water), Overhauser dynamic nuclear polarization, ultrafast infrared spectroscopy, and grid-based solvation-theory analyses (GIST) of molecular dynamics trajectories. The authors proposed that a candidate solvent reservoir sits in the cleft between the C-terminal α5\alpha5 helices at the top of the homodimer, more populated in the apo-state ensemble and expelled upon Zn(II) coordination, and framed site-resolved water dynamics combined with NMR and calorimetry as the route to a complete mechanistic description of how internal dynamics and solvent reorganization jointly produce allostery in CzrA and related metalloregulatory proteins.

Problem Statement

For the Staphylococcus aureus CzrA homodimer, whose Zn(II)-mediated allosteric inhibition of DNA binding is dynamically driven with a large favorable solvent entropy contribution that cannot be predicted from crystal structures, determine at atomistic and residue-specific resolution: (i) where in the solvation shell the water molecules that contribute most to the solvent entropy change upon Zn(II) binding reside, and (ii) their site-specific dynamical properties, including whether they are long-lived or exchangeable and their residence lifetimes; and (iii) establish the residue-specific coupling mechanisms linking the Zn(II)-induced changes in methyl side-chain internal dynamics to changes in local hydration, testing the hypothesis that internal dynamics and solvent reorganization are directly coupled. These three parts (i)-(iii) are jointly required; a result on any one part alone does not resolve the task. Taken together, they must yield the overarching objective: a complete mechanistic, atomistic description of how internal dynamics and solvent reorganization jointly produce allosteric regulation in CzrA, with the site-resolved hydration results reconciled quantitatively with the calorimetric and NMR-derived thermodynamics.

References

  1. Capdevila DA, Edmonds KA, Campanello GC, Wu H, Gonzalez-Gutierrez G, Giedroc DP. Functional Role of Solvent Entropy and Conformational Entropy of Metal Binding in a Dynamically Driven Allosteric System. Journal of the American Chemical Society 2018, 140, 9108-9119. DOI 10.1021/jacs.8b02129. https://doi.org/10.1021/jacs.8b02129
  2. Capdevila DA, Braymer JJ, Edmonds KA, Wu H, Giedroc DP. Entropy redistribution controls allostery in a metalloregulatory protein. Proceedings of the National Academy of Sciences USA 2017, 114, 4424-4429. DOI 10.1073/pnas.1620665114. https://doi.org/10.1073/pnas.1620665114
  3. Villarruel Dujovne M, Bringas M, Felli IC, Ravera E, Di Lella S, Capdevila DA. Introducing NMR strategies to define water molecules that drive metal binding in a transcriptional regulator. Journal of Magnetic Resonance Open 2023, 16-17, 100114. DOI 10.1016/j.jmro.2023.100114. https://doi.org/10.1016/j.jmro.2023.100114
  4. Rigoli M, Potestio R, Menichetti R. A Multiscale Analysis of the CzrA Transcription Repressor Highlights the Allosteric Changes Induced by Metal Ion Binding. Journal of Physical Chemistry B 2025, 129, 611-625. DOI 10.1021/acs.jpcb.4c06106. https://doi.org/10.1021/acs.jpcb.4c06106
  5. Gorman SD, Winston DS, Sahu D, Boehr DD. Different Solvent and Conformational Entropy Contributions to the Allosteric Activation and Inhibition Mechanisms of Yeast Chorismate Mutase. Biochemistry 2020, 59, 2528-2540. DOI 10.1021/acs.biochem.0c00277. https://doi.org/10.1021/acs.biochem.0c00277
  6. Capdevila DA, Rondon JJ, Edmonds KA, Rocchio JS, Villarruel Dujovne M, Giedroc DP. Bacterial Metallostasis: Metal Sensing, Metalloproteome Remodeling, and Metal Trafficking. Chemical Reviews 2024, 124, 13574-13659. DOI 10.1021/acs.chemrev.4c00264. https://doi.org/10.1021/acs.chemrev.4c00264
  7. Miller CA, Wankowicz SA. Binding Entropy Can Be Predicted by Crystallographic Ensembles. bioRxiv 2026. DOI 10.64898/2026.01.19.699807. https://doi.org/10.64898/2026.01.19.699807
  8. Antelo GT, Rondon JJ, Villarruel Dujovne M, Pis Diez CM, Cancian PG, Sastre S, Zeida A, Radi R, Wu H, Gonzalez-Gutierrez G, Giedroc DP, Capdevila DA. Evolution of allostery without shape shifting: Internal dynamics drives functional diversification of a transcriptional repressor superfamily. bioRxiv 2026. DOI 10.64898/2026.05.15.721447. https://doi.org/10.64898/2026.05.15.721447