Oligomerization endows enormous stability to soybean agglutinin: a comparison of the stability of monomer and tetramer of soybean agglutinin

Effect of temperature on structural configuration and immunoreactivity of pH-stressed soybean (Glycine max) agglutinin

Soybean agglutinin (SBA) was purified using ammonium sulfate precipitation and liquid chromatography. Purified SBA was used to produce monoclonal antibodies through hybridoma technology. SBA secondary structure was studied using circular dichroism. pH-stressed (pHs 3.0, 7.2, 8.5, and 9.6) SBA physical properties (particle size, ζ-potential, and aggregation temperature) were investigated. Gel electrophoresis (non-native and native) was used to study heat-induced structural configuration changes in SBA. The effect of pH and temperature on the immunoreactivity of SBA was analyzed using enzyme-linked immunosorbent assay and immunoblots probed with two anti-SBA monoclonal antibodies with either linear or conformational epitopes. The hemagglutinating activity of heated SBA was measured by hemagglutination assay. Our results indicated that SBA had the least thermostability at pH 3.0 and the highest at pH 8.5. Temperature-induced structural configuration change on pH-stressed SBA led to immunoreactivity change. Heat-induced (70 and 80 °C) soluble SBA aggregation was proportionally related to hemagglutinating activity reduction.

Computational insights into the circular permutation roles on ConA binding and structural stability

The mechanisms behind Concanavalin A (ConA) circular permutation have been under investigation since 1985. Although a vast amount of information is available about this lectin and its applications, the exact purpose of its processing remains unclear. To shed light on this, this study employed computer simulations to compare the unprocessed ProConA with the mature ConA. This approach aimed to reveal the importance of the post-translational modifications, especially how they affect the lectin stability and carbohydrate-binding properties. To achieve these goals, we conducted 200 ns molecular dynamics simulations and trajectory analyses on the monomeric forms of ProConA and ConA (both unbound and in complex with D-mannose and the GlcNAc2Man9 N-glycan), as well as on their oligomeric forms. Our findings reveal significant stability differences between ProConA and ConA at both the monomeric and tetrameric levels, with ProConA exhibiting consistently lower stability parameters compared to ConA. In terms of carbohydrate binding properties, however, both lectins showed remarkable similarities in their interaction profiles, contact numbers, and binding free energies with D-mannose and the high-mannose N-glycan. Overall, our results suggest that the processing of ProConA significantly enhances the stability of the mature lectin, especially in maintaining the tetrameric oligomer, without substantially affecting its carbohydrate-binding properties.

The four subunits of rabbit skeletal muscle lactate dehydrogenase do not exert their catalytic action additively

Oligomeric enzymes containing multiple active sites are usually considered to perform their catalytic action at higher rates when compared with their monomeric counterparts. This implies, in turn, that the activity performed by different holoenzyme subunits features additivity. Nevertheless, the extent of this additivity occurring in holoenzymes is far from being adequately understood. To tackle this point, we used tetrameric rabbit lactate dehydrogenase (rbLDH) as a model system to assay the reduction of pyruvate catalysed by this enzyme at the expense of β-NADH under pre-steady-state conditions. In particular, we observed the kinetics of reactions triggered by concentrations of β-NADH equimolar to 1, 2, 3, or all 4 subunits of the rbLDH holoenzyme, in the presence of an excess of pyruvate. Surprisingly, when the concentration of the limiting reactant exceeded that of a single holoenzyme subunit, we observed a sharp slowdown of the enzyme conformational rearrangements associated to the generation and the release of l-lactate. Furthermore, using a model to interpret the complex kinetics observed under the highest concentration of the limiting reactant, we estimated the diversity of the rates describing the action of the different rbLDH subunits.

