Interaction Of Monovalent Cations Research Articles

Improved polarizable force field for modeling Mg2+ binding to proteins and nucleotides.

Magnesium (Mg2+) is an indispensable regulatory cation for numerous cellular enzymes and membrane proteins that participate in a variety of biological processes, including cell signaling, glycolysis, cell differentiation, and genome stability. Molecular mechanics (MM) simulations have the potential to provide detailed insights into the mechanisms of these Mg-dependent proteins. However, MM force fields continue to suffer from large inaccuracies when dealing with divalent cations. The inclusion of explicit polarization in MM models, such as in CHARMM Drude and AMOEBA models, has been promising, but despite substantial improvements over non-polarizable models, errors in interactions of proteins with divalent cations remain in excess of 10 kcal/mol. Such errors are present even with ‘nonbonded-fix (NB-fix)’ corrections. Modeling becomes even more problematic when dealing with enzymes like kinases and phosphatases, where Mg2+ ions interact directly with both proteins and nucleotides. Toward addressing this long-standing challenge, we recently presented a recalibration of the AMOEBA model (Delgado et al. J Chem. Info. Model. 2022). Notably, polarization terms of cation-coordinating groups were improved to respond better to the high electric fields present near cations. This systematically reduced errors in interactions of monovalent cations with proteins. Here we show that this revised model, along with our many body NB-fix (MB-NB-fix) corrections, reduces errors in Mg2+-protein interaction energies to 5 kcal/mol. MB-NB-fix corrections also improve predictions of Mg2+-ATP interactions substantially. Errors are computed with respect to estimates from vdW-inclusive density functional theory that we benchmark against "gold standard" CCSD(T) calculations and experiments. We will also present the ramifications of these improvements in local interactions on Mg2+-ATP binding free energies and binding modes in bulk water, and on complexation of Mg2+ and ATP with proteins.

Interaction of Tetraalkylammonium+ and DNA

We are interested in the interactions of monovalent cations with duplex DNA as observed by capillary electrophoresis. Here we compare the interactions of Na+ and of tetraalkylammonium+ (TXA+) with a model hairpin oligonucleotide having the sequence GA5C4T5C. As the size of the monovalent cation increases, its association with the coil conformation is enhanced and its association with the hairpin conformation is diminished. These changes can be ascribed to the hydrophobic surface of TXA+, increasingly attracted to the hydrophobic coil conformation and increasingly rejected by the hydrophilic hairpin conformation. Thus TXA+ acts as a denaturant, markedly lowering the melting temperatures of duplex DNA.

Interaction of monovalent cations with acetonitrile

AbstractSolvation of monovalent cations (Me+) of alkali metalsNa+, K+, Rb+, and Cs+, coinage metalsCu+, Ag+, Au+, and p‐block elements Ga+, In+, and Tl+ with acetonitrile was studied by means of ab initio calculations and time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS). The intermolecular interactions in the complexes Me+···CH3CN were investigated using the coupled clusters theory including single, double, and noniterative triple substitutions (CCSD(T)) in conjunction with the Pol and Pol‐dk basis sets. The binding energies of these donor‐acceptor complexes were estimated; taking into account the basis set superposition error, zero‐point vibrations, correlation contribution, and scalar relativistic corrections. The theoretical ΔG0298 K values based on CCSD(T)/Pol and/or CCSD(T)/Pol‐dk binding energies correlated well with experimental transfer Gibbs energies (from water to acetonitrile) for the series of cations. In the case of Au monocation, relativistic correction turned out to be extremely important. Composition of the complex of Ag+ and Na+ with acetonitrile was determined by using SIMS supporting both theoretical and experimental transfer Gibbs energies. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Cation binding and thermostability of FTHFS monovalent cation binding sites and thermostability of N10-formyltetrahydrofolate synthetase from Moorella thermoacetica.

