Saccharide binding to transition metal ion free concanavalin A

Role of second metal ion in establishing active conformations of concanavalin A

The stoichiometry of Mn2+ binding to concanavalin A was found to be influenced by temperature, pH, and the presence or absence of saccharide. Demetalized concanavalin A binds one Mn2+ (S1 site) at 5 degrees C, pH 6.5, and two Mn2+ at 25 degrees C (S1 and S2 sites). The association constants for Mn2+ are 6.2 x 10(5) and 3.7 x 10(4) M-1 for the S1 and S2 sites, respectively, at 25 degrees C. Concanavalin A with one Mn2+ bound per monomer remains in an open conformation and exhibits a relatively high water proton relaxation rate. Concanavalin A with two Mn2+ ions remains in a closed conformation characterized by a lower relaxation rate. The rate of binding of the second Mn2+ to concanavalin A as determined by ESR and the rate of conversion of open form to closed form (folding over) as determined by proton relaxation rate measurements gave an identical rate constant of 80.0 +/- 5.8 M-1 h-1 at 17 degrees C. Ca2+, Sr2+, and high levels of methyl alpha-D-mannopyranoside also induce folding of concanavalin A. Ca2+ is not catalytic but stoichiometric in causing the folding. Mn2+ in the S1 site can be displaced by Ni2+, Co2+, and Zn2+, and Mn2+ in the S2 site can be displaced by Ca2+ and Sr2+. Concanavalin A with Ni2+, Co2+, Zn2+, or Mn2+ in the S1 site and Ca2+ or Sr2+ in the S2 site has a higher affinity for methylumbelliferyl alpha-D-mannopyranoside than Ni-Mn-, Co-Mn-, Zn-Mn-, and Cd-Cd-concanavalin A.

Models of binding of 4'-nitrophenyl alpha-D-mannopyranoside to the lectin concanavalin A

Molecular models for the complex formed between the lectin concanavalin A (Con A) and the saccharide derivative 4'-nitrophenyl-alpha-D-mannopyranoside (alpha-PNM) are presented, combining evidence from 1H-n.m.r. measurements, semi-empirical energy calculations and interactive graphics modelling. The models are in good agreement with the experimental data. Close examination of the models suggests that hydrophobic interactions together with van der Waals interactions and hydrogen bonds contribute to the stability of the complexes. It appears that there is a limited number of possible modes of binding of alpha-PNM to Con A.

Effect of the conformation of concanavalin A on its affinity for manganous ion

The stoichiometry of Mn2+ binding to concanavalin A at pH 6.4-7 which had been established in two independent studies [J.A. Sophianopoulos, A.J. Sophianopoulos, and W.C. MacMahon (1983) Arch. Biochem. Biophys. 223, 350-359; D.J. Christie, G.R. Munske, and J.A. Magnuson (1979) Biochemistry 18, 4638-4644] was challenged [C.F. Brewer, R.D. Brown, III, and S.H. Koenig (1983) Biochemistry 22, 3691-3702] on grounds of possible experimental errors. Additional evidence is presented in this study in support of the previous finding that at pH 6.4 only one Mn2+ binds per concanavalin A monomer of Mr 25,550. Also, evidence is presented showing that the results of Sophianopoulos et al. could not have been due to contamination by Ca2+. A comparison is made of the results in the three studies cited above which indicates that the concanavalin A used by Brewer et al. had decreased affinity for Mn2+ and it contained an appreciable fraction of concanavalin A incompetent of binding saccharides.

