Figure 1. The sizes of protein structures. Protein subunits are usually 10 kDa or larger. Modules have sizes below 80 amino acids. Domains vary more in size, depending on how they are defined. A size scale permits comparison for both the number of amino acids in a protein and its size in kDa. Simple enzymes have only a single type of catalytic activity, and are not subject to sophisticated regulatory controls. Complex enzymes are always larger, and may contain two or more catalytic centers, and frequently have additional binding sites for one or more regulatory effectors.

Figure 2. Sizes of human proteins.

Figure 3. A model for the evolution of new proteins. Modules, at an average size of 50 amino acids, are coded by exons, and are large enough to have tertiary structure. Domains, at an average size of 150 amino acids, are formed by two or more modules. Simple enzymes generally are formed by one domain. Gene duplication leads to copies for some enzymes, which become altered by mutation and selection if a new or better activity evolves. Fusion of two or more genes leads to multidomain proteins with multiple activities.

Figure 4. Examples of proteins with various assortments of modules or domains. Domains or modules are identified and their size in amino acids is in parentheses. A number following a domain element denotes the frequency of repetition for that domain. SH: Sarc homology; THSP: thrombospondin: VWA: von Willebrand factor type A.

Figure 5. Model for the structure of fatty acid synthetase. Each subunit of the protein dimer contains the separate protein domains: KS: -ketoacyl synthetase; MT: malonyl transacylase; AT: acetyl transferase; DH: dehydratase; ER: enoyl reductase; KR: -ketoacyl reductase; ACP: acyl carrier protein; TE: thioesterase.

Figure 6. Reactions catalyzed by the bifunctional OPET decarboxylase/HHDD isomerase. OPET: 5-oxopent-3-ene-1,2,5-tricarboxylate; HHDD: 2-hydroxyhepta-2,4-diene-1,7-dioate; OHED: 2-oxohept-3-enedioate.