InterPro domain: IPR020858

A number of serum transport proteins are known to be evolutionarily related, including albumin, alpha-fetoprotein, vitamin D-binding protein and afamin [ 1 , 2 , 3 ]. Albumin is the main protein of plasma; it binds water, cations (such as Ca ²⁺ , Na ⁺ and K ⁺ ), fatty acids, hormones, bilirubin and drugs - its main function is to regulate the colloidal osmotic pressure of blood. Alphafeto- protein (alpha-fetoglobulin) is a foetal plasma protein that binds various cations, fatty acids and bilirubin. Vitamin D-binding protein binds to vitamin D and its metabolites, as well as to fatty acids. The biological role of afamin (alpha-albumin) has not yet been characterised. The 3D structure of human serum albumin has been determined by X-ray crystallography to a resolution of 2.8A [ 4 ]. It comprises three homologous domains that assemble to form a heart-shaped molecule [ 5 ]. Each domain is a product of two subdomains that possess common structural motifs [ 5 ]. The principal regions of ligand binding to human serum albumin are located in hydrophobic cavities in subdomains IIA and IIIA, which exhibit similar chemistry. Structurally, the serum albumins are similar, each domain containing five or six internal disulphide bonds, as shown schematically below:

 +---+ +----+ +-----+ | | | | | | xxCxxxxxxxxxxxxxxxxCCxxCxxxxCxxxxxCCxxxCxxxxxxxxxCxxxxxxxxxxxxxxCCxxxxCxxxx | | | | | | +-----------------+ +-----+ +---------------+

1. Amphibian albumins as members of the albumin, alpha-fetoprotein, vitamin D-binding protein multigene family. J. Mol. Evol. 29, 344-54
2. Complete amino acid sequence of human vitamin D-binding protein (group-specific component): evidence of a three-fold internal homology as in serum albumin and alpha-fetoprotein. Biochim. Biophys. Acta 871, 189-98
3. Afamin is a new member of the albumin, alpha-fetoprotein, and vitamin D-binding protein gene family. J. Biol. Chem. 269, 18149-54
4. Atomic structure and chemistry of human serum albumin. Nature 358, 209-15