pH-dependent changes in properties of concanavalin A in the acid pH range

Properties characteristic of the structure and function of dimeric concanavalin A have been studied as a function of pH in the acid pH range using preparations comprising intact subunits or enriched in fragmented chains. For intact subunits, the glycogen binding ability falls to zero with a midpoint of pH 4.7, the release of Mn+2, Ca+2 and the fluorescent ligand 4-methylumbelliferyl-alpha-D-mannopyranoside from the lectin coincides over a pH range centered at pH 3.9, and the CD spectra of the aromatic amino acid residues increase sharply in amplitude between pH 4.0 and 1.5. Nevertheless, the sedimentation coefficient and peptide CD spectrum change insignificantly in the pH range 5 to 2, indicating that dimeric concanavalin A retains its secondary structure and overall hydrodynamic shape essentially unchanged upon acidification. The behavior of concanavalin comprising primarily fragmented chains is not significantly different from that of intact subunits, although it precipitates glycogen less efficiently. It is concluded that dimeric concanavalin A does not undergo a concerted change in structure upon acidification, but rather that it passes through a series of states differing from one another in their local conformations. The distinction in binding between the monosaccharide and the polysaccharide is attributed to participation of a secondary binding site in the latter case. A change in optical activity at 283 nm in the pH range 5-6 is ascribed to disruption of intersubunit interactions of Tyr 67 as the protein undergoes the dimer-tetramer equilibrium.

A fluorimetric study of the lanthanides binding to concanavalin A

The binding of Tb3+ and other lanthanides to Con A has been studied by sensitized Tb3+ luminescence, by quenching of intrinsic fluorescence and by activity measurements. In all the experimental conditions tested, it was found that holo and apo Con A bind lanthanide ions at a site different from the binding sites of the constitutive metals, Mn2+ and Ca2+. The bound lanthanide did not affect the saccharide binding ability and the hemoagglutinating ability of Con A. The intrinsic fluorescence of Con A is quenched by the binding of Tb3+ and Gd3+. The same quenching is obtained by shifting the pH of Con A from pH 6.5 to 4.5. It is proposed that H+ and Ln3+ completely quench a tryptophan, perhaps the residue 88 or 182.

Metal ion binding and conformational transitions in concanavalin A: a structure-function study

The affinity of the lectin Concanavalin A (Con A) for saccharides, and its requirement for metal ions such as Mn2+ and Ca2+, have been known for about 50 years. However the relationship between metal ion binding and the saccharide binding activity of Con A has only recently been examined in detail. Brown et al. (Biochemistry 16, 3883 (1977)) showed that Con A exists as a mixture of two conformational states: a "locked" form and an "unlocked" form. The unlocked form of the protein weakly binds metal ions and saccharide, and is the predominate conformation of demetallized Con A (apo-Con A) at equilibrium. The locked form binds two metal ions per monomer with the resulting complex(es) possessing full saccharide binding activity. Brown and coworkers measured the kinetics of the transition of the unlocked form to the fully metallized locked conformation containing Mn2+ and Ca2+. They also demonstrated that Mn2+ alone could form a locked ternary complex with Con A, and that rapid removal of the ions resulted in a metastable form of apo-Con A in the locked conformation which slowly (hours at 25 degrees C) reverted back to (predominantly) the unlocked conformation. The ability to form either conformation in the absence or presence of metal ions has thus allowed us to explore the relationship between metal ion binding and conformational transitions in Con A as determinants of the saccharide binding activity of the lectin. Based on the kinetics of the transition of unlocked apo-Con A to fully metallized locked Con A, and X-ray crystallographic data, it appears that the transition between the two conformations of Con A involves a cis-trans isomerization of an Ala-Asp peptide bond in the backbone of the protein, near one of the two metal ion binding sites. The relatively large activation energy for the transition (approximately 22 kcal M-1) results in relatively slow interconversions between the conformations (from minutes to days), whereas the equilibria with metal ions and saccharide are rapid. Thus, many metastable complexes can be formed and a variety of transition pathways between the two conformations studied. We have identified and characterized binary, ternary, and quaternary complexes of both conformations of Con A containing Mn2+ and saccharide, and have determined both metal ion and saccharide dissociation constants for all of them, as well as equilibrium and kinetic values for the conformational transitions between them.(ABSTRACT TRUNCATED AT 400 WORDS)

43Ca and 67Zn NMR spectra of Ca2+, Zn2+-concanavalin A solutions

The half-band width of 43Ca NMR of free aqueous Ca2+ was scarcely increased by adding more than equal molar apo-concanavalin A(apo-Con A), suggesting that slow chemical exchange, koff less than 10 s-1, occurs for the Ca2+ ion from Con A. In contrast with the 43Ca NMR findings, the half-band width of 67Zn NMR of free aqueous Zn2+ was markedly increased by adding apo-Con A. The 67Zn NMR half-band width of Zn2+ in the presence of apo-Con A was decreased by adding excess Ca2+, but was increased by adding excess D-mannose. These changes of the half-band width were influenced mutually by D-mannose or Ca2+, respectively. The broadened half-band widths of Zn2+ in the presence of Con A were decreased by adding Mn2+, suggesting that Mn2+ was substituted for Zn2+ at a metal binding site of Con A.

