Proline residues kinetics

Cytochrome c possesses three phyllogenetically-conserved proline residues that are presumably involved in the correct folding of the protein to form the native structure. The effects of substitutions at one of these sites, Pro-71 (Fig. 4), on the equilibrium and kinetics of yeast iso-2-cytochrome c unfolding have been studied by Nall and co-workers through comparison of the properties of a Thr... 

The biased pucker of the proline residue significantly influences conformational stability when additional interactions are absent. However, it has been shown that preorganized proline units can also weaken native interactions in certain cases. While most studies have focused on the folding kinetics and stability of the resulting proteins, effects of proline derivatives on protein function (i.e. ligand-binding) have also been investigated. ... 

Fig. 7.9 (pp. 368-369). Comparative studies of unfolding-refolding of three carp parv-albumins (from Lin and Brandts, 1978) at pH 5.8, 25°C. Values of the kinetic parameters, relaxation time, and amplitude are given for each protein. Unfolding experiments are represented by filled symbols and refolding experiments by open symbols (A), (B), and (C) correspond to different parvalbumins, only that studied in (B) contained proline residues (courtesy of Brandts). 

Several attempts were made to detect and identify nucleation centers, but until recently no direct evidence for their existence was obtained. The slow step observed in earlier fast reaction kinetic studies was at first attributed to a nucleation process (Tsong et ai, 1972a), and subsequently to cis-trans isomerization of proline residues. However, this last statement is still controversial. 

Chyraotrypsin inhibitor 2 (CI2) folds rapidly by simple two-state kinetics that is, D N, with a r1/2of 13 ms.18,19 CI2 is a small 64-residue protein that has all its peptidyl-proline bonds in the favorable trans conformation.20 (There are, of course, additional slow cis —> trans peptidyl-prolyl isomerization events, which account for about 20-30% of the refolding amplitudes.) The occurrence of two-state kinetics does not prove that there are no intermediates on the folding pathway there could be intermediates that are present at high energy and are kineti-cally undetectable (see section B4). Two-state behavior has subsequently been found for many other small proteins. The simplicity of two-state folding kinetics provides the ideal starting point for the analysis and illumination of the basic principles of folding. 

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