Stability and stabilization of globular proteins in solution

Proteins are multifunctional: their amino acid sequences simultaneously det ermine folding, function and turnover. Correspondingly, evolution selected for compromises between rigidity (stability) and flexibility (folding/funct ion/ degradation), to the result that generally the free energy of stabiliz ation of globular proteins in solution is the equivalent to only a few weak intermolecular interactions. Additional increments may come from extrinsic factors such as ligands or specific compatible solutes. Apart from the ent halpic effects, entropy may play a role by reducing the flexibility (cystin e bridges, increased proline content), or by water release from residues bu ried upon folding and association. Additional quaternary interactions and c loser packing are typical characteristics of proteins from thermophiles. In halophiles, protein stability and function are maintained by increased ion binding and glutamic acid content, both allowing the protein inventory to compete for water at high salt. Acidophiles and alkalophiles show neutral i ntracellular pH; proteins facing the outside extremes of pH possess anomalo usly high contents in ionizable amino acids. Global comparisons of the amin o acid compositions and sequences of proteins from mesophiles and extremoph iles did not result in general rules of protein stabilization, even after i ncluding complete genome sequences into the search. Obviously, proteins are individuals that optimize internal packing and external solvent interactio ns by very different mechanisms, each protein in its own way. Strategies de duced from specific ultrastable proteins allow stabilizing point mutations to be predicted. (C) 2000 Elsevier Science B.V. All rights reserved.