Kinetic studies of the demetallization and inactivation of concanavalin A

The demetallization of various metallo derivatives of Concanavalin A (i.e., MnMnPL, CoMnPL, CaCaPL, CoCaPL and MnCaPL, where PL represents protein in a locked conformation) has been examined by three separate procedures. These include the treatment of the protein with the metal ion chelators, EDTA and terpyridine, and subjecting the protein to low pH (i.e., pH 1.2). In all three procedure and for all five species examined, the immediate product of protein demetallization was the PL conformation previously described by Brown, R.D., III, Brewer, C.F. and Koenig, S.H. (Biochemistry (1977) 16, 3883-3896). The rates of dissociation of the metals from the different protein species, as measured spectrophotometrically using terpyridine, were found to be identical to the rates (k1) of loss of protein sugar binding affinity in the presence of EDTA as measured by assays with the fluorescent sugar, 4-methylumbelliferyl alpha-D-mannoside. The kinetic and thermodynamic data associated with the inactivation of the protein species have allowed the different metallo derivatives to be classed into two general categories. Class I forms include MnMnPL, CoMnPL and CaCaPL and possess an average k1 (25 degrees C) value of 3.88 X 10(-2) s-1 and an average Ea of 14.2 kcal X mol-1. Class II forms CoCaPL and MnCaPL have average values for k1 (25 degrees C) and Ea of 3.67 X 10(-5) s-1 and 21.6 kcal X mol-1, respectively.

Effects of manganese and calcium on conformational stability of concanavalin A: a differential scanning calorimetric study

The effect of degree of saturation of concanavalin A with Mn2+ or Ca2+, or both, on its thermal denaturation was investigated by differential scanning calorimetry. Acid-demetallized concanavalin A was partly or fully remetallized in acetate buffer (pH 5.0) containing 0.4 to 0.5 M NaCl. Under these conditions, native dimeric concanavalin A is highly stable, undergoing heat denaturation at 101 degrees C, with an enthalpy of denaturation of 7.4 cal/g. Removal of metal ions lowered stability considerably; concanavalin A with 0.06 Mn2+/monomer and 0.23 Ca2+/monomer (mol/mol) was denatured at 74 degrees C with an enthalpy of denaturation of 3.2 cal/g. Added Mn2+ stabilized demetallized concanavalin A, but added Ca2+ alone (up to 2 mol/mol monomer) did not. The Ca2+/ concanavalin A ratio influenced stabilization by Mn2+. In the presence of 1 to 2 Mn2+/ monomer and 0.5 or less Ca2+/monomer (mol/mol), stabilized concanavalin A was denatured at 85-88 degrees C and at 94-97 degress C, indicating presence of two stabilized metallo-concanavalin A species. At 1.0 or more mole each of Mn2+ and Ca2+ per monomer, one endotherm was observed at or above 98 degrees C and the enthalpy of denaturation was increased to 5.3 cal/g from less than 3.6 cal/g at lower metal ion/protein ratios. Stabilization was greater with Mn2+ plus Ca2+ than with Mn2+ alone, consistent with intrasubunit cooperativity in metal ion-induced stabilization of concanavalin A.

Calorimetric study of concanavalin A binding to saccharides

The binding of concanavalin A in the dimer form to various saccharides has been studied by calorimetry, and estimates of the binding enthalpy and binding constants have been calculated. Methyl alpha-D-mannoside and methyl alpha-D-glucoside have a -- delta H0 of 21.5 and 11.5 kJ/mol, respectively, at both pH 4 and 4.5. The p-nitrophenyl derivatives react with enthalpic values of 15.6 and 14.6 kJ/mol. The galactosepyranosides show no heat effects during mixing with the protein solutions. The apparent binding enthalpies calculated from the variations of the equilibrium constants with temperature are in good agreement with the values measured experimentally. The two binding sites of the dimer form of concanavalin A are equal and independent, and the low enthalpies obtained do not justify a large conformational change during the reaction. The binding reaction has also been estimated for other sugars normally contained in glycoproteins.

