pH-dependent conformational changes of concanavalin A

Directed Evolution of Aptamer Discovery Technologies

Although antibodies are a powerful tool for molecular biology and clinical diagnostics, there are many emerging applications for which nucleic acid-based aptamers can be advantageous. However, generating high-quality aptamers with sufficient affinity and specificity for biomedical applications is a challenging feat for most research laboratories. In this Account, we describe four techniques developed in our laboratory to accelerate the discovery of high-quality aptamer reagents that can achieve robust binding even for challenging molecular targets. The first method is particle display, in which we convert solution-phase aptamers into aptamer particles that can be screened via fluorescence-activated cell sorting (FACS) to quantitatively isolate individual aptamer particles based on their affinity. This enables the efficient isolation of high-affinity aptamers in fewer selection rounds than conventional methods, thereby minimizing selection biases and reducing the emergence of artifacts in the final aptamer pool. We subsequently developed the multiparametric particle display (MPPD) method, which employs two-color FACS to isolate aptamer particles based on both affinity and specificity, yielding aptamers that exhibit excellent target binding even in complex matrixes such as serum. The third method is an alkyne-azide chemistry ("click chemistry")-based particle display (click-PD) that enables the generation and screening of "non-natural" aptamers with a wide range of base modifications. We have shown that these base-modified aptamers can achieve robust affinity and specificity for targets that have proven challenging or inaccessible with natural nucleotide-based aptamer libraries. Finally, we describe the non-natural aptamer array (N2A2) platform in which a modified benchtop sequencing instrument is used to characterize base-modified aptamers in high throughput, enabling the efficient identification of molecules with excellent affinity and specificity for their targets. This system first generates aptamer clusters on the flow-cell surface that incorporate alkyne-modified nucleobases and then performs a click reaction to couple those nucleobases to an azide-modified chemical moiety. This yields a sequence-defined array of tens of millions of base-modified sequences, which can then be characterized for affinity and specificity in a high-throughput fashion. Collectively, we believe that these advancements are helping to make aptamer technology more accessible, efficient, and robust, thereby enabling the use of these affinity reagents for a wider range of molecular recognition and detection-based applications.

Concanavalin A: coordination diversity to xenobiotic metal ions and biological consequences

The binding ability of lectins has gained attention owing to the carbohydrate-specific interactions of these proteins. Such interactions can be applied to diverse fields of biotechnology, including the detection, isolation, and concentration of biological target molecules. The physiological aspects of the lectin concanavalin A (ConA) have been intensively studied through structural and functional investigations. X-ray crystallography studies have proven that ConA has two β-sheets and a short α-helix and that it exists in the form of a metalloprotein containing Mn²⁺ and Ca²⁺. These heterometals are coordinated with side chains located in a metal-coordinated domain (MCD), and they affect the structural environment in the carbohydrate-binding domain (CBD), which interacts with carbohydrates through hydrogen bonds. Recent studies have shown that ConA can regulate biophysical interactions with glycoproteins in virus envelopes because it specifically interacts with diverse polysaccharides through its CBD (Tyr, Asn, Asp, and Arg residues positioned next to the MCD). Owing to their protein-protein interaction abilities, ConA can form diverse self-assembled complexes including monomers, dimers, trimers, and tetramers, thus affording unique results in different applications. In this regard, herein, we present a review of the structural modifications in ConA through metal-ion coordination and their effect on complex formation. In recent approaches, ConA has been applied for viral protein detection, on the basis of the interactions of ConA. These aspects indicate that lectins should be thoroughly investigated with respect to their biophysical interactions, for avoiding unexpected changes in their interaction abilities.

