Through the analyses of the Miyazawa-Jernigan matrix it has been shown that the hydrophobic effect generates the dominant driving force for protein folding. By using both lattice and off-lattice models, it is shown that hydrophobic-type potentials are indeed efficient in inducing the chain through nativelike configurations, but they fail to provide sufficient stability so as to keep the chain in the native state. However, through comparative Monte Carlo simulations, it is shown that hydrophobic potentials and steric constraints are two basic ingredients for the folding process. Specifically, it is shown that suitable pairwise steric constraints introduce strong changes on the configurational activity, whose main consequence is a huge increase in the overall stability condition of the native state; detailed analysis of the effects of steric constraints on the heat capacity and configurational activity are provided. The present results support the view that the folding problem of globular proteins can be approached as a process in which the mechanism to reach the native conformation and the requirements for the globule stability are uncoupled.
Globular proteins are produced as a linear chain of aminoacids in water solution in the cell and, in the same aqueous environment, fold into their respective unique and functional native structures. In spite of this, many theoretical studies have tried to explain the folding process in vacuum, but in this paper we adopt an alternative point of view: the folding problem of heteropolymers is analyzed from the solvent perspective. The thermodynamics of the folding process is discussed for a non homogeneous system composed by the chain and solvent together; hydrophobic effects, modulated by the polar/nonpolar attributes of the residue sequence and by its corresponding steric specificities, are proposed as basic ingredients for the mechanisms of the folding process. These ideas are incorporated in both lattice and off-lattice models and treated by Monte Carlo simulations. Configurational and thermodynamical results are compared with properties of real proteins. The results suggest that the folding problem of small globular protein can be considered as a process in which the mechanism to reach the native structure and the requirements for the globule stability are uncoupled.
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