Are proteins made of modules?

Analysis of a set of well characterized enzymes shows that the size of a protein subunit is directly related to the number of unique ligand binding functions described for the particular enzyme. The average size increment is about 5 000 Da per ligand binding function. This value corresponds very well to: (a) the amount of polypeptide chain required to form a stable folded structure, and (b) the size of polypeptide coded by the average exon. This reinforces the hypothesis that exon-coded modules are basic architectural units for proteins. Key predictive elements of this hypothesis are: 1) generally each module has a unique function, such as the ability to bind a specific ligand; 2) the size of an enzyme subunit should be determined by the number of modules required to accomplish the enzyme's biological role.