Structural and biochemical analyses of concanavalin A circular permutation by jack bean asparaginyl endopeptidase

Over 30 years ago, an intriguing posttranslational modification was found responsible for creating concanavalin A (conA), a carbohydrate-binding protein from jack bean (Canavalia ensiformis) seeds and a common carbohydrate chromatography reagent. ConA biosynthesis involves what was then an unprecedented rearrangement in amino-acid sequence, whereby the N-terminal half of the gene-encoded conA precursor (pro-conA) is swapped to become the C-terminal half of conA. Asparaginyl endopeptidase (AEP) was shown to be involved, but its mechanism was not fully elucidated. To understand the structural basis and consequences of circular permutation, we generated recombinant jack bean pro-conA plus jack bean AEP (CeAEP1) and solved crystal structures for each to 2.1 and 2.7 Å, respectively. By reconstituting conA biosynthesis in vitro, we prove CeAEP1 alone can perform both cleavage and cleavage-coupled transpeptidation to form conA. CeAEP1 structural analysis reveals how it is capable of carrying out both reactions. Biophysical assays illustrated that pro-conA is less stable than conA. This observation was explained by fewer intermolecular interactions between subunits in the pro-conA crystal structure and consistent with a difference in the prevalence for tetramerization in solution. These findings elucidate the consequences of circular permutation in the only posttranslation example known to occur in nature.

Concanavalin A photocross-linked affinity cryogels for the purification of horseradish peroxidase

The present study describes an easy and efficient procedure for the purification of horseradish peroxidase from horseradish roots. For this purpose, supermacroporous cryogels having Concanavalin A were prepared by photosensitive cross-linking polymerization. Horseradish peroxidase binding and elution from the prepared cryogels were carried out changing various parameters such as initial peroxidase concentration and pH. The best binding performance was obtained at pH 7.0. The maximum horseradish peroxidase binding of the cryogels was found to be 3.85 mg g−1 cryogel. Horseradish peroxidase purification from crude extract resulted in 115.1-fold. SDS-PAGE analysis and circular dichroism measurements indicated that the horseradish peroxidase purification from horseradish roots was successfully carried out.

Glycosylated Conductive Polymer: A Multimodal Biointerface for Studying Carbohydrate-Protein Interactions

Carbohydrate-protein interactions occur through glycoproteins, glycolipids, or polysaccharides displayed on the cell surface with lectins. However, studying these interactions is challenging because of the complexity and heterogeneity of the cell surface, the inherent structural complexity of carbohydrates, and the typically weak affinities of the binding reactions between the lectins and monovalent carbohydrates. The lack of chromophores and fluorophores in carbohydrate structures often drives such investigations toward fluorescence labeling techniques, which usually require tedious and complex synthetic work to conjugate fluorescent tags with additional risk of altering the reaction dynamics. Probing these interactions directly on the cell surface is even more difficult since cells could be too fragile for labeling or labile dynamics could be affected by the labeled molecules that may interfere with the cellular activities, resulting in unwanted cell responses. In contrast, label-free biosensors allow real-time monitoring of carbohydrate-protein interactions in their natural states. A prerequisite, though, for this strategy to work is to mimic the coding information on potential interactions of cell surfaces onto different biosensing platforms, while the complementary binding process can be transduced into a useful signal noninvasively. Through carbohydrate self-assembled monolayers and glycopolymer scaffolds, the multivalency of the naturally existing simple and complex carbohydrates can be mimicked and exploited with label-free readouts (e.g., optical, acoustic, mechanical, electrochemical, and electrical sensors), yet such inquiries reflect only limited aspects of complicated biointeraction processes due to the unimodal transduction. In this Account, we illustrate that functionalized glycosylated conductive polymer scaffolds are the ideal multimodal biointerfaces that not only simplify the immobilization process for surface fabrication via electrochemical polymerization but also enable the simultaneous analysis of the binding events with orthogonal electrical, optical, or mass sensing label-free readouts. We established this approach using polyaniline and polythiophene as examples. Two general methods were demonstrated for glycosylated polymer fabrications (i.e., electropolymerization of monomer bearing α-mannoside residues or click chemistry based mannose conjugation to electrochemically preformed quinone fused polymer with potential to introduce different carbohydrate moieties and construct glycan arrays in a similar manner). Their conjugated π system extending over a large number of recurrent monomer units renders them sensitive optoelectronic materials. The carbohydrate-protein interactions on the side chain could disrupt the electrostatic, H-bonding, steric, or van der Waals interactions within or between polymers, leading to a change of conductivity or optical absorption of the conductive polymers. This will allow concurrent interrogation of these interactions with adjoining biological processes and mechanisms in multimodal fashion. Furthermore, the functionalized glycosylated conductive polymers can be designed and synthesized with controlled oxidation states, desired ionic dopants, and the imperative density and orientation of the sugar ligands that enable the assessment of differential receptor binding profiles of carbohydrate-protein interactions with much more detailed information and high accuracy. Finally, the glycosylated biosensing interfaces were successfully validated for their applications in Gram-negative bacterial detection, antibiotic resistance studies, and antimicrobial susceptibility assays, all based on inferring carbohydrate-protein interactions directly on cell surfaces, thus illustrating their potential uses in infectious disease research, clinical diagnostics, and environmental monitoring of harmful pathogens.