Formyltetrahydrofolate synthetase (FTHFS) from the thermophilic homoacetogen, Moorella thermoacetica, has an optimum temperature for activity of 55-60 degrees C and requires monovalent cations for both optimal activity and stabilization of tetrameric structure at higher temperatures. The crystal structures of complexes of FTHFS with cesium and potassium ions were examined and monovalent cation binding positions identified. Unexpectedly, NH(4)(+) and K(+), both of which are strongly activating ions, bind at a different site than a moderately activating ion, Cs(+), does. Neither binding site is located in the active site. The sites are 7 A apart, but in each of them, the side chain of Glu 98, which is conserved in all known bacterial FTHFS sequences, participates in metal ion binding. Other ligands in the Cs(+) binding site are four oxygen atoms of main chain carbonyls and water molecules. The K(+) and NH(4)(+) binding site includes the carboxylate of Asp132 in addition to Glu98. Mutant FTHFS's (E98Q, E98D, and E98S) were obtained and analyzed using differential scanning calorimetry to examine the effect of these mutations on the thermostability of the enzyme with and without added K(+) ions. The addition of 0.2 M K(+) ions to the wild-type enzyme resulted in a 10 degrees C increase in the thermal denaturation temperature. No significant increase was observed in E98D or E98S. The lack of a significant effect of monovalent cations on the stability of E98D and E98S indicates that this alteration of the binding site eliminates cation binding. The thermal denaturation temperature of E98Q was 3 degrees C higher than that of the wild-type enzyme in the absence of the cation, indicating that the removal of the unbalanced, buried charge of Glu98 stabilizes the enzyme. These results confirm that Glu98 is a crucial residue in the interaction of monovalent cations with FTHFS.

Permeation and interaction of monovalent cations with the cGMP-gated channel of cone photoreceptors.

We measured the ion selectivity of cGMP-dependent currents in detached membrane patches from the outer segment of cone photoreceptors isolated from the retina of striped bass. In inside-out patches excised from either single or twin cones the amplitude of these currents, under symmetric ionic solutions, changed with the concentration of cGMP with a dependence described by a Hill equation with average values, at +80 mV, of Km = 42.6 microM and n = 2.49. In the absence of divalent cations, and under symmetric ionic solutions, the I-V curves of the currents were linear over the range of -80 to +80 mV. The addition of Ca altered the form of the I-V curve to a new function well described by an empirical equation that also describes the I-V curve of the photocurrent measured in intact photoreceptors. The monovalent cation permeability sequence of the cGMP-gated channels in the absence of divalent ions was PK > PNa = PLi = PRb > PCs (1.11 > 1.0 = 0.99 = 0.96 > 0.82). The conductance selectivity sequence at +80 mV was GNa = GK > GRb > GCs > GLi (1.0 = 0.99 > 0.88 > 0.74 > 0.60). The organic cations tetramethylammonium (TMA) and arginine partially blocked the current, but the larger ion, arginine, was permeant, whereas the smaller ion, TMA, was not. The amplitude of the outward current through the channels increased with the concentration of monovalent cations on the cytoplasmic membrane surface, up to a saturating value. The increase was well described by the adsorption isotherm of a single ion binding site within the channel with average binding constants, at +80 mV, of 104 mM for Na and 37.6 mM for Li. By assuming that the ion channel contains a single ion binding site in an energy trough separated from each membrane surface by an energy barrier, and using Eyring rate theory, we simulated I-V curves that fit the experimental data measured under ionic concentration gradients. From this fit we conclude that the binding site interacts with one ion at a time and that the energy barriers are asymmetrically located within the membrane thickness. Comparison of the quantitative features of ion permeation and interaction between the cGMP-gated channels of rod and cone photoreceptors reveals that the ion binding sites are profoundly different in the two types of channels. This molecular difference may be particularly important in explaining the differences in the transduction signal of each receptor type.

Interaction of monovalent cations with tetrodotoxin and saxitoxin binding at sodium channels of frog myelinated nerve

Na currents and Na-current fluctuations were measured in myelinated frog nerve fibres to study interactions between monovalent externally applied cations and the binding of the Na-channel blockers tetrodotoxin (TTX) or saxitoxin (STX). Adding 110 mM NaCl to Ringer's solution increased the maximum peak Na conductance by a factor of 2.51 in the presence of 12 nM TTX and by a factor of 2.43 in the presence of 4 nM STX. According to the analysis of Na-current fluctuations this increase of the Na conductance is mainly caused by an increase of the number N of unblocked Na channels per node, while the conductance of a single channel saturates in the hyperosmolar solutions. The increase of N is interpreted by displacement of TTX or STX from Na channels by external Na+. Relief of TTX blockage was also observed by adding 110 mM chloride salts of Li+, hydrazine+, guanidine+ and K+ to Ringer, but not in Ringer + 110 mM tetramethylammonium chloride or 250 mM sucrose. The increase of N by the external cations is a saturating function of the permeability of the Na channel to these ions. The results are interpreted by a toxin receptor in a superficial prefilter to the Na channel, which contributes to cation discrimination at the outer channel region.