Low-pH Molten Globule-Like Form of CIA17, a Chitooligosaccharide-Specific Lectin from the Phloem Exudate of Coccinia indica, Retains Carbohydrate-Binding Ability

pH-induced changes in the conformation, structural dynamics, and carbohydrate-binding activity of Coccinia indica agglutinin (CIA17), a PP2-type lectin, were investigated employing biophysical approaches. The secondary structure of CIA17 remains nearly unaltered over a wide pH range (2.0-8.5), while the tertiary structure of the protein exhibits considerable changes. A decrease in the fluorescence intensity and excited-state lifetime at low pH indicated perturbation in the local conformation (near Trp residues) of CIA17, which was further supported by enhancement in the Trp accessibility toward charged quenchers under acidic conditions. Fluorescence correlation spectroscopic studies indicated that at pH 2.0, CIA17 exists as a monomer over the concentration range of 10-200 nM and forms dimers at higher concentrations (K_D ∼ 387 nM) but could not form higher oligomers even at ∼150-fold higher concentrations, unlike under native conditions at pH 7.4. Thermal unfolding of the low pH intermediate involves two distinct steps: dissociation of a dimer to a monomer, followed by the unfolding of the monomer. These results strongly suggest that the acid-induced unfolding pathway of CIA17 involves the formation of a monomeric molten globule-like intermediate, which retains appreciable carbohydrate-binding ability. These observations are of great physiological significance since the PP2 proteins are involved in plant defense responses.

Apolipoprotein E4 exhibits intermediates with domain interaction

ApoE4(C112R) is the strongest risk factor for Alzheimer's disease, while apoE3(C112) is considered normal. The C112R substitution is believed to alter the interactions between the N-terminal (NTD) and the C-terminal domain (CTD) leading to major functional differences. Here we investigate how the molecular property of the residue at position 112 affects domain interaction using an array of C112X substitutions with arginine, alanine, threonine, valine, leucine and isoleucine as 'X'. We attempt to determine the free energy of domain interaction (∆G_INT) from stabilities of the NTD (∆G_NTD) and CTD (∆G_CTD) in the full-length apoE, and the stabilities of fragments of the NTD (∆G_NTF) and CTD (∆G_CTF), using the relationship, ∆G_INT = ∆G_NTD + ∆G_CTD - ∆G_NTF - ∆G_CTF. We find that although ∆G_NTD is strongly dependent on the C112X substitutions, ∆G_NTD - ∆G_NTF is small. Furthermore, ∆G_CTD remains nearly the same as ∆G_CTF. Therefore, ∆G_INT is estimated to be small and similar for the apoE isoforms. However, stability of domain interaction monitored by urea dependent changes in interdomain Forster Resonance Energy Transfer (FRET) is found to be strongly dependent on C112X substitutions. ApoE4 exhibits the highest mid-point of denaturation of interdomain FRET. To resolve the apparently contradictory observations, we hypothesize that higher interdomain FRET in apoE4 in urea may involve 'intermediate' states. Enhanced fluorescence of bis-ANS and susceptibility to proteolytic cleavage support that apoE4, specifically, the NTD of apoE4 harbor 'intermediates' in both native and mildly denaturing conditions. The intermediates could hold key to the pathological functions of apoE4.

Delineating the role of ionic interactions in structural and functional integrity of B. malayi Guanylate kinase

The present work deals with investigating the role of ionic interactions in the native conformation of BmGK by altering pH and salt concentration as well as by disruption of inter-subunit region. The study on structural and functional properties of BmGK as a function of pH showed that the secondary and tertiary elements of the protein were disturbed at low pH with loss of its native oligomerization and functional activity. High concentration of NaCl also changed the native conformation of BmGK with dissociation of its dimeric form. We also mutated dimeric interface of BmGK and identified intersubunit residues, Arg105 and Glu140, essential for dimer stability as double mutation at both positions hinders dimerization. The quaternary structure is found to be essential for full enzymatic activity and stability. In vitro results were supported by in silico molecular dynamics simulation studies through conformational stability analysis. Thus, the work carried out points toward new approach of targeting dimeric interface of BmGK in lieu of its similar active site region to its counterpart human enzyme. This may lead to the design of inhibitors targeted to key parasitic enzyme (BmGK) specifically.

