Mechanisms of protein folding and unfolding

Case study 7.1 j Mechanisms of protein folding and unfolding 

The rate-determining step is thought to accoimt for the relaxation time of 160 ns measured with a laser-induced temperatme jiunp from 280 K to 300 K in an alanine-rich polypeptide containing 21 residues. It is thought that the limitation on the rate of the helix-coil transition in this peptide arises from an activation energy barrier of 1.7 kj mol associated with initial events of the form. .. hhhh hhch... in the middle of the chain. Therefore, initiation is 

The kinetics of unfolding have also been measured in naturcJly occurring proteins. In the engrculed homeodomcun (En-HD) protein, which contains three short hehcal segments (Fig. 7.11), unfolding occurs with a hcdf-life of about 630 ps at 298 K. It is difficult to interpret these results because we do not yet know how the cunino acid sequence or interactions between hehces in a folded protein affect the hehx-coil relaxation time. 

A key feature of the framework and nucleation-condensation models is the formation of secondary structure—which might or might not be coupled to the formation of tertiary structure—early in the folding process. It follows that a full description of the mechanism of protein folding also requires an understanding of the rules that stabilize molecular interactions in polypeptides. We consider these rules in Chapter 11. 

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