Interaction of choline salts with artificial biological membranes: DSC studies elucidating cellular interactions.

To better understand the relationship between the relative cytotoxicity of diluted ionic liquids and their specific interaction with biological membranes, the thermotropic behavior of model lipid membrane systems formulated in a series of choline based organic salts was investigated. Unilamellar vesicles prepared from dipalmitoylphosphatidylcholine were exposed to a series of choline phosphate salts at a concentration of 10mM at pH7.40, and the gel to liquid-crystalline state transition was examined using differential scanning calorimetry. The choline salts that were observed to have a low relative toxicity in previous studies induced minimal changes in the lipid phase transition behavior of these model membranes. In contrast, the salts choline bis(2,4,4-trimethylpentyl)phosphinate and choline bis(2-ethylhexyl)phosphate, both of which were observed to have high relative toxicity, caused distinct disruptions in the lipid phase transition behavior, consistent with penetration of the salts into the acyl chains of the phospholipids. choline bis(2,4,4-trimethylpentyl)phosphinate reduced the Tm and enthalpy of the main transition of dipalmitoylphosphatidylcholine while choline bis(2-ethylhexyl)phosphate induced the equilibration of alternate phases.

Structure and function of proteins in hydrated choline dihydrogen phosphate ionic liquid.

Ionic liquids are being intensely studied as promising media for the stabilization of proteins and other biomolecules. Choline dihydrogen phosphate (CDHP) has been identified as one of the most promising candidates for this application. In this work we have probed in more detail the effects that CDHP may have on the thermodynamics, structure, and stability of proteins, including one of therapeutic interest. Microcalorimetry and circular dichroism spectropolarimetry (CD) were used to assess the thermal stability of protein solutions in CDHP/water mixtures at various concentrations. Increasing thermal stability of lysozyme and interleukin-2 in proportion to CDHP concentration was observed. Isothermal titration calorimetry (ITC) was used to quantify binding interactions, and indicate that the mechanism for stability does not appear to be dependent upon CDHP binding to protein. CD and small angle X-ray scattering (SAXS) analyses were used to probe for structural changes due to the presence of CDHP. SAXS indicates charge effects on the surface of the protein play a role in protein stability in ionic liquids, and no significant alteration of the overall tertiary conformation of lysozyme was observed at 25 °C. However, after incubation at 37 °C or at higher concentrations of CDHP, small changes in protein structure were seen. Effects on protein activity were monitored using turbidity assays, and CDHP decreases protein activity but does not eliminate it. Protein solubility was also monitored using a turbidity assay and was found to be inversely proportional to the concentration of CDHP in solution.

Role of citrate and phosphate anions in the mechanism of iron(III) sequestration by ferric binding protein: kinetic studies of the formation of the holoprotein of wild-type FbpA and its engineered mutants.

Ferric binding protein A (FbpA) plays a central role in the iron acquisition processes of pathogenic Neisseria gonorrheae, Neisseria meningitidis, and Haemophilus influenzae. FbpA functions as an iron shuttle within the periplasmic space of these Gram-negative human pathogens. Iron is picked up by FbpA at the periplasmic aspect of the outer membrane with concomitant acquisition of a synergistic anion. Here we report the kinetics and mechanisms involved with loading of iron(III) into iron-free FbpA using iron(III) citrate as an iron source in the presence of excess citrate or phosphate (physiologically available anions) at pH 6.5. In the presence of excess phosphate, iron(III) citrate loads into apo-FbpA in three kinetically distinguishable steps, while in the presence of excess citrate, only two steps are discernible. A stable intermediate containing iron(III) citrate-bound FbpA is observed in each case. The observation of an additional kinetic step and moderate increase in apparent rate constants suggests an active role for phosphate in the iron insertion process. To further elucidate a mechanism for iron loading, we report on the sequestration kinetics of iron(III) citrate in the presence of phosphate with binding site mutant apo-FbpAs, H9E, E57D, E57Q, Q58A, Y195F, and Y196H. Tyrosine mutations drastically alter the kinetics and hamper iron sequestration ability. H9E, E57D, and E57Q have near native iron sequestration behavior; however, iron binding rates are altered, enabling assignment of sequential side chain interactions. Additionally, this investigation elaborates on the function of FbpA as a carrier for iron chelates as well as "naked" or free iron as originally proposed.

In vitro assessment of choline dihydrogen phosphate (CDHP) as a vehicle for recombinant human interleukin-2 (rhIL-2).