Isolation and partial characterization of 3 nontoxic d-galactose-specific isolectins from seeds of Momordica balsamina

Three isolectins denoted hereforth MBaL-30, MBaL-60, and MBaL-80 were isolated from seeds extract of Momordica balsamina by 30%, 60%, and 80% ammonium sulfate saturations, respectively. The native molecular weights of these lectins, as judged by gel filtration, were 108, 56, and 160 kDa, respectively. On SDS-PAGE, under reduced condition, 27 kDa band was obtained for all isolectins. The lectins hemagglutinating activities were variably inhibited by d-galactose (minimum inhibitory concentrations = 12.5mM, 50mM, and 0.391mM, respectively). MBaL-30 and -60 could agglutinate all human blood types with slight preference for the A and O blood groups, whereas MBaL-80 did not agglutinate B and AB blood types. The 3 isolectins were purified from crude seeds extract, collectively, in a single step on the affinity matrix Lactamyl-Seralose 4B; this purified lectin fraction, which contains all isolectins, is termed MBaL. The N-terminal of MBaL till the 25th amino acid was NLSLSELDFSADTYKSFIKNLRKQL, which shares 88% sequence identity with Momordica charantia lectin type-2 ribosomal inactivating protein from Momordica charantia and 50% with momordin II from Momordica balsamina. MBaL retained 100% activity at up to 50°C for 30 minutes. MBaL-30 and MBaL-60 exhibited maximum activities in the pH range between 4 and 8, while MBaL-80 was showing maximum activity in the pH range between 3 and 5. Treatment of MBaL-30 and MBaL-60 with EDTA completely abolished their hemagglutinating activities. Addition of Zn and Fe ions to the ethylenediaminetetraacetic acid-treated MBaL-30 and MBaL-60 lectins did not only regained the loss of activity but also resulted in 200% to 300% increase in activity, respectively. MBaL-30 and -60 agglutinated gram positive Listeria monocytogenes and Staphylococcus aureus, whereas MBaL-30 could merely agglutinate Escherichia coli. None of these lectins could arrest bacterial growth. Addition of MBaL to cancer cell lines (Gastric cancer cell line (AGS) and Gastric cencer cell line (MKN45), Glioblastoma (ECV-304), and Human urinary bladder cancer cell line (U87-MG)) at varying concentrations did not cause statistically significant changes on cell growth and viability.

Nanoscale materials for probing the biological functions of the glycocalyx

Glycans are among the most intriguing carriers of biological information in living systems. The structures of glycans not only convey the cells' physiological state, but also regulate cellular communication and responses by engaging receptors on neighboring cells and in the extracellular matrix. The assembly of simple monosaccharide building blocks into linear or branched oligo- and polysaccharides gives rise to a large repertoire of diverse glycan structures. Despite their structural complexity, individual glycans rarely engage their protein partners with high affinity. Yet, glycans modulate biological processes with exquisite selectivity and specificity. To correctly evaluate glycan interactions and their biological consequences, one needs to look beyond individual glycan structures and consider the entirety of the cell-surface landscape. There, glycans are presented on protein scaffolds, or are linked directly to membrane lipids, forming a complex, hierarchically organized network with specialized functions, called the glycocalyx. Nanoscale glycomaterials, which can mimic the various components of the glycocalyx, have been instrumental in revealing how the presentation of glycans can influence their biological functions. In this review, we wish to highlight some recent developments in this area, while placing emphasis on the applications of glycomaterials providing new insights into the mechanisms through which glycans mediate cellular functions.

