The folding problem simplified: proteins with nearly identical sequence but different structure and function

The ability of a protein to fold rapidly and efficiently into its intricate and highly specific structure is an essential part of the conversion of genetic information into cellular activity. Despite over five decades of work, understanding protein folding still remains one of the major challenges in modern biochemistry and biophysics. Pioneering work in the 1950s by Christian Anfinsen, on the folding of ribonuclease, demonstrated that the primary structure of a protein "encodes" the information necessary for a nascent polypeptide to fold into its native, physiologically active, three-dimensional conformation (Anfinsen, et al. 1961). After denaturation, indeed, small globular proteins can fold back, into proper shape, simply removing the denaturant agent. The spontaneity of the folding reaction led to the concept that the native conformation of a protein is the most stable accessible state and corresponds to the lowest free energy (Pace 1990). Afterwards, Cyrus Levinthal pointed out that folding can not occur by a stochastic search among all possible conformations; because otherwise even a small protein would need a unrealistically long time to fold (Levinthal 1968). The attractive and repulsive forces between neighbouring amino acid residues, favouring certain conformations of individual amino acids in the polypeptide chain, dramatically reduce the conformational space and the number of possible folding pathways available. Therefore, to understand how proteins fold, it is necessary to understand the thermodynamics of the forces stabilizing the native state and also the dynamic mechanism whereby the conformational search to the native state is achieved.