Surface Behaviour of β-FeOOH: Point of Zero Change and Interactions with Anions and Cations

The acid-base dissociation behaviour of the surface of β-FeOOH in aqueous suspension has been investigated by potentiometric titration. The zero point of charge of the electrical double layer of the oxide/solution interface (pH = 7·6 ± 0·1 in KNO3, NaNO3 and LiNO3 electrolytes) coincides with the pH of the isoelectric point, determined by the solid addition method. Chloride and sulphate anions are strongly adsorbed on the β-FeOOH surface and thus modify the balance of positive and negative groups on the surface. The sequence of interaction of monovalent cations with the surface is K+ = ≤ Na+ ≤ Li+. β-FeOOH appears to behave essentially in the same manner as found previously for α-FeOOH.

Interaction of valinomycin and monovalent cations with the (Ca2+,Mg2+)-ATPase of skeletal muscle sarcoplasmic reticulum.

The interactions of monovalent cations and of the K+-specific ionophore, valinomycin, with the Ca2+-ATPase of skeletal muscle of sarcoplasmic reticulum have been studied in the absence of cation gradients by their effects on enzyme turnover and on the ATP plus Ca2+-dependent enhanced fluorescence of the ATP analogue, 2',3'-O-(2,4,6-trinitrocyclohexyldienylidine)-adenosine 5'-triphosphate (TNP-ATP) (Watanabe, T., and Inesi, G. (1982) J. Biol. Chem. 257, 11510-11516). Monovalent cations decreased turnover-dependent TNP-ATP fluorescence in the series K+ greater than Rb+ approximately equal to Cs+ greater than Na+ greater than Li+ (K0.5 = 49, 73, 75, 94, and 246 mM, respectively), consistent with the known specificity of the monovalent cation binding site that stimulates turnover and E-P hydrolysis. Valinomycin (200 nmol/mg), in the absence of monovalent cations, decreased ATPase activity by 30% and abolished the stimulatory effects of 150 mM KCl or NaCl on turnover. The ionophore alone enhanced TNP-ATP fluorescence by 20% and altered the specificity and affinity of the site that inhibited TNP-ATP fluorescence to Cs+ greater than Rb+ greater than K+ approximately equal to Na+ greater than Li+ (K0.5 = 79, 111, 134, 136, and 270 mM, respectively), which follows the Hofmeister series for effectiveness of monovalent lyotropic cations. TNP-ATP binding was not affected by either monovalent cations or valinomycin. Inhibition of turnover-dependent TNP-ATP fluorescence appears to be a useful parameter for monitoring monovalent cation binding to the Ca2+-ATPase. It is concluded that the ionophore interacts directly with the Ca2+-ATPase, independent of its K+ conductance effects on the lipid bilayer, and modifies the affinity and specificity of the monovalent cation site, either by direct interaction or by the formation of a valinomycin-monovalent cation-enzyme complex.

Interactions of monovalent cations with sodium channels in squid axon. II. Modification of pharmacological inactivation gating.

The time-, frequency-, and voltage-dependent blocking actions of several cationic drug molecules on open Na channels were investigated in voltage-clamped, internally perfused squid giant axons. The relative potencies and time courses of block by the agents (pancuronium [PC], octylguanidinium [C8G], QX-314, and 9-aminoacridine [9-AA]) were compared in different intracellular ionic solutions; specifically, the influences of internal Cs, tetramethylammonium (TMA), and Na ions on block were examined. TMA+ was found to inhibit the steady state block of open Na channels by all of the compounds. The time-dependent, inactivation-like decay of Na currents in pronase-treated axons perfused with either PC, 9-AA, or C8G was retarded by internal TMA+. The apparent dissociation constants (at zero voltage) for interaction between PC and 9-AA with their binding sites were increased when TMA+ was substituted for Cs+ in the internal solution. The steepness of the voltage dependence of 9-AA or PC block found with internal Cs+ solutions was greatly reduced by TMA+, resulting in estimates for the fractional electrical distance of the 9-AA binding site of 0.56 and 0.22 in Cs+ and TMA+, respectively. This change may reflect a shift from predominantly 9-AA block in the presence of Cs+ to predominantly TMA+ block. The depth, but not the rate, of frequency-dependent block by QX-314 and 9-AA is reduced by internal TMA+. In addition, recovery from frequency-dependent block is not altered. Elevation of internal Na produces effects on 9-AA block qualitatively similar to those seen with TMA+. The results are consistent with a scheme in which the open channel blocking drugs, TMA (and Na) ions, and the inactivation gate all compete for a site or for access to a site in the channel from the intracellular surface. In addition, TMA ions decrease the apparent blocking rates of other drugs in a manner analogous to their inhibition of the inactivation process. Multiple occupancy of Na channels and mutual exclusion of drug molecules may play a role in the complex gating behaviors seen under these conditions.

