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Anion binding to ubiquitin and its relevance to the Hofmeister effects.

ABSTRACT: Although the non-covalent interactions between proteins and salts contributing to the Hofmeister effects have been generally mapped, there are many questions regarding the specifics of these interactions. We report here studies involving the small protein ubiquitin and salts of polarizable anions. These studies reveal a complex interplay between the reverse Hofmeister effect at low pH, the salting-in Hofmeister effect at higher pH, and six anion binding sites in ubiquitin at the root of these phenomena. These sites are all located at protuberances of preorganized secondary structure, and although stronger at low pH, are still apparent when ubiquitin possesses no net charge. These results demonstrate the traceability of these Hofmeister phenomena and suggest new strategies for understanding the supramolecular properties of proteins.

SUBMITTER: Yao W

PROVIDER: S-EPMC8178748 | biostudies-literature | 2020 Nov

REPOSITORIES: biostudies-literature

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Anion binding to ubiquitin and its relevance to the Hofmeister effects.

Yao Wei W Wang Kaiyu K Wu Aide A Reed Wayne F WF Gibb Bruce C BC

Chemical science 20201104 1

Although the non-covalent interactions between proteins and salts contributing to the Hofmeister effects have been generally mapped, there are many questions regarding the specifics of these interactions. We report here studies involving the small protein ubiquitin and salts of polarizable anions. These studies reveal a complex interplay between the reverse Hofmeister effect at low pH, the salting-in Hofmeister effect at higher pH, and six anion binding sites in ubiquitin at the root of these ph ...[more]

PMID: 34163600

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Project description:Quantitative interpretation and prediction of Hofmeister ion effects on protein processes, including folding and crystallization, have been elusive goals of a century of research. Here, a quantitative thermodynamic analysis, developed to treat noncoulombic interactions of solutes with biopolymer surface and recently extended to analyze the effects of Hofmeister salts on the surface tension of water, is applied to literature solubility data for small hydrocarbons and model peptides. This analysis allows us to obtain a minimum estimate of the hydration b1 (H2O A(-2)), of hydrocarbon surface and partition coefficients Kp, characterizing the distribution of salts and salt ions between this hydration water and bulk water. Assuming that Na+ and SO4(2-) ions of Na2SO4 (the salt giving the largest reduction in hydrocarbon solubility as well as the largest increase in surface tension) are fully excluded from the hydration water at hydrocarbon surface, we obtain the same b1 as for air-water surface (approximately 0.18 H2O A(-2)). Rank orders of cation and anion partition coefficients for nonpolar surface follow the Hofmeister series for protein processes, but are strongly offset for cations in the direction of exclusion (preferential hydration). By applying a coarse-grained decomposition of water accessible surface area (ASA) into nonpolar, polar amide, and other polar surface and the same hydration b1 to interpret peptide solubility increments, we determine salt partition coefficients for amide surface. These partition coefficients are separated into single-ion contributions based on the observation that both Cl- and Na+ (also K+) occupy neutral positions in the middle of the anion and cation Hofmeister series for protein folding. Independent of this assignment, we find that all cations investigated are strongly accumulated at amide surface while most anions are excluded. Cation and anion effects are independent and additive, allowing successful prediction of Hofmeister salt effects on micelle formation and other processes from structural information (ASA).

Project description:Selective recognition and removal of sulfate and phosphates from aqueous media in the presence of highly competing anions is very demanding because of their biological and environmental implications. In this paper, we present the anion recognition approach for the selective and efficient extraction of sulfate by nitrophenyl-functionalized tris-urea receptors (L1-L2) from highly competitive aqueous media with an equivalent concentration of nitrate and other anions. Tetrabutylammonium hydroxide has been used for the first time as a phase transfer anionic extractant for sulfate-exchange from the aqueous phase to the organic phase (dichloromethane) containing a tris-urea receptor (L1-L3). The sulfate extraction efficacy of L2 (≈84-90%) was observed to be higher than those of L1 (≈76-82%) and L3 (≈68-75%) in competitive extraction experiments. In contrast, an analogous nitrophenyl-functionalized tris-thiourea receptor (L4) has been recognized for the selective and efficient extraction of phosphates from aqueous media in the presence of several competing anions including sulfate and nitrate, with ≈85-92% extraction efficiency. In this case, tetrabutylammonium acetate has been used as a phase transfer anionic extractant for phosphate exchange between the two immiscible phases. Due to the higher acidity of tris-thiourea -NH groups in comparison to the analogous tris-urea, tetrabutylammonium hydroxide could deprotonate a hydrogen bond donating -NH group of the thiourea receptor and phosphate extraction was observed to be inefficient in such a case. Several liquid-liquid extraction (LLE) experiments have been carried out to establish the selective removal of sulfate and phosphates by the tripodal receptors from competitive aqueous media having different combinations of two or more anions. The LLE products obtained from organic phases were characterized by NMR (1H, 13C, 31P, and 19F) spectroscopy to affirm the oxoanion selectivity of the receptors and purity of the complexes. The tripodal receptors can easily be recycled for successive extraction cycles by simply washing the LLE products (oxoanion complexes) with a methanol-water (1 : 1, v/v) solvent system followed by filtration.

Project description:Host-mediated liquid-liquid extraction is a convenient method for the separation of inorganic salts. However, selective extraction of an anion, regardless of its hydrophilicity or lipophilicity as qualitatively described by its place in the Hofmeister series, remains challenging. Herein we report the complete disruption of the Hofmeister-based ordering of anions in host-mediated extraction by a rigidified tweezer-type receptor possessing remarkably strong anion-binding affinity under the conditions examined. Experiments introduce a convenient new method for determination of anion binding using phosphorus inductively coupled plasma mass spectrometry (ICP-MS) to measure extraction of tetra-n-butylphosphonium (TBP+) salts from water into nitrobenzene, specifically examining the disrupting effect of the added arylethynyl bisurea anion receptor. In the absence of the receptor, the salt partitioning follows the expected Hofmeister-type ordering favoring the larger, less hydrated anions; the analysis yields the value -24 kJ mol-1 for the standard Gibbs energy of partitioning of TBP+ cation from water into nitrobenzene at 25 °C. Selectivity is markedly changed by the addition of receptor to the nitrobenzene and is concentration dependent, giving rise to three selectivity regimes. We then used SXLSQI liquid-liquid equilibrium analysis software developed at Oak Ridge National Laboratory to fit host-mediated extraction equilibria for TBP+ salts of Cl-, Br-, I-, and NO3- to the distribution data. While the reverse-Hofmeister 1 : 1 binding of the anions by the receptor effectively cancels the Hofmeister selectivity of the TBPX partitioning into nitrobenzene, formation of unexpected 2 : 1 receptor : anion complexes favoring Cl- and Br- dominates the selectivity at elevated receptor concentrations, producing the unusual order Br- > Cl- > NO3- > I- in anion distribution wherein a middle member of the series is selected and the most lipophilic anion is disfavored. Density functional theory calculations confirmed the likelihood of forming 2 : 1 complexes, where Cl- and Br- are encapsulated by two receptors adopting energetically competitive single or double helix structures. The calculations explain the rare non-Hofmeister preference for Br-. This example shows that anion receptors can be used to control the selectivity and efficiency of salt extraction regardless of the position of the anion in the Hofmeister series.

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Anion binding to ubiquitin and its relevance to the Hofmeister effects.

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Anion binding to ubiquitin and its relevance to the Hofmeister effects.

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