Analysis of Receptor-Ligand Interactions

Abstract

Molecules have an innate affinity for one another due to electrostatic forces, such as Coulombic attractions, hydrogen bonds, and dispersion forces. The noncovalent interactions that result from this affinity are of particular importance in biological processes, including the catalysis of chemical reactions (by enzymes), neutralization of foreign toxins (by antibodies), and stimulation of cellular activities (by hormones). To initiate these processes, receptors and ligands exchange interactions with solvent and solute molecules for interactions with each other. Much effort is currently being made by biological chemists to understand the molecular details of receptor-ligand interactions, and by medicinal chemis ts to exploit this understanding in developing useful pharmaceutics (1). In addition, organic chemists are attempting to develop s synthetic systems that mimic the biological interactions (2, 3). Each of these efforts requires knowledge of the number of potential binding sites on a receptor and the affinity of each binding site for its ligand. In this article, we describe how this knowledge can be obtained, and draw parallels, when appropriate, to the analysis of the kinetics of enzymatic catalysis (4, 5). Properties of Receptor-Ligand InteractionsThe interactions of all biological receptors with their natural ligands share several properties. SpecificityBiological receptors generally bind tightly to a single natural ligand. (This specificity need not be absolute. For example, a number of natural toxins, such as α-bungarotoxin, carry out their mischief by binding to the acetylcholine receptor.) Ligand specificity can be readily assessed through a competitive-binding assay. Here, the amount of ligand bound to a receptor is measured in the presence of other putative ligands. If the receptor is indeed specific for the original ligand, the amount of ligand bound is not affected by the presence of the other ligands. For example, the addition of a 1000-fold molar excess of serum albumin does not decrease the amount of diphtheria toxin bound to its cell-surface receptor. AffinityMolecules interact noncovalently with other molecules. For example, proteins tend to stick to glass, due in part to polar surfaces interacting with one another. The surface of a cell is also quite polar, due largely to the extensive amount of carbohydrate that extends from membrane proteins and membrane lipids. Consequently, all proteins have some affinity for cell surfaces. Receptor-ligand interactions are distinguished from other noncovalent interactions between molecules by their high affinity.

Spectral investigations of the interaction of some porphyrins with bovine serum albumin

“…Figure 6 shows these processes with dissociation constants of 1.3 Â 10 À7 and 2.5 Â 10 À4 and n H values of 1.6 and 3.5 respectively. The non-integral values of n H are consistent with cooperativity [41]. For the T4CPP-BSA and T3CPP-BSA complexes the binding constant (K) and number of binding sites (n) obtained from the Scatchard plots (see Figs 3 and 4) are given in Table 2.…”

“…Figures 3-5 show Scatchard plots for the interaction of T4CPP, T3CPP and T3, 4BCPP respectively with BSA. The linear plots in Figs 3 and 4 imply the existence of independent and equivalent binding sites for T4CPP and T3CPP on BSA [41,45]. However, T3, 4BCPP displayed an S-shaped Scatchard plot (see Fig.…”

“…A Hill plot was used to determine the cooperativity in the binding process [40,41] The fluorescence experiments with BSA were performed by measuring the contact energy transfer from three aromatic amino acid residues, namely tryptophan, tyrosine and phenylalanine with excitation wavelengths at 281, 274 and 257 nm respectively, to the bound porphyrin at ! = 655 nm.…”

Assessing the inhibitory potency of galectin ligands identified from combinatorial (glyco)peptide libraries using surface plasmon resonance spectroscopy

“…The glycoprotein was immobilized on a flow cell of a CM5 chip (106 RU), and an untreated flow cell was used as the reference surface for h-Gal-1. The IC 50 values of the (glyco)peptides and lactose were derived from the K i values, which were determined by analyzing the SPR data using the following equation [21]:…”

Affinity-Based Inhibition of β-Amyloid Toxicity

“…This fitting procedure afforded K rel values, a nomenclature used to distinguish them from true affinities. To determine affinities for compounds listed in Table 2, a nonspecific binding term was added to eq 1 (35), and data were fit by linear regression to eq 3:…”