Some Basic Principles of Protein Structure

As more protein structures became available it was observed that some contained more than one distinct region, with each region often having a separate function. Each of these regions is usually known as a domain, a domain being defined as a polypeptide chain that can fold independently into a stable three-dimensional structure. 

Water-soluble globular proteins usually have an interior composed almost entirely of nonpolar, hydrophobic amino acids such as phenylalanine, tryptophan, valine and leucine with polar and charged amino acids such as lysine and arginine located on the surface of the molecule. This packing of hydrophobic residues is a consequence of the hydrophobic effect, which is the most important factor that contributes to protein stability. The molecular basis for the hydrophobic effect continues to be the subject of some debate but is generally considered to be entropic in origin. Moreover, it is the entropy change of the solvent that is 

Fig 10 6 The hydrophobic effect Wafer molecules around a rum-polar solute form a cage-like structure, which reduces the entropy When two non-polar groups associate, water molecules are liberated, increasing the entropy 

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