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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 most proteins have two or more domains. A domain is typically 100-250 amino acids in length[1], but can sometimes be shorter or longer.


  • 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 insulin synthesis and 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.

For more information see Protein Domain in Wikipedia.

See also [1] and this summary of it.


  1. 1.0 1.1 Evolution of the protein repertoire. Cyrus Chothia, Julian Gough, Christine Vogel, Sarah A. Teichmann (2003). Science 300:1701-3. PMID:12805536

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