Glycosylated aniline polymer sensor: amine to imine conversion on protein-carbohydrate binding

In this report, functionalized mannosylated aniline polymer (manno-PANI) was investigated as an electrochemical platform to study carbohydrate-protein interactions by exploiting the conductivity change of manno-PANI when the specific lectin binding occurs. A systematic study was performed to characterize the interconversion of polyaniline content (from amine to imine) in manno-PANI by UV-vis spectroscopy during its binding with concanavalin A (Con A). Both X-ray photoelectron spectrometry (XPS) and UV-vis results suggest that Con A binding with the manno-PANI film triggers the switching of amine functionalities in the polyaniline backbone, converting them to imine forms. Electrochemical impedance spectroscopy (EIS) was used to quantify the specific interactions between Con A and mannose by measuring the impedance change of manno-PANI film for the detection of Con A. A linear relationship between the impedance and Con A concentration was obtained, and the detection limit reaches to 0.12 nM Con A in a buffer solution (pH=7.4), whereas the addition of nonspecific control lectins to the same manno-PANI film gave very little impedance variations. Stability characterization of the manno-PANI film over 20 weeks shows a maximum drift of only 3% from the original signal. Thus, the uniquely constructed carbohydrate-PANI hybrid is a promising new carbohydrate recognition moiety for studying carbohydrate-protein interactions, presumably leading to a new electrochemical method for characterization of carbohydrate-protein interactions and carbohydrate-mediated intercellular recognitions.

Crystal cookery - using high-throughput technologies and the grocery store as a teaching tool

Crystallography is a multidisciplinary field that links divergent areas of mathematics, science and engineering to provide knowledge of life on an atomic scale. Crystal growth, a key component of the field, is an ideal vehicle for education. Crystallization has been used with a 'grocery store chemistry' approach and linked to high-throughput remote-access screening technologies. This approach provides an educational opportunity that can effectively teach the scientific method, readily accommodate different levels of educational experience, and reach any student with access to a grocery store, a post office and the internet. This paper describes the formation of the program through the students who helped develop and prototype the procedures. A summary is presented of the analysis and preliminary results and a description given of how the program could be linked with other aspects of crystallography. This approach has the potential to bridge the gap between students in remote locations and with limited funding, and access to scientific resources, providing students with an international-level research experience.

