Coiled coil structures folding/stability

Most nonfibrous proteins have a very precise and compact three-dimensional or tertiary structure formed when the a-helix and random coil of the polypeptide chain bends, twists, and folds over and back upon itself. The tertiary structure is stabilized by interactions of amino acid R-groups (Fig. 2-4a), and thus, is dictated by the primary structure. The biochemical function of a protein is intimately tied to its tertiary structure. That is, to function in a certain way, a protein must have the correct tertiary structure. Stated conversely only one specific tertiary structure will permit a protein to serve optimally a specific function (see also Figs. 4-3 and 4-4). 

Fig. 40. Attempt to schematically represent the ordered mesophase of PDES juxtaposed to the isotropic phase. The coUapsed coil structure assumed in the mesophase entails chain folds that provide additional motional degrees of freedom and is therefore thermodynamically stabilized in a certain temperature range (compare the statistical physics treatment in Ref. [124]). The shaded ellipses indicate somewhat closer packed, more ordered and less mobile areas in contrast to the more mobile, more distorted chain conformations in regions where many folds accumulate...

Despite their lack of stabilizing disulfide bridges Potl inhibitors feature a common, stable fold. The N-terminus is coiled, although in some structures a small /3-strand has been identified. After a turn the structure adopts an a-helical structure, followed by a turn and an other /3-strand. The sequence then features an extended turn or loop motif that contains the reactive site of the inhibitor before it proceeds with a /3-strand running almost parallel to the /3-strand after the a-helix. After another turn and coiled motif a short /3-strand antiparallel to the other /3-strands precedes the coiled C-terminus. Usually the N-terminal residue in the reactive site is an acidic residue followed by an aromatic amino acid, that is, tyrosine or phenylalanine. Figure 11 shows the complex of chymotrypsin inhibitor (Cl) 2 with subtilisin, the hexamer of Cl 2 from H. vulgare and a structural comparison with a trypsin inhibitor from Linum usitatissimum ... 

Incorporation of an imine unit into a mPE began with a Monte Carlo search of dodecamer (58e), which indicated that the oligomer adopted a six-turn helical structure [86]. To verify these results solvent denaturation studies were performed on 58e which showed a helix coil transition with a AG(CH3CN) very similar to that of the native oligomers (3.0 0.2 for 58e vs 3.2 0.1 for 15). The small difference in the AG(CH3CN) indicates that the imine bond has a negligible effect on the stability of the folded state of the oligomer. With this... 

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