Urea Unfolding Study of E. coli Alanyl-tRNA Synthetase and Its Monomeric Variants Proves the Role of C-Terminal Domain in Stability

E. coli alanyl-tRNA exists as a dimer in its native form and the C-terminal coiled-coil part plays an important role in the dimerization process. The truncated N-terminal containing the first 700 amino acids (1–700) forms a monomeric variant possessing similar aminoacylation activity like wild type. A point mutation in the C-terminal domain (G674D) also produces a monomeric variant with a fivefold reduced aminoacylation activity compared to the wild type enzyme. Urea induced denaturation of these monomeric mutants along with another alaRS variant (N461 alaRS) was studied together with the full-length enzyme using various spectroscopic techniques such as intrinsic tryptophan fluorescence, 1-anilino-8-naphthalene-sulfonic acid binding, near- and far-UV circular dichroism, and analytical ultracentrifugation. Aminoacylation activity assay after refolding from denatured state revealed that the monomeric mutants studied here were unable to regain their activity, whereas the dimeric full-length alaRS gets back similar activity as the native enzyme. This study indicates that dimerization is one of the key regulatory factors that is important in the proper folding and stability of E. coli alaRS.

A quaternary mechanism enables the complex biological functions of octameric human UDP-glucose pyrophosphorylase, a key enzyme in cell metabolism

In mammals, UDP-glucose pyrophosphorylase (UGP) is the only enzyme capable of activating glucose-1-phosphate (Glc-1-P) to UDP-glucose (UDP-Glc), a metabolite located at the intersection of virtually all metabolic pathways in the mammalian cell. Despite the essential role of its product, the molecular basis of UGP function is poorly understood. Here we report the crystal structure of human UGP in complex with its product UDP-Glc. Beyond providing first insight into the active site architecture, we describe the substrate binding mode and intermolecular interactions in the octameric enzyme that are crucial to its activity. Importantly, the quaternary mechanism identified for human UGP in this study may be common for oligomeric sugar-activating nucleotidyltransferases. Elucidating such mechanisms is essential for understanding nucleotide sugar metabolism and opens the perspective for the development of drugs that specifically inhibit simpler organized nucleotidyltransferases in pathogens.

Thermal, chemical and pH induced unfolding of turmeric root lectin: modes of denaturation

Curcuma longa rhizome lectin, of non-seed origin having antifungal, antibacterial and α-glucosidase inhibitory activities, forms a homodimer with high thermal stability as well as acid tolerance. Size exclusion chromatography and dynamic light scattering show it to be a dimer at pH 7, but it converts to a monomer near pH 2. Circular dichroism spectra and fluorescence emission maxima are virtually indistinguishable from pH 7 to 2, indicating secondary and tertiary structures remain the same in dimer and monomer within experimental error. The tryptophan environment as probed by acrylamide quenching data yielded very similar data at pH 2 and pH 7, implying very similar folding for monomer and dimer. Differential scanning calorimetry shows a transition at 350.3 K for dimer and at 327.0 K for monomer. Thermal unfolding and chemical unfolding induced by guanidinium chloride for dimer are both reversible and can be described by two-state models. The temperatures and the denaturant concentrations at which one-half of the protein molecules are unfolded, are protein concentration-dependent for dimer but protein concentration-independent for monomer. The free energy of unfolding at 298 K was found to be 5.23 Kcal mol⁻¹ and 14.90 Kcal mol⁻¹ for the monomer and dimer respectively. The value of change in excess heat capacity upon protein denaturation (ΔC_p) is 3.42 Kcal mol⁻¹ K⁻¹ for dimer. The small ΔC_p for unfolding of CLA reflects a buried hydrophobic core in the folded dimeric protein. These unfolding experiments, temperature dependent circular dichroism and dynamic light scattering for the dimer at pH 7 indicate its higher stability than for the monomer at pH 2. This difference in stability of dimeric and monomeric forms highlights the contribution of inter-subunit interactions in the former.