Preparation of homogeneous concanavalin A

Concanavalin A (Con A), obtained either commercially or by affinity chromatography, was further purified by incubating at 6-8 hr at pH 3.0-3.2 in 1 M NaCl, 0.08 M glycine and 3 mM each Ca2+ and Mn+2, heat treating at 45 degrees C for 2 hr and centrifuging. The supernatant was neutralized to pH 5 and stored in the cold. Te overall yield was 70-80%. Some of the properties of Con A at pH 5 are: The absorption coefficient of a l g/dl solution is 13.7 at 280 nm; the mean residue ellipticity at 224.5 nm is -9,300 degrees to -9,800 degrees; by sedimentation equilibrium, its molecular weight is 53,000 between pH 3.0 and pH 5.2. Con A solutions standing at room temperature at pH 7 for ten days lose through precipitation only 5-8% of the protein in 0.2 M NaCl and 15% of the protein in 0.1 M NaCl. In the solution conditions of SDS and urea-SDS gels, Con A not only unfolds slowly and incompletely, but it also forms high molecular weight aggregates. Thus, electrophoresis of Con a in such gels is unsuitable for tests of homogeneity. However, as judged by sedimentation equilibrium in 6.5 M quanidine at pH 8.1, purified Con A was monodisperse.

Kinetics of interaction of some alpha- and beta-D-monosaccharides with concanavalin A

The rates of formation and dissociation of concanavalin A with some 4-methylumbelliferyl and p-nitrophenyl derivatives of alpha- and beta-D-mannopyranosides and glucopyranosides were measured by fluorescence and spectral stopped-flow methods. All processes examined were uniphasic. The second-order formation rate constants varied only from 6.8 x 10(4) to 12.8 x 10(4) M-1 x s-1, whereas the first-order dissociation rate constants ranged from 4.1 to 220 s-1, all at pH 5.0, I=0.3 M, and 25 degrees C. Dissociation rates thus controlled the value of the binding constant. The effect of temperature on these reactions was examined, from which enthalpies and entropies of activation and of reaction could be calculated. The effects of pH at 25 degrees C on the reaction rates of 4-methylumbelliferyl alpha-D-mannopyranoside and 4-methylumbelliferyl alpha-D-glucopyranoside with concanavalin A were examined. The value of the binding constant Kap (derived from the kinetics) at any pH could be related to the intrinsic binding constant K by the expression Kap = KaK(Ka + [H+])-1. The values of Ka, the ionization constant of the protein segment responsive to sugar binding, were 3 x 10(-4) M and 1 x 10(-4) M for 4-methylumbelliferyl alpha-D-mannopyranoside and 4-methylumbelliferyl alpha-D-glucopyranoside, respectively. The binding constant of p-nitrophenyl alpha-D-mannopyranoside is surprisingly much less sensitive to a pH change from 5.0 to 2.7. Ionic strength had little effect on the binding characteristics of 4-methylumbelliferyl alpha-D-mannopyranoside to concanavalin A at pH 5.2 and 25 degrees C.

A study of structurally related binding properties of concanavalin A using differential scanning calorimetry

Differential scanning calorimetry has been used to study the interactions between concanavalin A and different carbohydrates. It was found that the binding of carbohydrate to concanavalin A stabilizes the structure of the protein as judged by an increase in transition temperature of the lectin. Furthermore, the degree of stabilization is shown to be a function of the association constant for each sugar moiety. Interactions between concanavalin A and the glycoprotein horseradish peroxidase were also studied. On changing the molar ratio of peroxidase to concanavalin A from 0.1 to 16, an initial dramatic increase in the transition temperature for concanavalin A was observed, but at ratios above four the degree of stabilization decreased; no stabilization of peroxidase was found in molar ratios above one. Results are also presented on the importance of Mn2+ and Ca2+ for the stabilization of the concanavalin A structure and for its ability to bind carbohydrates. In the presence of Mn2+ ions alone, the lectin could not form the necessary conformation for binding the carbohydrate. However, with Ca2+ ions alone the lectin was able to bind the carbohydrate ligands, as judged by the shift in its transition temperature.