Interactions of monovalent cations with sodium channels in squid axon. I. Modification of physiological inactivation gating.

Inactivation of Na channels has been studied in voltage-clamped, internally perfused squid giant axons during changes in the ionic composition of the intracellular solution. Peak Na currents are reduced when tetramethylammonium ions (TMA+) are substituted for Cs ions internally. The reduction reflects a rapid, voltage-dependent block of a site in the channel by TMA+. The estimated fractional electrical distance for the site is 10% of the channel length from the internal surface. Na tail currents are slowed by TMA+ and exhibit kinetics similar to those seen during certain drug treatments. Steady state INa is simultaneously increased by TMA+, resulting in a "cross-over" of current traces with those in Cs+ and in greatly diminished inactivation at positive membrane potentials. Despite the effect on steady state inactivation, the time constants for entry into and exit from the inactivated state are not significantly different in TMA+ and Cs+. Increasing intracellular Na also reduces steady state inactivation in a dose-dependent manner. Ratios of steady state INa to peak INa vary from approximately 0.14 in Cs+- or K+-perfused axons to approximately 0.4 in TMA+- or Na+-perfused axons. These results are consistent with a scheme in which TMA+ or Na+ can interact with a binding site near the inner channel surface that may also be a binding or coordinating site for a natural inactivation particle. A simple competition between the ions and an inactivation particle is, however, not sufficient to account for the increase in steady state INa, and changes in the inactivation process itself must accompany the interaction of TMA+ and Na+ with the channel.

Interaction of monovalent cations with salicylaldehyde: Evidence for excited-state ion-pair dissociation

Interaction of monovalent cations with salicylaldehyde resulted in the formation of tight ion-pairs. In these ion-pairs salicylaldehyde acts as a bidentate anion through the phenolate and the carbonyl oxygen atoms. Para substituted phenols, on the other hand, form less tight ion-pairs with monovalent cations. Red shifts in both absorption and emission spectra of these ion-pairs were observed as the ionic radius of the metal ion is increased. In presence of 18-crown-6, solventseparated ion-pairs are formed at the expense of contact ion-pairs. Evidence for excited-state dissociation of the solvent-separated ion-pairs is reported.

Interactions of monovalent cations with phosphatidylserine bilayer membranes.

Interactions of monovalent cations with phosphatidylserine bilayer membranes.

Kinetic studies of the role of monovalent cations in the amidolytic activity of activated bovine plasma protein C.

The interaction of monovalent cations with activated bovine plasma protein C (APC) has been examined by kinetic methods, with H-D-phenylalanylpipecolylarginine-p-nitroanilide (S-2238) being employed as the substrate. By use of a kinetic model in which it is assumed that two monovalent cation sites (or classes of sites) need to be occupied for the catalytic event to occur, it has been shown that the Km,app +/- 10% of S-2238, at saturating cation concentrations, is dependent upon the nature of the cation and decreases in parallel with increasing alkali cation radius, according to the following series (Km,app for S-2238 in parentheses): Li+ (630 microM) greater than Na+ (220 microM) greater than K+ (189 microM) greater than Cs+ (70 microM). For the cation NH4+ at saturating cation levels, the Km,app for S-2238 is 270 microM. The Km,app +/- 10% for the cation has been determined at saturating S-2238 levels and a similar trend is noted. On the basis of the values obtained, the cation order (Km,app for cation at saturating S-2238 levels in parentheses) is as follows: Li+ (182 mM) greater than Na+ (129 mM) greater than K+ (55 mM) greater than Cs+ (41 mM). The Km,app for NH4+ at saturating S-2238 concentrations is 70 mM. These data indicate that an active participation of the cation in the amidolytic activity of APC exists, which is correlated with the ionic radius of the cation.

Interaction of monovalent cations with Rb + and Na + uptake in yeast

The concentration dependence of both Rb + uptake and Na + uptake by yeast can be described by a quadratic rate equation. This equation is derived for translocation of cations via a two-site translocation system. In accordance with predictions made for such a two-site translocation system the shape of the uptake isotherm depends both upon the substrate cation species and upon the concentration of other added competing cations. On plotting the rate of Rb + uptake against the quotient of that rate and the Rb + concentration concave, convex and also linear curves are found depending upon the type and the concentration of added monovalent cations. The Na + uptake isotherm plotted in a similar way shows a shift from a concave curve to a straight line on adding increasing amounts of Rb + to the yeast suspension. Decreasing the pH of the medium leads to a more pronounced convex

The interaction of monovalent cations with the sodium pump of low-potassium goat erythrocytes.

