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Receptors 2
Published in James E. Ferrell, Systems Biology of Cell Signaling, 2021
Many receptors, including receptor tyrosine kinases, are multimeric, and this can generate a variety of possible behaviors. The ligand binding curves can become sigmoidal rather than hyperbolic, through positively cooperative binding, where the first molecule of ligand to bind makes the next binding event more favorable. The MWC model, which is built on the idea of two interconverting conformation states in the receptor and the preferential binding of the ligand to one of the two, can account for such sigmoidal binding curves and so can the KNF model, which assumes that the first binding event causes the second binding site to have a higher (for positive cooperativity) or lower (for negative cooperativity) affinity for ligand. Both the MWC and KNF models can account for the equilibrium binding of oxygen to hemoglobin, but only the KNF model can account for the binding of EGF to the Drosophila EGFR, which bears all the hallmarks of negative cooperativity. Positive cooperativity can make receptor activation more switch-like and decisive; negative cooperativity can make receptor activation less decisive but allows the receptor to operate over a greater range of ligand concentrations without becoming saturated.
The Emergence of Temporal Order within a Living Being
Published in Pier Luigi Gentili, Untangling Complex Systems, 2018
The enzymes having more than one site may have an exciting feature: that of showing indirect interactions between the distinct binding sites. In such cases, the enzymes are defined allosteric. This term was introduced by Jacques Monod and François Jacob to characterize the end-product (L-isoleucine) inhibition of the enzyme L-threonine deaminase (Cui and Karplus 2008). L-isoleucine does not compete with the reactant L-threonine in binding at the catalytic site; it instead binds at a regulatory site, inhibiting the reaction. The term “allosteric” comes from the Greek αλλος-στερεος meaning “other-space” and refers to the influence the binding at one site has on the binding at a remote location in the same macromolecule. The ligand that brings about the allosteric regulation of the binding of another ligand is called effector, or modulator.4 If the effectors are identical to the substrate molecule, we speak about “homotropic interactions” between the ligands and the enzyme. If the binding of a substrate to the first site of the protein facilitates binding to the second, and so on, the interaction is defined as “positive cooperativity.” On the other hand, if the binding of the substrate molecule to the first site inhibits the binding to the second, and so on, the interaction is defined as “negative cooperativity.” When the effectors are ligands having a structure different from that of the substrate, we speak about “heterotropic interactions.” In heterotropic interactions, the effectors can play as either activators or inhibitors of enzymatic activity. Depending on how ligands affect the enzymatic activity, their actions are also typified as “competitive” or “uncompetitive” concerning the substrate binding to the enzyme. Ligands are defined as competitive when their binding changes the K constant of equation [7.10] (in fact, they are also named as “K systems;” see Figure 7.1a). When the binding of a ligand changes the maximum velocity of the enzymatic reaction (i.e., kh in equation [7.10]), they are referred to as uncompetitive or “V systems” (where V stands for Velocity; see Figure 7.1b) (Hammes and Wu 1974).
Experimental and theoretical insight into the cooperativity effect in composite wax powder and ternary complex of coronene with CH4 and Mn+ (Mn+ = Li+, Na+, K+, Be2+, Mg2+ or Ca2+)
Published in Molecular Physics, 2018
Le-tao Jiang, Pei-kang Bai, Jian-hong Wang, Bin Liu, Yu-xin Li
In complex, when the noncovalent interactions operate simultaneously and mutually enhance each others strength, they are termed as cooperativity (or synergetic) effect [12]. Due to the more favourable binding energies of charged complexes than neutral, the cooperativity effects involving molecule···ion (i.e. σ-type) or ion···π interaction are more notable [13], and they influence remarkably the physical and chemical properties of charged complexes [14–18]. Since composite wax powder is a mixture of paraffin wax, SDBS, graphite powder, etc., its superior properties may be attributed to the cooperativity effect of the intermolecular, molecule ion or ion···π interactions among various components. Properly characterising the cooperativity (or synergetic) effect in composite wax powder is crucial for understanding its property.
Micellar catalyzed hydrolysis of mono-2,3-dichloroaniline phosphate
Published in Journal of Dispersion Science and Technology, 2018
Nisha Chhetri, S. A. Bhoite, A. K. Singh
Micelle-catalyzed reactions resemble with enzyme-catalyzed reaction in many ways. For example, micelle-catalyzed reactions exhibit a sigmoid-shaped curve for the plot of rate constant versus surfactant concentration, which is typical to enzyme-catalyzed reactions. This means that after attaining a rate maximum, the rate decreases at the higher concentration of the surfactant. For enzyme-catalyzed reactions, Hill model[25] is in vogue for a very long time which is based on positive cooperativity. Taking a cue from Hill model, Piszkiewicz developed a kinetic model to explain the micellar effect.[26]