title: Are proteins made of modules?
abstract: 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.
mesh_terms: D001483:Base Sequence; D001665:Binding Sites; D005075:Biological Evolution; D004798:Enzymes / Q000032:analysis* / Q000235:genetics; D005796:Genes; D008024:Ligands / Q000378:metabolism; D008957:Models, Genetic; D010455:Peptides / Q000235:genetics / Q000502:physiology; D011487:Protein Conformation; D011494:Protein Kinases / Q000032:analysis / Q000235:genetics; D014961:X-Ray Diffraction
chemical_list: D004798:Enzymes; D008024:Ligands; D010455:Peptides; D011494:Protein Kinases