A relationship between 13C-chemical-shift displacements and counterion-condensation theory, in the binding of calcium ion by heparin

Theoretical assessment of the oligolysine model for ionic interactions in protein-DNA complexes

The observed salt dependence of charged ligand binding to polyelectrolytes, such as proteins to DNA or antithrombin to heparin, is often interpreted by means of the "oligolysine model." We review this model as derived entirely within the framework of the counterion condensation theory of polyelectrolytes with no introduction of outside assumptions. We update its comparison with experimental data on the structurally simple systems for which it was originally intended. We then compute the salt dependence of the binding free energy for a variety of protein-DNA complexes with nonlinear Poisson-Boltzmann (NLPB) simulation methods. The results of the NLPB calculations confirm the central prediction of the oligolysine model that the net charge density of DNA remains invariant to protein binding. Specifically, when a cationic protein residue penetrates the layer of counterions condensed on DNA, a counterion is released to bulk solution, and when an anionic protein residue penetrates the condensed counterion layer, an additional counterion is condensed from bulk solution. We also conclude, however, that the cumulative effect of charged protein residues distant from the binding interface makes a significant contribution to the salt dependence of binding, an observation not accommodated by the oligolysine model.

Thermodynamic characterization of the interaction between the C-terminal domain of extracellular superoxide dismutase and heparin by isothermal titration calorimetry

Extracellular superoxide dismutase (ECSOD) interacts with heparin through its C-terminal domain. In this study we used isothermal titration calorimetry (ITC) to get detailed thermodynamic information about the interaction. We have shown that the interaction between ECSOD and intestinal mucosal heparin (M(w) 6000-30000 Da) is exothermic and driven by enthalpy at physiological salt concentration. However, the contribution from entropy is favorable for binding of small isolated heparin fragments. By studying different size-defined heparin fragments, we also concluded that a hexasaccharide moiety is sufficient for strong binding to ECSOD. The binding involves proton transfer from the buffer to the ECSOD-heparin complex, and the results indicate that the number of ionic interactions made between ECSOD and heparin upon binding varies from three to five for heparin and an octasaccharide fragment, respectively. Surprisingly and despite the many charges found on both the protein and the polysaccharide, our results indicate that the nonionic contribution to the binding is large. From the temperature dependence we have calculated the constant pressure heat capacity change (DeltaC(p)) of the interaction to -644 J K(-1) mol(-1) and -306 J K(-1) mol(-1) for heparin and an octasaccharide, respectively.

Towards a model of the mineral-organic interface in bone: NMR of the structure of synthetic glycosaminoglycan- and polyaspartate-calcium phosphate composites

We have synthesised three materials-chondroitin sulphate (ChS, a commercial product derived from shark cartilage and predominantly chondroitin-6-sulphate (Ch-6-S)) bound to pre-formed hydroxyapatite (HA, Ca(10)(PO(4))(6)(OH)(2)), HA formed in the presence of ChS and poly-Asp bound to pre-formed HA-to generate models for the mineral-organic interface in bone. The three materials have been investigated by (13)C cross polarisation magic-angle spinning (CPMAS) NMR, (13)C{(31)P} rotational echo double resonance (REDOR) and powder x-ray diffraction (XRD) in order to verify their composition and to determine the nature of their binding to HA. Our results show that for HA formed in the presence of Ch-6-S, all carbon atoms in the Ch-6-S having contact with mineral phosphate. We propose that HA in this case forms all around the Ch-6-S polymer rather than along one face of it as is more commonly supposed in cases of templating by organic molecules. However, Ch-6-S binding to pre-formed HA probably occurs via a surface layer of water on the mineral rather than to the mineral directly. In contrast, poly-Asp binds closely to the pre-formed HA surface and so is clearly able to displace at least some of the surface-bound water.

A new radiochemical method to investigate ion binding with polyelectrolytes

A new method for investigating the binding of ions with polyelectrolytes has been developed. This method, based on Donnan equilibrium and an isotope exchange between the electrolyte and polyelectrolyte, can distinguish territorial from specific binding of ions and can determine fractions of ions bound with the polyion. This method can determine ion binding with polyelectrolytes in a wide range of polyelectrolyte concentrations in multicomponent solutions. The method was tested with radioactive tracers 22Na+, 36Cl- and heparin sodium salt. The influence of the ionic strength on the Na+ binding with heparin was investigated at 310 K. In the limit of zero ionic strength, all Na+ ions are bound to heparin, but only 45% of them are exchangeable. Thus Na+ ions can be bound both territorially and specifically. The fraction of bound ions decreases rapidly with increasing ionic strength. The fraction of the specifically bound ions becomes negligible when the ionic strength exceeds 0.01 M, whereas the fraction of territorially bound ions can be neglected at ionic strengths higher than 0.45 M.

Multinuclear magnetic resonance studies of the interaction of inorganic cations with heparin

The interaction of Na+, Ca2+, Mg2+, Zn2+ and La3+ with heparin, a highly negatively charged glycosaminoglycan, was studied by 1H and 23Na nuclear magnetic resonance spectroscopy. 1H chemical shift and nuclear Overhauser effect (NOE) data indicate that the counter ions Na+, Ca2+ and Mg2+ interact with the low pH, carboxylic acid form of heparin by delocalized, long-range electrostatic interactions. At higher pH, 1H chemical shift and NOE data indicate that Na+ and Mg2+ continue to interact with heparin in the same manner, even upon deprotonation of the carboxylic acid group; however, there is a site-specific contribution to the binding of Ca2+, Zn2+ and La3+ under these conditions. Acid dissociation constants for heparin carboxylic acid groups and heparin-metal binding constants were determined from the pH dependence of 1H chemical shifts and 23Na spin-lattice (T1) relaxation times. Equilibrium constants for exchange of M2+ for heparin-bound Na+ were obtained from 23Na T1 data. The acid dissociation constants show a strong dependence on Na+ concentration due to the polyelectrolyte character of heparin.

