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Proteopedia Page Contributors and Editors (what is this?)Proteopedia Page Contributors and Editors (what is this?)

Jaime Prilusky, Eric Martz

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-A protein ''domain'' is a sequence of amino acids that can fold, independently of the remainder of the full-length sequence, into a compact stable structure. Water-soluble domains typically have hydrophobic cores. Some small full-length proteins consist of a single domain, but most proteins have two or more domains. A domain is typically 100-250 amino acids in length<ref name="EPR" />, but can sometimes be shorter or longer.
+A protein ''domain'' usually means a sequence of amino acids that folds, independently of the remainder of the full-length sequence, into a compact stable structure. Water-soluble domains typically have hydrophobic cores. Some small full-length proteins consist of a single domain, but many proteins have two or more domains. A domain is typically 100-250 amino acids in length<ref name="EPR" />, but can sometimes be shorter or longer.
+In 2006, chains with a single domain in the CATH database had an average length of 159<ref name="cath2006">PMID: 17135200</ref>. In 2006, 64% of all protein structures in CATH were single domain chains. 27% were two-domain chains<ref name="cath2006" />.
 Examples:
+* [[9ins]] (porcine): Insulin, with 51 amino acids, is one of the smallest stably folded protein domains, on the boundary between a protein and a [[peptide]]. It has a hydrophobic core. Human preproinsulin is synthesized with 110 amino acids. After removal of a 24 amino acid signal sequence, the remaining 86 amino acid proinsulin is cleaved in two places, forming a mature disulfide-linked dimer of two protein chains. Chain B is residues 1-30. Chain A is residues 66-86 (length 21). "C-peptide", residues 33-63 (length 31), is released as a separate bio-active peptide. Mature human insulin differs from porcine in a single amino acid: the terminal residue in chain B is Thr vs. Ala, respectively. Wikipedia has good articles on [https://en.wikipedia.org/wiki/Insulin#Synthesis insulin synthesis] and [https://en.wikipedia.org/wiki/C-peptide C-peptide]. See also [[Insulin Structure & Function]].
+* [[1lzs]]: Lysozyme functions as a single chain of 130 amino acids. It folds to single domain with a hydrophobic core.
 * [[2hhd]]: Each of the 4 chains that form hemoglobin (a tetramer) folds into a single domain composed of alpha-helices. Each domain (chain) is 141-146 amino acids in length for human hemoglobin.
-* [[1igy]]: Immunoglobulin G (antibody) consists of 12 domains in 4 chains. These domains are composed of beta-sheets. Each of the two heavy chains has 4 domains, and each of the 2 light chains has 2 domains. These domains are about 110 amino acids in length. Each heavy chain has two pairs of domains connected by a flexible linker about 20 amino acids in length.
+* [[1igy]]: Immunoglobulin G (antibody) consists of 12 domains in 4 chains. These ''immunoglobulin superfamily'' domains are composed of beta-sheets. Each of the two heavy chains has 4 domains, and each of the 2 light chains has 2 domains. These domains are about 110 amino acids in length. Each heavy chain has two pairs of domains connected by a flexible linker about 20 amino acids in length.
 For more information see [http://www.wikipedia.com/wiki/Protein_domain Protein Domain in Wikipedia].