Binding of dengue virus particles and dengue proteins onto solid surfaces

The interaction between dengue virus particles (DENV), sedimentation hemagglutinin particles (SHA), dengue virus envelope protein (Eprot), and solid surfaces was investigated by means of ellipsometry and atomic force microscopy (AFM). The surfaces chosen are bare Si/SiO2 wafers and Si/SiO2 wafers covered with concanavalin A (ConA), jacalin (Jac), polystyrene (PS), or poly(styrene sulfonate) (PSS) films. Adsorption experiments at pH 7.2 and pH 3 onto all surfaces revealed that (i) adsorption of DENV particles took place only onto ConA under pH 7.2, because of specific recognition between glycans on DENV surface and ConA binding site; (ii) DENV particles did not attach to any of the surfaces at pH 3, suggesting the presence of positive charges on DENV surface at this pH, which repel the positively charged lectin surfaces; (iii) SHA particles are positively charged at pH 7.2 and pH 3 because they adhered to negatively charged surfaces at pH 7.2 and repelled positively charged layers at pH 3; and (iv) SHA particles carry polar groups on the surface because they attached to silanol surfaces at pH 3 and avoided hydrophobic PS films at pH 3 and pH 7.2. The adsorption behavior of Eprot at pH 7.2 revealed affinity for ConA>Jac>PSS>PS≈bare Si/SiO2 layers. These findings indicate that selectivity of the Eprot adsorption is higher when it is part of virus structure than when it is free in solution. The correlation between surface energy values determined by means of contact angle measurements and DENV, SHA, or Eprot adsorption behavior was used to understand the intermolecular forces at the interfaces. A direct correlation was not found because the contributions from surface energy were probably surpassed by specific contributions.

Oxide-dependent adhesion of the Jurkat line of T lymphocytes

The adhesion force of Jurkat cells was measured using atomic force microscopy (AFM) in aqueous solution at pH 7.2 on six metal oxide surfaces, namely, two quartz (alpha-SiO2) crystal faces, amorphous SiO2 glass, rutile (alpha-TiO2), muscovite mica (KAl2(AlSi3O10)(OH)2), and polycrystalline corundum (alpha-Al2O3). We show quantitatively for the first time that the T lymphocyte adhesion force and adhesion work correlates with substrate point of zero charge, indicating greater adsorption on surfaces with smaller negative charge. Adhesion events also exhibited sawtooth-shaped force-distance profiles indicative of protein bonds. No significant correlations were found with oxide Hamaker constants, indicating negligible contributions from van der Waals forces, nor with surface roughness. These results suggest that, when cell-surface receptors are not activated, Jurkat cell adhesion is dominated by specific interactions related to the unfolding of modular glycoproteins or other proteins that are not unique to T-cell surfaces and by electrostatic forces between negatively charged glycoproteins and variably charged oxide surfaces. Our results have implications for the interactions of immune system cells with metal oxides present in the human body either by design as in biomedical applications or inadvertently such as inhaled mineral dust particles in the lung.

Surface recognition and fluorescence sensing of histone by dansyl-appended cyclophane-based resorcinarene trimer

A cyclophane-based resorcinarene trimer (3) bearing a dansyl moiety as an environmentally sensitive fluorophore was prepared by stepwise condensation of a tetraaza[6.1.6.1]paracyclophane skeleton with a dansyl moiety and three resorcinarene derivatives having heptacarboxylic acid residues in this sequence. The dansyl-appended cyclophane exhibited the following fluorescence properties regarding solvent polarity dependency and histone surface recognition: With increasing dioxane contents in dioxane/water solvents, the fluorescence intensity originating from the dansyl moiety of 3 increased along with a concomitant blue shift of the fluorescence maximum (lambdaem). The microenvironmentally sensitive fluorescence properties of dansyl fluorophore were maintained, even when the dansyl moiety was covalently attached to a cyclophane. Most interestingly, the cyclophane-based resorcinarene trimer exhibited recognition and fluorescence sensing capabilities toward histone, a small basic protein of eukaryotic chromatins. The fluorescence intensity originating from 3 increased along with a concomitant blue shift of lambdaem upon the addition of histone, reflecting the formation of 3-histone complexes. A relatively large fluorescence polarization (P) value was obtained for the 3-histone complexes (0.15), reflecting highly restricted conformations of 3, and the obtained P value was much larger than that of 3 alone in aqueous medium (0.07). The binding constant (K) of 3 with histone (unit basis) was estimated to be 2.1 x 106 M-1. On the other hand, upon the addition of acetylated histone (Ac-histone) to an aqueous solution containing 3, the extent of change in fluorescence intensity originating from the dansyl group of 3 was almost negligible, indicating that the electrostatic interactions between 3 and Ac-histone were weak. In addition, the fluorescence spectral changes were also small or negligible upon the addition of other proteins such as albumin, ovalbumin, peanut agglutinin, myoglobin, concanavalin A, cytochrome c, and lysozyme, having isoelectric points of 4.7, 4.8, 5.7-6.7, 6.8, 7.1, 9, and 11.0, respectively, to an aqueous solution containing 3.