Regioselectivity in the sulfation of some chemically-modified heparins, and observations on their cation-binding characteristics

Two modified forms of heparin, polymers A and B, have been prepared, one containing residues of nonsulfated alpha-L-idopyranosyluronic acid (3) and the other residues of alpha-L-galactopyranosyluronic acid (7), in place of the normal alpha-L-idopyranosyluronic acid 2-sulfate (1). In addition, both A and B contained 2-acetamido-2-deoxy-alpha-D-glucopyranosyl 6-sulfate residues (6) in place of the corresponding N-sulfated residues (2) of the original heparin. These polymers were subjected to sulfation under various conditions. Examination of the products by NMR spectroscopy showed that polymer A was sulfated initially at position-3 of residue 3, and that slower substitution occurred at position-3 of 6. By contrast, polymer B exhibited low regioselectivity, as sulfation occurred with about equal facility at positions-2 and -3 of 7, and -3 of 6. The sulfation products had no significant anti Xa activity. Based on the paramagnetic effects of Cu2+ and chemical shift displacements induced by Ca2+, NMR spectroscopy was used to compare cation-binding properties of A and B with those of heparin. In contrast to heparin, which forms a complex with Cu2+ detectable at a level of < 10(-3) mol per dimeric unit of the polymer, neither A nor B exhibited an interaction with the cation. However, polymer A was found to bind Ca2+, in this respect being distinct from the related modification, 1-->6, which contains a 2-sulfate group in 1, as well as from polymer B.

Ionic modulation of the effects of heparin on plasminogen activation by tissue plasminogen activator: the effects of ionic strength, divalent cations, and chloride

Ionic strength, divalent cations, and Cl- modulate the ability of the glycosaminoglycan heparin to stimulate the activation of human plasminogen (Pg) by tissue-type Pg activator. Kinetic analysis of Pg activation indicates that heparin is inhibitory, stimulatory, or nonstimulatory as a function of ionic strength. While increasing ionic strength inhibits Pg activation in the absence of heparin, in it presence an activation phase followed by an inhibitory phase is observed. Divalent cations, inhibitors of activation in the absence of heparin, increase the rate of activation in its presence. Kinetic analysis demonstrates that divalent cations augment the heparin stimulatory effect a maximum of 60-fold due to increases in kcat without changes in Km of the reaction. This effect is heparin-specific, since activation is not affected by Ca2+ in the presence of heparan sulfate or de-N-sulfated heparin. Also, Cl- inhibits Pg activation in the presence of heparin by acting as a competitive inhibitor (Kic of 100 mM). Furthermore, inhibition by Cl- reduces the overall magnitude of heparin stimulation of Pg activation. These results suggest that physiologic ions in combination with heparin may be significant effectors of Pg activation in the vascular microenvironment.

Characterization and cellular distribution of acidic peptide and oligosaccharide metal-binding compounds from kidneys

Two low-molecular-mass Ni-binding fractions first isolated from human kidneys [Templeton & Sarkar (1985) Biochem. J. 230, 35-42] are further characterized. Both components are acidic and are readily separated from each other by gel chromatography on Bio-Gel P-2. After equilibration with 63Ni the largest complex constitutes about 30% of the radioactive 63Ni and is an approx. 3.5 kDa peptide and the smallest species comprise short oligosaccharides containing 70% of the radioactivity. Both of these components are found in human, bovine and porcine kidneys as well as in a porcine proximal tubule-like cell line LLC-PK1. There is a small variation in amino acid composition between species. The oligosaccharides are reducing sugars and contain sulphate, glucosamine, glucuronic acid and iduronic acid with two to four overall negative charges. The monosaccharide composition was determined by h.p.l.c. with pulsed amperometric detection of the acid hydrolysates and by gas chromatography. In the LLC-PK1 cell line the acidic peptide is both intracellular and extracellular, whereas the oligosaccharides are only intracellular. The concentration of extracellular peptide, as measured by 63Ni binding, is found to increase after exposure of the cells to low micromolar concentrations of Ni, whereas the oligosaccharide concentrations, also measured by 63Ni binding, remain constant. The oligosaccharide component is decreased by 40% in the presence of NH4Cl, suggesting that is derived from degradation of internalized heparan sulphate.

Marked stereoselectivity in the binding of copper ions by heparin. Contrasts with the binding of gadolinium and calcium ions

Heparin forms a complex with cupric ion (Cu2+) at a level of less than or equal to 10(-3) mol of the metal ion per dimeric unit of the polymer, as evidenced by paramagnetic relaxation effects on its 1H- and 13C-n.m.r. spectra. No interaction occurred with heparin derivatives modified either by desulfation of the residues of alpha-L-iduronic acid 2-sulfate, or by hydrolysis of the sulfamino group of the residues of 2-deoxy-2-sulfamino-alpha-D-glucose 6-sulfate, although binding was induced by N-acetylation of the latter derivative. Under the same experimental conditions, no alternative type of glycosyluronic acid structure tested, including the other glycosaminoglycans, showed significant relaxation enhancement by Cu2+. These results are in contrast to those obtained with gadolinium ion (Gd3+), another paramagnetic probe, or with calcium ion (Ca2+), which promotes chemical-shift displacements. The binding selectivities of those two cations are much broader than that of Cu2%, although they also differ notably in their relationship to the structure of heparin.