Choline dihydrogen phosphate (CDHP) is an ionic liquid reported to increase thermal stability of model proteins. The current work investigated CDHP effect on structural integrity and biological activity of recombinant human interleukin-2 (rhIL-2), a therapeutic protein used for treating advanced melanoma. In vitro CDHP biocompatibility was also evaluated using primary cell cultures, or B16-F10 cell line, chronically exposed to the ionic liquid. Formulation of rhIL-2 in an aqueous 680mM CDHP pH 7.4 solution resulted in a 12.5°C increase in the Tm of rhIL-2 compared to a basic buffer formulation, and provided conformational rhIL-2 stabilization when the solution was heated to 23.3°C above the Tm. CDHP solutions (≤80mM), exhibited no cytotoxic activity toward primary splenocytes or B16-F10 cells in culture. However, a 10-fold loss in biological activity was observed when rhIL-2 was used in a 30mM CDHP aqueous solution with NaHCO3 (pH≥7.2) compared to controls without CDHP. While increased Tm is associated with a diminished rhIL-2 biological activity, the therapeutic protein remains structurally intact and functional.

Ionic liquid "buffers"-pH control in ionic liquid systems.

We describe hydrated ionic liquids which have an intrinsic proton buffering action and hence an ability to control proton activity in an ionic medium.

Hijacking transferrin bound iron: protein-receptor interactions involved in iron transport in N. gonorrhoeae.

Neisseria gonorrhoeae has the capacity to acquire iron from its human host by removing this essential nutrient from serum transferrin. The transferrin binding proteins, TbpA and TbpB constitute the outer membrane receptor complex responsible for binding transferrin, extracting the tightly bound iron from the host-derived molecule, and transporting iron into the periplasmic space of this Gram-negative bacterium. Once iron is transported across the outer membrane, ferric binding protein A (FbpA) moves the iron across the periplasmic space and initiates the process of transport into the bacterial cytosol. The results of the studies reported here define the multiple steps in the iron transport process in which TbpA and TbpB participate. Using the SUPREX technique for assessing the thermodynamic stability of protein-ligand complexes, we report herein the first direct measurement of periplasmic FbpA binding to the outer membrane protein TbpA. We also show that TbpA discriminates between apo- and holo-FbpA; i.e. the TbpA interaction with apo-FbpA is higher affinity than the TbpA interaction with holo-FbpA. Further, we demonstrate that both TbpA and TbpB individually can deferrate transferrin and ferrate FbpA without energy supplied from TonB resulting in sequestration by apo-FbpA.

Ex vivo analysis of synergistic anion binding to FbpA in Gram-negative bacteria.

Ferric binding protein, FbpA, is a member of the transferrin superfamily whose function is to move an essential nutrient, iron, across the periplasm and into the cytosol through formation of a ternary complex containing Fe (3+) and a synergistic anion, X. Here we utilize SUPREX ( stability of unpurified proteins from rates of H/D exchange) to determine the identification and distribution of the synergistic anion in FeFbpA-X species in periplasmic preparations from Gram-negative bacteria. SUPREX is a mass spectrometry-based technique uniquely suited for thermodynamic analyses of protein-ligand complexes in complex biological mixtures such as periplasmic preparations. Model binary mixtures of FeFbpA-Cit and FeFbpA-PO 4 were initially characterized by SUPREX due to the likely presence of citrate and phosphate ions in the periplasm. Ex vivo SUPREX analyses were performed on FeFbpA-X species overexpressed in an Escherichia coli cell line and on endogenous FeFbpA-X species in Neisseria gonorrheae. Detected in the E. coli periplasmic extract were two distinct populations of FbpA, including one in which the protein was unliganded (i.e., apoFbpA) and one in which the protein was bound to iron and the synergistic anion, phosphate (i.e., FeFbpA-PO 4). FeFbpA-PO 4 was the only population of FbpA molecules detected in the N. gonorrheae periplasmic extract. This work provides the first determination of the identity of the in vivo anion bound to FeFbpA-X in the periplasm and substantiates the hypothesis that the synergistic anion plays a structural and functional role in FbpA-mediated transport of iron across the periplasm and into the cytosol.

Ga3+ as a mechanistic probe in Fe3+ transport: characterization of Ga3+ interaction with FbpA.

The obligate human pathogens Haemophilus influenzae, Neisseria gonorrhoeae, and N. meningitidis utilize a highly conserved, three-protein ATP-binding cassette transporter (FbpABC) to shuttle free Fe(3+) from the periplasm and across the cytoplasmic membrane. The periplasmic binding protein, ferric binding protein (FbpA), is capable of transporting other trivalent cations, including Ga(3+), which, unlike Fe(3+), is not redox-active. Because of a similar size and charge as Fe(3+), Ga(3+) is widely used as a non-redox-active Fe(3+) substitute for studying metal complexation in proteins and bacterial populations. The investigations reported here elucidate the similarities and differences in FbpA sequestration of Ga(3+) and Fe(3+), focusing on metal selectivity and the resulting transport function. The thermodynamic binding constant for Ga(3+) complexed with FbpA at pH 6.5, in 50 mM 4-morpholineethanesulfonic acid, 200 mM KCl, 5 mM KH(2)PO(4) was determined by UV-difference spectroscopy as log K'eff=13.7+/-0.6. This represents a 10(5)-fold weaker binding relative to Fe(3+) at identical conditions. The unfolding/refolding behavior of Ga(3+) and Fe(3+) holo-FbpA were also studied using a matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy technique, stability of unpurified proteins from rates of H/D exchange (SUPREX). This analysis indicates significant differences between Fe(3+) and Ga(3+) sequestration with regard to protein folding behavior. A series of kinetic experiments established the lability of the Ga(3+)FbpA-PO(4) assembly, and the similarities/differences of stepwise loading of Fe(3+) into apo- or Ga(3+)-loaded FbpA. These biophysical characterization data are used to interpret FbpA-mediated Ga(3+) transport and toxicity in cell culture studies.