1. The activation by Na ions and the effect of the anti-L antibody on the sodium pump of low-potassium type (LK) erythrocytes, have been studied by measuring ouabain-sensitive ATPase activity of red cell membranes of LK goats. The experimental data were first corrected for incomplete occupation of the external K sites of the pump, using a saturation function obtained from influx experiments.2. Double-reciprocal plots of the corrected rates against Na concentration at various fixed K concentrations, yield a pattern of competitive K inhibition when it is assumed that three equivalent sodium sites take part in the internal activation of LK-(Na+K)-ATPase. The dissociation constant of Na at each site (K(m)) lies between 10 and 20 mM and that of K as competitive inhibitor (K(i)), between 1.5 and 4.5 mM.3. The maximal rate of hydrolysis of LK goat (Na + K)-ATPase is not different from those usually obtained with the high-potassium type (HK) red cell enzyme. Then, the low pumping rate of LK erythrocytes in physiological conditions is only reflecting the poor Na affinity, both absolute and relative, at the internal Na sites of their sodium pumps.4. The stimulation of the ouabain-sensitive ATPase activity by sensitization of the membranes with anti-L serum, is mediated by a threefold reduction of the K(m)/K(i) ratio at each site. K(m) decreases by a factor of 10, but there is also a smaller diminution of K(i). The maximal rate of hydrolysis, however, is unchanged by the anti-L treatment. The least-squares fitting of the pooled data by the rate equation, converges better with less than three and more than two equivalent sodium sites.5. The affinity sequence at two external K sites of the LK goat erythrocyte sodium pump, determined in the presence of 100 mM external Na, is Rb > K > Cs. It is obtained from the concentration dependence in influx experiments, and is the same as reported for human red cells.6. Cubic-root Dixon plots of the corrected ouabain-sensitive ATPase activity against the concentration of K and its congeners, show the sequence Tl > K > Rb > Na > Cs for the affinities at the internal cation sites of the LK sodium pump. Anti-L treatment decreases the relative magnitude of Na and Cs selectivities, it being not certain whether a Rb-Na transition then occurs.7. The results are discussed in terms of possible mechanisms whereby the sodium pump of LK and HK red cells may adjust the properties of their cation sites upon translocation of monovalent cations.

Monovalent cation activation of tryptophanase.

The interaction of monovalent cations with holotryptophanase has been examined by spectral and kinetic methods. Using S-orthonitrophenyl-L-cysteine as a substrate, activation by the following monovalent cations was demonstrated; values of KA (mM, in italics) and Vmax (mumol min-1 mg) aare given in parentheses: Li+ (54 +/- 11.6, 4.3 +/- 0.28), Na+ (40 +/- 0.03, 18) K+ (1.44 +/- 0.06, 41.1 +/- 3.5), Tl+ (0.95 +/- 0.1, 39 +/- 4.4), NH4+ (0.23 +/- 0.01, 57.9 +/- 2.6), Rb+ (3.5 +/- 0.3, 33.5 +/- 1.8), Cs+ (14.6 +/- 2.6, 21 +/- 2.3). It was demonstrated by circular dichroic spectra that the competitive inhibitor, ethionine, interacts with the holoenzyme in the absence of activating monovalent cations, although it does not undergo labilization of the alpha proton. On addition of monovalent cation to the holoenzyme-ethionine complex, a marked increase occurs in absorption of 508 nm resulting from labilization of the alpha proton with formation of the quinoid form of the pyridoxal phosphate moiety of the enzyme-substrate complex at the catalytic center (Morino, Y., and Snell, E.E. (1967) J. Biol. Chem; 242, 2800-2809. The extent of formation of this quinoid intermediate was linearly related to the maximum velocity observed with each cation except NH4+, which was anomalously active. When measured at 500 nm, the change in absorption ranged from deltaA = 0.45 mg-1 of tryptophanase for NH4+ to 0.06 mg-1 for Li+. Two moles of thallium (I) were bound per mole of subunit. The data are most consistent with the interaction of monovalent cation at or near the catalytic center in such a way that it either participates directly in the reaction or is required for the critical alignment of one or more functional groups necessary for catalysis.