Nonlinearly additive forces in multivalent ligand binding to a single protein revealed with force spectroscopy.
ABSTRACT We present evidence of multivalent interactions between a single protein molecule and multiple carbohydrates at a pH where the protein can bind four ligands. The evidence is based not only on measurements of the force required to rupture the bonds formed between concanavalin A (ConA) and alpha-D-mannose but also on an analysis of the polymer-extension force curves to infer the polymer architecture that binds the protein to the cantilever and the ligands to the substrate. We find that although the rupture forces for multiple carbohydrate connections to a single protein are larger than the rupture force for a single connection, they do not scale additively with increasing number. Specifically, the most common rupture forces are approximately 46, 68, and 85 pN at a loading rate of 650 +/- 25 pN/s, which we argue corresponds to 1, 2, and 3 ligands being pulled simultaneously from a single protein as corroborated by an analysis of the linkage architecture. As in our previous work polymer tethers allow us to discriminate between specific and nonspecific binding. We analyze the binding configuration (i.e., serial vs parallel connections) through fitting the polymer stretching data with modified wormlike chain (WLC) models that predict how the effective stiffness of the tethers is affected by multiple connections. This analysis establishes that the forces we measure are due to single proteins interacting with multiple ligands, the first force spectroscopy study that establishes single-molecule multivalent binding unambiguously.
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ABSTRACT: New classes of antibiotics, such as antimicrobial peptides or proteins (AMPs), are crucial to deal with threatening bacterial diseases. rBPI21 is an AMP based on the human neutrophil BPI protein, with potential clinical use against meningitis. We studied the membrane perturbations promoted by rBPI21 on Gram-negative Escherichia coli and Gram-positive Staphyloccoccus aureus. Its interaction with bacteria was also studied in the presence of lipopolysaccharide (LPS), rBPI21 major ligand. Flow cytometry analysis of both bacteria shows that rBPI21 induces membrane depolarization. rBPI21 increases the negative surface charge of both bacteria toward positive values, as shown by zeta-potential measurements. This is followed by surface perturbations, culminating in cell lysis, as visualized by atomic force microscopy (AFM). Force spectroscopy measurements show that soluble LPS decreases the interaction of rBPI21 with bacteria, specially with S. aureus. This suggests that the rBPI21 LPS-binding pocket may also participate on the binding to Gram-positive bacteria.Nanomedicine: nanotechnology, biology, and medicine 11/2013; · 6.93 Impact Factor
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ABSTRACT: Atomic force microscopy (AFM) can be used to make measurements in vacuum, air, and water. The method is able to gather information about intermolecular interaction forces at the level of single molecules. This review encompasses experimental and theoretical data on the characterization of ligand-receptor interactions by AFM. The advantage of AFM in comparison with other methods developed for the characterization of single molecular interactions is its ability to estimate not only rupture forces, but also thermodynamic and kinetic parameters of the rupture of a complex. The specific features of force spectroscopy applied to ligand-receptor interactions are examined in this review from the stage of the modification of the substrate and the cantilever up to the processing and interpretation of the data. We show the specificities of the statistical analysis of the array of data based on the results of AFM measurements, and we discuss transformation of data into thermodynamic and kinetic parameters (kinetic dissociation constant, Gibbs free energy, enthalpy, and entropy). Particular attention is paid to the study of polyvalent interactions, where the definition of the constants is hampered due to the complex stoichiometry of the reactions.Biochemistry (Moscow) 12/2012; 77(13):1536-52. · 1.15 Impact Factor
- Australian Journal of Chemistry - AUST J CHEM. 01/2010; 63(4).