Dielectric properties of Bauhinia monandra and concanavalin A lectin monolayers, part I

The dielectric properties of the galactose-binding lectins Bauhinia monandra (BmoLL) and Concanavalin A (Con A) were assessed by surface potential measurements of their spread monolayers on an aqueous subphase containing a monovalent electrolyte. For both lectins the curves of surface potential versus mean molecular area (DeltaV-A) and the independently recorded isotherms of surface pressure versus mean molecular area (Pi-A) were shown to be pH-dependent. As the subphase pH changed from 2 to 9, a noticeable trend to higher surface pressures accompanied the compression of the monolayers. Conversely, the surface potentials values of both monolayers decreased with increasing pH. For Con A, with the single exception of the pH 9 case, lowering the pH yielded DeltaV values higher than those for BmoLL. The contribution of the electric double layer (Psi0) to the overall DeltaV values at a given Pi (15 mN/m) was calculated using a modified Davies equation and assuming that at this surface pressure the monolayers of both studied lectins were stable. While at all studied pHs the Psi0 values for Con A exceeded those calculated for BmoLL, for both lectins they were insensitive to pH changes. This provided evidence that the reorientation of lectin molecules, during compression predominantly contributed to the alteration of the overall DeltaV values. The calculated Psi0 values made possible the evaluation of the dipole moments for BmoLL and Con A, and it has been estimated that the decrease in the pH of the subphase from 9 to 2 produced a 1.6-fold (twofold) increase in the value of for BmoLL (Con A). The differences in dielectric properties between the two film-forming lectins have been attributed to the differences in their structures. Indeed, the circular dichroism (CD) spectrum of Con A showed the predominance of beta-plated sheet structures while that of BmoLL was typically rich in alpha-helix structures.

Isolation, purification, and physicochemical characterization of a D-galactose-binding lectin from seeds of Erythrina speciosa

