Multidomain Proteins

Abstract Protein domains that have a necessary function are often used in many different proteins. Protein architecture has evolved to utilise a large set of such domains in forming the thousands of different proteins necessary for any living organism. Definitions for domains are still somewhat variable, and therefore current databases show domain sizes ranging from a few amino acids to more than 800 amino acids, with the great majority at 50–150 amino acids, or approximately 5–16 kDa. Simple proteins normally contain only one or two domains, whereas larger proteins may have incorporated more than 30 domains needed for the more complex cellular functions. At least two‐thirds of mammalian proteins have more than one domain. Only multicellular eukaryotes have a significant proportion of proteins with repeating domains. Key Concepts: Protein domains may represent the earliest or simplest proteins formed at the beginning of life. Protein domains are usually the individual units for protein folding. Protein domains often serve as structural units that define a protein. By having useful functions that may be selected, protein domains become central units in the evolution of diverse proteins. Almost all protein domains have a specific ligand‐binding function. Conformational changes in a protein may involve a reorientation of two or more domains to form or disrupt a ligand‐binding site, and thereby control an enzyme's catalytic activity.