Square-wave voltammetry assays for glycoproteins on nanoporous gold

Electrochemical enzyme-linked lectinsorbent assays (ELLA) were developed using nanoporous gold (NPG) as a solid support for protein immobilization and as an electrode for the electrochemical determination of the product of the reaction between alkaline phosphatase (ALP) and p-aminophenyl phosphate (p-APP), which is p-aminophenol (p-AP). Glycoproteins or concanavalin A (Con A) and ALP conjugates were covalently immobilized onto lipoic acid self-assembled monolayers on NPG. The binding of Con A - ALP (or soybean agglutinin - ALP) conjugate to glycoproteins covalently immobilized on NPG and subsequent incubation with p-APP substrate was found to result in square-wave voltammograms whose peak difference current varied with the identity of the glycoprotein. NPG presenting covalently bound glycoproteins was used as the basis for a competitive electrochemical assay for glycoproteins in solution (transferrin and IgG). A kinetic ELLA based on steric hindrance of the enzyme-substrate reaction and hence reduced enzymatic reaction rate after glycoprotein binding is demonstrated using immobilized Con A-ALP conjugates. Using the immobilized Con A-ALP conjugate, the binding affinity of immunoglobulin G (IgG) was found to be 105 nM, and that for transferrin was found to be 650 nM. Minimal interference was observed in the presence of 5 mg mL-1 BSA as a model serum protein in both the kinetic and competitive ELLA. Inhibition studies were performed with methyl D-mannoside for the binding of TSF and IgG to Con A-ALP; IC50 values were found to be 90 μM and 286 μM, respectively. Surface coverages of proteins were estimated using solution depletion and the BCA protein concentration assay.

Monomeric banana lectin at acidic pH overrules conformational stability of its native dimeric form

Banana lectin (BL) is a homodimeric protein categorized among jacalin-related family of lectins. The effect of acidic pH was examined on conformational stability of BL by using circular dichroism, intrinsic fluorescence, 1-anilino-8-napthalene sulfonate (ANS) binding, size exclusion chromatography (SEC) and dynamic light scattering (DLS). During acid denaturation of BL, the monomerization of native dimeric protein was found at pH 2.0. The elution profile from SEC showed two different peaks (59.65 ml & 87.98 ml) at pH 2.0 while single peak (61.45 ml) at pH 7.4. The hydrodynamic radii (R h) of native BL was 2.9 nm while at pH 2.0 two species were found with R h of 1.7 and 3.7 nm. Furthermore at, pH 2.0 the secondary structures of BL remained unaltered while tertiary structure was significantly disrupted with the exposure of hydrophobic clusters confirming the existence of molten globule like state. The unfolding of BL with different subunit status was further evaluated by urea and temperature mediated denaturation to check their stability. As inferred from high Cm and ΔG values, the monomeric form of BL offers more resistance towards chemical denaturation than the native dimeric form. Besides, dimeric BL exhibited a Tm of 77°C while no loss in secondary structures was observed in monomers even up to 95°C. To the best of our knowledge, this is the first report on monomeric subunit of lectins showing more stability against denaturants than its native dimeric state.

Trifluoroethanol-induced conformational change of tetrameric and monomeric soybean agglutinin: role of structural organization and implication for protein folding and stability

2,2,2-Trifuoroethanol (TFE)-induced conformational structure change of a β-sheet legume lectin, soybean agglutinin (SBA) has been investigated employing its exclusive structural forms in quaternary (tetramer) and tertiary (monomer) states, by far- and near-UV CD, FTIR, fluorescence, low temperature phosphorescence and chemical modification. Far-UV CD results show that, for SBA tetramer, native atypical β-conformation transforms to a highly α-helical structure, with the helical content reaching 57% in 95% TFE. For SBA monomer, atypical β-sheet first converts to typical β-sheet at low TFE concentration (10%), which then leads to a nonnative α-helix at higher TFE concentration. From temperature-dependent studies (5-60 °C) of TFE perturbation, typical β-sheet structure appears to be less stable than atypical β-sheet and the induced helix entails reduced thermal stability. The heat induced transitions are reversible except for atypical to typical β-sheet conversion. FTIR results reveal a partial α-helix conversion at high protein concentration but with quantitative yield. However, aggregation is detected with FTIR at lower TFE concentration, which disappears in more TFE. Near-UV CD, fluorescence and phosphorescence studies imply the existence of an intermediate with native-like secondary and tertiary structure, which could be related to the dissociation of tetramer to monomer. This has been further supported by concentration dependent far-UV CD studies. Chemical modification with N-bromosuccinimide (NBS) shows that all six tryptophans per monomer are solvent-exposed in the induced α-helical conformation. These results may provide novel and important insights into the perturbed folding problem of SBA in particular, and β-sheet oligomeric proteins in general.