A lectin was isolated from the saline extract of Erythrina speciosa seeds by affinity chromatography on lactose-Sepharose. The lectin content was about 265 mg/100g dry flour. E. speciosa seed lectin (EspecL) agglutinated all human RBC types, showing no human blood group specificity; however a slight preference toward the O blood group was evident. The lectin also agglutinated rabbit, sheep, and mouse blood cells and showed no effect on horse erythrocytes. Lactose was the most potent inhibitor of EspecL hemagglutinating activity (minimal inhibitory concentration (MIC)=0.25 mM) followed by N-acetyllactosamine, MIC=0.5mM, and then p-nitrophenyl alpha-galactopyranoside, MIC=2 mM. The lectin was a glycoprotein with a neutral carbohydrate content of 5.5% and had two pI values of 5.8 and 6.1 and E(1%)(1 cm) of 14.5. The native molecular mass of the lectin detected by hydrodynamic light scattering was 58 kDa and when examined by mass spectroscopy and SDS-PAGE it was found to be composed of two identical subunits of molecular mass of 27.6 kDa. The amino acid composition of the lectin revealed that it was rich in acidic and hydroxyl amino acids, contained a lesser amount of methionine, and totally lacked cysteine. The N-terminal of the lectin shared major similarities with other reported Erythrina lectins. The lectin was a metaloprotein that needed both Ca(2+) and Mn(2+) ions for its activity. Removal of these metals by EDTA rendered the lectin inactive whereas their addition restored the activity. EspecL was acidic pH sensitive and totally lost its activity when incubated with all pH values between pH 3 and pH 6. Above pH 6 and to pH 9.6 there was no effect on the lectin activity. At 65 degrees C for more than 90 min the lectin was fairly stable; however, when heated at 70 degrees C for 10 min it lost more than 80% of its original activity and was totally inactivated at 80 degrees C for less than 10 min. Fluorescence studies of EspecL indicated that tryptophan residues were present in a highly hydrophobic environment, and binding of lactose to EspecL neither quenched tryptophan fluorescence nor altered lambda(max) position. Treating purified EspecL with NBS an affinity-modifying reagent specific for tryptophan totally inactivated the lectin with total modification of three tryptophan residues. Of these residues only the third modified residue seemed to play a crucial role in the lectin activity. Addition of lactose to the assay medium did not provide protection against NBS modification which indicated that tryptophan might not be directly involved in the binding of haptenic sugar D-galactose. Modification of tyrosine with N-acetylimidazole led to a 50% drop in EspecL activity with concomitant acetylation of six tyrosine residues. The secondary structure of EspecL as studied by circular dichroism was found to be a typical beta-pleated-sheet structure which is comparable to the CD structure of Erythrina corallodendron lectin. Binding of lactose did not alter the EspecL secondary structure as revealed by CD examination.

Purification of Cajanus cajan root lectin and its interaction with rhizobial lipopolysaccharide as studied by different spectroscopic techniques

A lectin present in roots of Cajanus cajan seedlings was isolated and purified by affinity chromatography. Sugar specificity assayed by hemagglutination-inhibition activity indicated that lectin belongs to glucose/mannose-specific group. The root lectin was found to be mannose-specific from the second day onwards as it was reconfirmed by specific elution of different days' sample from mannose agarose matrix. The maximum interaction of lectin with goat IgM was obtained in 10-day-old sample, indicating the highest crude lectin content. Lectin (total amount of eluted protein) from different days soil sample showed a maximum amount in 10-day-old sample. For further studies, the lectin has been isolated from the roots of 10-day C. cajan seedlings and purified on mannose-CL agarose column by affinity chromatography. Lectin was found to be a dimer of 18.5-kDa subunit as revealed by SDS-PAGE. Tryptophan quenching fluorescence was studied for C. cajan root lectin. Secondary structure of C. cajan root lectin as studied by circular dichroism was found to be a typical beta-pleated sheet structure. The interaction of purified root lectin with C. cajan-specific rhizobial lipopolysaccharide and its inhibition by specific and nonspecific sugars was demonstrated by fluorescence and circular dichroism. Results discussed in this paper were studied for the first time by different spectroscopic methods, suggesting that C. cajan root lectin-lipopolysaccharide interaction is specific.

Immunological significance of metal induced conformational changes in the mitogenic achatininH binding to carbohydrate ligands

9-O-Acetyl neuraminic acid specific lectin (AchatininH) was isolated from the hemolymph of the land snail Achatina fulica by affinity chromatography on sheep submaxillary mucin (SSM) coupled cyanogen bromide activated Sepharose 4B. The molecular weight of the native protein was 2.42 kDa. UV-Vis absorption, fluorescence and circular dichroism spectroscopic studies on AchatininH revealed the importance of divalent metal ions (Ca2 +, Mg2+ and Mn2+) on lectin conformational change associated with activity of lectins. The binding of these cations changes lambdamax to shorter wavelength in the far UV region (blue shift) and longer wavelength in UV region (red shift), indicating substantial contribution of aromatic side chain in the far UV region on binding with metal ions. The results infer that divalent cations cause conformational changes in lectin which may be responsible for affinity with their carbohydrate moiety.

