Protein stability and protein folding.

Proteins show only marginal free energies of stabilization. Mutative adaptations to extremes of physical conditions (high temperature, pressure and salt concentration) tend to maintain 'corresponding states' regarding overall topology, flexibility and hydration. Since enhanced stability requires only minute local changes in the structure of a given protein, general strategies of adaptation cannot be established. Apart from alterations at the protein level, extrinsic factors such as ions, cofactors or specific ligands may serve to enhance in vivo and in vitro protein stability. Protein folding and association reflect the hierarchy of protein structure, with the formation of secondary/supersecondary structure, subdomains/domains and structured monomers as consecutive steps. The process requires highly specified environmental conditions; e.g. active mesophilic or halophilic proteins cannot be expressed in thermophilic and non-halophilic hosts. On the other hand, a given protein may tolerate extreme sequence variability without substantially altering its three-dimensional structure and stability. Significant rate-determining steps in the overall reaction, that is, formation of disulphide bridges, proline isomerization and oligomerization, are catalysed by specific enzymes or directed by 'helper proteins' (protein disulphide isomerase, peptidyl-prolyl cis-trans isomerase and chaperones). Physiological stress conditions, (site-directed) mutations, and in vitro studies may be used to unravel the significance of the three 'shuffling reactions'.