Protein denaturation, ligand effect

The thermodynamic model which describes the effect of sub-saturating amount of a ligand follows the ideas and formalism suggested by Robert, Gill and Wyman [267], When a protein denatures according to a one-step mechanism and has a single binding site in the native conformation. 

The use of differential scanning microcalorimetry for measuring the thermal denaturation of proteins is described in Chapter 17, section Ale. Typically, 0.5-1 mg of protein in 1 mL of buffer, or 0.1-0.2 mg in 0.5 mL with the most sensitive apparatus, is required for an accurate determination of the enthalpy of denaturation. The thermodynamics of dissociation of a reversibly bound ligand may be calculated from its effects on the denaturation curve of a protein.14 The binding of ligands always raises the apparent Tm (temperature at 50% denaturation) of a protein because of the law of mass action the ligand does not bind to the denatured state of the protein, and so binding displaces the denaturation equilibrium toward the native state. 

Early ORD measurements on beef liver catalase showed large Cotton effects in the Soret region, which disappeared upon denaturation (255). No evidence for previously reported (256) Cotton effects near 600 nm could be obtained (255). ORD and CD measurements on bovine liver catalase included also the liganded hemoprotein. Conformational changes in the protein, occurring upon ligand addition, were deduced from data in the ultraviolet region (257). 

Weber et al. [110] have presented a model that accounts for the concentration independence of the pressure dissociation of virus particles and the partial restoration of the concentration dependence in the presence of urea concentrations that are below the concentration that denatures the protein. Under certain conditions a transition may be observed from the deterministic assembly of the virus particle from the subunits towards the normal stochastic assembly process. A combined effect of specific ligands, pressure and temperature may therefore help in designing new strategies for the design of vaccination procedures. 

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