Denaturation of concanavalin A by urea at acid pH

The denaturation of dimeric concanavalin A induced by urea at pH 3 has been studied using optical activity and sedimentation velocity. Under the conditions employed Mn+2 and Ca+2 are dissociated from the protein, but the basic structural elements are little changed from those prevailing in the functional lectin at pH 5.5 [H.E. Auer and T. Schilz, preceding paper in this issue]. The protein passes through three stages as the urea concentration is varied from 0 to 10 M. Below 4 M urea the only effect observed is the loss of optical activity of the aromatic amino acid residues. At 4 M, a conformational change occurs producing extensive aggregation, which persists to 7 M. At 8-10 M urea a disordered monomeric protein molecule prevails. The protein could be reactivated provided that dilution to native conditions was very rapid and the protein concentration remained very low. Kinetics of denaturation were monitored by optical activity at 218, 225 and 283 nm. Transients with one, two or three components were observed, which were resolved by nonlinear regression according to sequential first-order decay laws. First order character was confirmed by independence of the kinetic parameters from protein concentration over a two- to four-fold range. Enthalpies and entropies of activation for the various steps were also determined. The transients at the three wavelengths monitor changes in beta structure, beta turns and aromatic groups, respectively. The urea dependence of the rate constants is unique in most cases. It is concluded that different structural elements of the concanavalin A molecule unfold independently from one another.

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.

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.

Stabilization of concanavalin A by metal ligands

Metal-free concanavalin A is readily and irreversibly inactivated by temperatures above 60 degrees. Manganese ion completely prevents the thermal aggregation of the protein at 60 and 70 degrees, and partially protects at 80 degrees, but shows no protective properties at 90 degrees. Managanese protection against thrermal aggregation was found to be maximal at pH 4-8. The precipitation between glycogen and Mn2+-stabilized conanavian A is partially inhibited at temperatures greater than 30 degrees, but can be reversed by cooling to room temperature...

Self-association, conformation and binding equilibria of concanavalin A

The discovery of distinct intact and fragmented forms of Con A, together with the observation that Con A self-associates near neutrality raises questions that may be important when interpreting experiments concerned with the biological actions of the protein. Do intact and fragmented units have the same affinity for carbohydrate? Do intact and fragmented units differ in conformation? Are all dimeric units of a homologous type or do hybrid dimers consisting of one intact and one fragmented unit also exist? Can all dimeric types self-associate to the tetramer form? Do dimer and tetramer species differ in their affinity for carbohydrate? These questions have been made amenable to investigation by the development of a method which separates intact and fragmented species under conditions which do not cause time-dependent or irreversible changes in protein conformation. It is found that intact dimeric units preferentially associate to the tetramer form. Under appropriate conditions of pH and ionic strength, dimer and tetramer species, and therefore fragmented and intact forms, can be separated by chromatography on Bio Gel P-100. Hybrid dimers are not present in appreciable amounts. Both types of homologous dimers (intact and fragmented) have similar affinity for carbohydrate, but dimer and tetramer species show significant differences. The results of near UV circular dichroism studies indicate that fragmented units possess slightly different conformation than intact units. An ionization-linked conformational transition in Con A does not appear to be linked directly with the self-association of the protein between pH 5 and 7. Ligand-induced changes in the conformation of Con A are now being examined in detail. Pflumm et al. (1971) have shown that occupation of the sugar binding site of Con A results in a perturbation of conformation as revealed by near UV circular dichroism measurements. The perturbation is relatively small and does not result in more than 1-2% increase in the rotational relaxation time (Shinitzky et al., 1973). On the other hand, removal of metal ions causes a hydrodynamic change sufficient to increase the frictional coefficient and to decrease the sedimentation coefficient (S20, w) from 3.98 S to 3.78 S. Differences between the native and the apoprotein conformation are now being examined using fluorescence polarization and the hydrophobic fluorescent probe 1-anilinonaphthalene-8-sulfonate.