Density variant glycan microarray for evaluating cross-linking of mucin-like glycoconjugates by lectins

Interactions of mucin glycoproteins with cognate receptors are dictated by the structures and spatial organization of glycans that decorate the mucin polypeptide backbone. The glycan-binding proteins, or lectins, that interact with mucins are often oligomeric receptors with multiple ligand binding domains. In this work, we employed a microarray platform comprising synthetic glycopolymers that emulate natural mucins arrayed at different surface densities to evaluate how glycan valency and spatial separation affect the preferential binding mode of a particular lectin. We evaluated a panel of four lectins (Soybean agglutinin (SBA), Wisteria floribunda lectin (WFL), Vicia villosa-B-4 agglutinin (VVA), and Helix pomatia agglutin (HPA)) with specificity for α-N-acetylgalactosamine (α-GalNAc), an epitope displayed on mucins overexpressed in many adenocarcinomas. While these lectins possess the ability to agglutinate A₁-blood cells carrying the α-GalNAc epitope and cross-link low valency glycoconjugates, only SBA showed a tendency to form intermolecular cross-links among the arrayed polyvalent mucin mimetics. These results suggest that glycopolymer microarrays can reveal discrete higher-order binding preferences beyond the recognition of individual glycan epitopes. Our findings indicate that glycan valency can set thresholds for cross-linking by lectins. More broadly, well-defined synthetic glycopolymers enable the integration of glycoconjugate structural and spatial diversity in a single microarray screening platform.

High-yield expression of recombinant soybean agglutinin in plants using transient and stable systems

Soybean agglutinin (SBA) is a specific N-acetylgalactosamine-binding plant lectin that can agglutinate a wide variety of cells. SBA has great potential for medical and biotechnology-focused applications, including screening and treatment of breast cancer, isolation of fetal cells from maternal blood for genetic screening, the possibility as a carrier system for oral drug delivery, and utilization as an affinity tag for high-quality purification of tagged proteins. The success of these applications, to a large degree, critically depends on the development of a highly efficient expression system for a source of recombinant SBA (rSBA). Here, we demonstrate the utility of transient and stable expression systems in Nicotiana benthamiana and potato, respectively, for the production of rSBA, with the transgenic protein accumulated to 4% of total soluble protein (TSP) in Nicotiana benthamiana leaves and 0.3% of TSP in potato tubers. Furthermore, we show that both plant-derived rSBAs retain their ability to induce the agglutination of red blood cells, are similarly glycosylated when compared with native SBA, retained their binding specificity for N-acetylgalactosamine, and were highly resistant to degradation in simulated gastric and intestinal fluids. Affinity column purification using N-acetylgalactosamine as a specific ligand resulted in high recovery and purity of rSBA. This work is the first step toward use of rSBA for various new applications, including the development of rSBA as a novel affinity tag for simplified purification of tagged proteins and as a new carrier molecule for delivery of oral drugs.

