Proteins flexible structure

The thermodynamic stability of a protein in its native state is small and depends on the differences in entropy and enthalpy between the native state and the unfolded state. From the biological point of view it is important that this free energy difference is small because cells must be able to degrade proteins as well as synthesize them, and the functions of many proteins require structural flexibility. 

The atoms of a protein s structure are usually defined by four parameters, three coordinates that give their position in space and one quantity, B, which is called the temperature factor. For well refined, correct structures these B-values are of the order of 20 or less. High B-values, 40 or above, in a local region can be due to flexibility or slight disorder, but also serve as a warning that the model of this region may be incorrect. 

Biopolymers e.g., polysaccharides, polynucleotides, unfolded protein molecules, that all attain expanded flexible structures in solution adsorb more or less according to the principles discussed above. 

HMGA proteins flexible players in a structured world... 

With the help of NMR measmement, it has been shown that the Ca Vcalmodulin complex has a flexible structure. Flexibility is probably of great importance for the function of Ca Vcalmodulin. In the complex with the protein substrate (Fig. 6.10b), Ca V calmodulin has a collapsed structme in which the two globular domains are much closer together than in free Ca Vcalmodulin. 

Protein kinases can exist in active and inactive forms, which is why they are able to perform the function of a switch in signaling pathways. Protein kinases are particularly suitable as switches in signal pathways due to their flexible structure of two domains that can adopt different orientations with respect to one another. Fmthermore, in the cleft between the two domains, it is possible to initiate signal-controlled conformational changes of great importance for substrate binding and catalytic activity. 

In the case of the rather porous and flexible structure of sodium caseinate nanoparticles, the data show that the interaction with surfactants causes a tendency towards the shrinkage of the aggregates, most likely due to the enhanced cross-linking in their interior as a result of the protein-surfactant interaction. This appears most pronounced for the case of the anionic surfactants (CITREM and SSL) interacting with the sodium caseinate nanoparticles. Consistent with this same line of interpretation, a surfactant-induced contraction of gelatin molecules of almost 30% has been demonstrated as a result of interaction with the anionic surfactant a-olefin sulfonate (Abed and Bohidar, 2004). 

---