| Primary Identifier | IPR000213 | Type | Family |
| Short Name | VitD-bd |
| description | A number of serum transport proteins are known to be evolutionarily related,including albumin, alpha-fetoprotein, vitamin D-binding protein and afamin[, , ]. Albumin is the main protein of plasma; it binds water, cations (suchas Ca2+, Na+and K+), fatty acids, hormones, bilirubin and drugs - its mainfunction is to regulate the colloidal osmotic pressure of blood. Alphafeto-protein (alpha-fetoglobulin) is a foetal plasma protein that binds variouscations, fatty acids and bilirubin. The biological role of afamin (alpha-albumin) has not yet been characterised.Vitamin D-binding protein (DBP) is an abundant serum glycoprotein secretedby the liver; the protein transports vitamin D sterols, binds to actin, and is found on the surface of B-lymphocytes and subpopulations of T-lymphocytes[]. The full length DBP contains 476-amino acids, including a 16-aminoacid signal sequence. Sequence analysis reveals 23% similarity to albuminand to alpha-fetoprotein []. DBP contains a characteristic placement ofcysteine residues, identical to that in albumin, suggesting a similarfolding structure. Albumin and alpha-fetoprotein contain three internallyrepeated domains []. DBP shows similarity to the first two domains andhas a truncated third domain, supporting the view that DBP is a member ofthe albumin/alpha-fetoprotein multigene family []. Within the sequence, regularly-spaced disulphide bridges generate a 3-domainfolding structure, each domain containing ~170 amino acids, with 5 or 6internal disulphide bonds, as shown schematically below: +---+ +----+ +-----+| | | | | |xxCxxxxxxxxxxxxxxxxCCxxCxxxxCxxxxxCCxxxCxxxxxxxxxCxxxxxxxxxxxxxxCCxxxxCxxxx| | | | | |+-----------------+ +-----+ +---------------+ |