Organization and dynamics of tryptophan residues in tetrameric and monomeric soybean agglutinin: studies by steady-state and time-resolved fluorescence, phosphorescence and chemical modification

We have investigated the organization and dynamics of tryptophan residues in tetrameric, monomeric and unfolded states of soybean agglutinin (SBA) by selective chemical modification, steady-state and time-resolved fluorescence, and phosphorescence. Oxidation with N-bromosuccinimide (NBS) modifies two tryptophans (Trp 60 and Trp 132) in tetramer, four (Trp 8, Trp 203 and previous two) in monomer, and all six (Trp 8, Trp 60, Trp 132, Trp 154, Trp 203 and Trp 226) in unfolded state. Utilizing wavelength-selective fluorescence approach, we have observed a red-edge excitation shift (REES) of 10 and 5 nm for tetramer and monomer, respectively. A more pronounced REES (21 nm) is observed after NBS oxidation. These results are supported by fluorescence anisotropy experiments. Acrylamide quenching shows the Stern-Volmer constant (K(SV)) for tetramer, monomer and unfolded SBA being 2.2, 5.0 and 14.6 M(-1), respectively. Time-resolved fluorescence studies exhibit biexponential decay with the mean lifetime increasing along tetramer (1.0 ns) to monomer (1.9 ns) to unfolded (3.6 ns). Phosphorescence studies at 77 K give more structured spectra, with two (0,0) bands at 408.6 (weak) and 413.2 nm for tetramer. However, a single (0,0) band appears at 411.8 and 407.2 nm for monomer and unfolded SBA, respectively. The exposure of hydrophobic surface in SBA monomer has been examined by 8-anilino-1-naphthalenesulfonate (ANS) binding, which shows approximately 20-fold increase in ANS fluorescence compared to that for tetramer. The mean lifetime of ANS also shows a large increase (12.0 ns) upon binding to monomer. These results may provide important insight into the role of tryptophans in the folding and association of SBA, and oligomeric proteins in general.

Attributes of glycosylation in the establishment of the unfolding pathway of soybean agglutinin

Soybean agglutinin (gSBA) is a tetrameric legume lectin, each of whose subunits are glycosylated. Earlier studies have shown that this protein shows exceptionally high stability in terms of free energy of unfolding when compared to other proteins from the same family. This article deals with the unfolding reactions of the nonglycosylated recombinant form of the protein rSBA and its comparison with the glycosylated counterpart gSBA. The nonglycosylated form features a lower stability when compared to the glycosylated form. Further, the unfolding pathways in the two are widely different. Although the glycosylated form undergoes a simple two-state unfolding, the nonglycosylated species unfolds via a compact monomeric intermediate that is not a molten globule. Representative isothermal and thermal denaturation profiles show that glycosylation accounts for a stabilization of approximately 9 kcal/mol of the tetramer, whereas the difference in T(m) between the two forms is 26 degrees C. Computational studies on the glycan-protein interactions at the noncanonical interface of the protein show that quite a number of hydrogen bond and hydrophobic interactions stabilize the glycoprotein tetramer.

A Two-Photon Fluorescence-Correlation Study of Lectins Interacting with Carbohydrated 20 nm Beads

We present results of a two-photon fluorescence-correlation study carried out with glycosylated and untreated 20 nm fluorescing spheres that interacted with the carbohydrate-binding proteins soybean agglutinin (SBA) and concanavalin A (Con A). The assay principle allows protein-carbohydrate binding interactions to be determined without protein labeling. This assay might serve as a simple model system for studying physical and chemical interactions between proteins and carbohydrates, for example, at cell or virus surfaces. In experiments with galactosylated 20 nm beads and SBA, several stages of protein-carbohydrate interactions could be clearly distinguished. Initially, only a few lectins bound to the nanospheres. At higher lectin concentrations polymerization occurred, and aggregates consisting of about 2.6 x 10(5) glycosylated nanospheres were formed. At very high lectin concentrations, the degree of polymerization dropped, and the size of single SBA-covered nanospheres increased to approximately 40 nm. When Con A was used instead of SBA, a significantly smaller degree of aggregation (4 x 10(4) spheres) was obtained. Treatment of unglycosylated 20 nm beads with SBA as a negative control sample resulted in a much lower unspecific aggregation (5 x 10(3) spheres). The assay principle can thus help to elucidate relative binding affinities.