The Haemophilus influenzae hFbpABC Fe3+ transporter: analysis of the membrane permease and development of a gallium-based screen for mutants.

The obligate human pathogen Haemophilus influenzae utilizes a siderophore-independent (free) Fe(3+) transport system to obtain this essential element from the host iron-binding protein transferrin. The hFbpABC transporter is a binding protein-dependent ABC transporter that functions to shuttle (free) Fe(3+) through the periplasm and across the inner membrane of H. influenzae. This investigation focuses on the structure and function of the hFbpB membrane permease component of the transporter, a protein that has eluded prior characterization. Based on multiple-sequence alignments between permease orthologs, a series of site-directed mutations targeted at residues within the two conserved permease motifs were generated. The hFbpABC transporter was expressed in a siderophore-deficient Escherichia coli background, and effects of mutations were analyzed using growth rescue and radiolabeled (55)Fe(3+) transport assays. Results demonstrate that mutation of the invariant glycine (G418A) within motif 2 led to attenuated transport activity, while mutation of the invariant glycine (G155A/V/E) within motif 1 had no discernible effect on activity. Individual mutations of well-conserved leucines (L154D and L417D) led to attenuated and null transport activities, respectively. As a complement to site-directed methods, a mutant screen based on resistance to the toxic iron analog gallium, an hFbpABC inhibitor, was devised. The screen led to the identification of several significant hFbpB mutations; V497I, I174F, and S475I led to null transport activities, while S146Y resulted in attenuated activity. Significant residues were mapped to a topological model of the hFbpB permease, and the implications of mutations are discussed in light of structural and functional data from related ABC transporters.

SUPREX (Stability of Unpurified Proteins from Rates of H/D Exchange) analysis of the thermodynamics of synergistic anion binding by ferric-binding protein (FbpA), a bacterial transferrin.

SUPREX (stability of unpurified proteins from rates of H/D exchange) is a H/D exchange- and matrix-assisted laser desorption/ionization (MALDI)-based technique for characterizing the equilibrium unfolding/refolding properties of proteins and protein-ligand complexes. Here, we describe the application of SUPREX to the thermodynamic analysis of synergistic anion binding to iron-loaded ferric-binding protein (Fe(3+)FbpA-X, X = synergistic anion). The in vivo function of FbpA is to transport unchelated Fe(3+) across the periplasmic space of certain Gram-negative bacteria, a process that requires simultaneous binding of a synergistic anion. Our results indicate that Fe(3+)FbpA-X is not a so-called "ideal" protein system for SUPREX analyses because it does not exhibit two-state folding properties and it does not exhibit EX2 H/D exchange behavior. However, despite these nonideal properties of the Fe(3+)FbpA-X protein-folding/unfolding reaction, we demonstrate that the SUPREX technique is still amenable to the quantitative thermodynamic analysis of synergistic anion binding to Fe(3+)FbpA. As part of this work, the SUPREX technique was used to evaluate the DeltaDeltaG(f) values of four synergistic anion-containing complexes of Fe(3+)FbpA (i.e., Fe(3+)FbpA-PO(4), Fe(3+)FbpA-citrate, Fe(3+)FbpA-AsO(4), and Fe(3+)FbpA-SO(4)). The DeltaDeltaG(f) value obtained for Fe(3+)FbpA-citrate relative to Fe(3+)FbpA-PO(4) (1.45 +/- 0.44 kcal/mol), is in good agreement with that reported previously (1.98 kcal/mol). The value obtained for Fe(3+)FbpA-AsO(4) (0.58 +/- 0.45 kcal/mol) was also consistent with that reported previously (0.68 kcal/mol), but the measurement error is very close to the magnitude of the value. This work (i) demonstrates the utility of the SUPREX method for studying anion binding by FbpA, (ii) provides the first evaluation of a DeltaDeltaG(f) value for Fe(3+)FbpA-SO(4), -1.43 +/- 0.17 kcal/mol, and (iii) helps substantiate our hypothesis that the synergistic anion plays a role in controlling the lability of iron bound to FbpA in the transport process.