Folded protein, configurational entropy

Ensembles 600 Enterokinase 480 Enthalpy 55 activation 56, 545-547 protein folding 509 -512 specific heat effects 511, 545 - 547 Enthalpy-entropy compensation 346 Enthalpy versus entropy in protein folding 509-512, 587, 599 Entropy 55, 68-72 activation 56, 545 -547 binding 324, 345 Boltzmann equation 510 chelate effect 345 configurational 510 configurational entropy of loops 535 effective concentration 68-72 equilibria on enzyme surface 118 hydrogen bond 338 hydrophobic bond 332, 510 importance in enzyme catalysis 72 importance in enzyme-substrate binding 72... 

The disulfide bond differs from other types of interactions in folded proteins, such as hydrogen bonds and hydrophobic, electrostatic and van der Waals interactions. The disulfide bond is a covalent bond that is able to significantly stabilize folded conformations by 2-5 kcal/ mol for each disulfide.11 The effect is presumed to be due mainly to a decrease in the configurational chain entropy of the unfolded polypeptide.21 On the other hand, another view is that the disulfide bond destabilizes folded structures entropically, but stabilizes them enthalpically to a greater extent.31... 

Tidor and Karplus have carried out a normal mode analysis of the protein bovine pancreatic trypsin inhibitor (BPTl) in order to calculate and compare the vibrational entropy of five different forms the native form three variants, each of which contained a reduced form of one of the three disulfide bonds found in native BPTI and a variant that lacked the dipeptide bond at the trypsin-reactive site. The existence of cross-links in proteins has been thought to contribute to their stability by reducing the configurational entropy of the unfolded state. The entropy of the folded state is also affected by the presence or absence of cross-links. 

At this temperature, the entropy change for dissolution of liquid hydrocarbons in water is zero. However, the entropy of protein denaturation is far from zero at this temperature but amounts to 17.6 J - K l per mole of amino acid residues (Privalov, 1979), a value that corresponds to an 8-fold increase of the number of possible configurations and is close to the value expected for the helix-coil transition of polypeptides (Schellman, 1955). This difference shows that an oil drop is an inadequate model for a globular protein. A more suitable model resembles that of a small crystal with a quite definite positive melting entropy (see also Bellow, 1977, 1978). 

Fig. 4. The curved solid line depicts the energy landscape funnel of the folding process. The a -axis is a measure of accessible configuration space. At high energies, many configurations are possible, hence the entropy is large. The landscape guides or funnels the protein to lower energies (indicated by the arrows). The lower the energy, the more the protein is confined to fewer conformations. The global minimum corresponds to the native state. Note that the transition states have to be put in manually...

---