Albumin Explained

Symbol:	Serum_albumin
Serum albumin family
Pfam:	PF00273
Pfam Clan:	CL0282
Interpro:	IPR014760
Smart:	SM00103
Prosite:	PS51438
Scop:	1ao6
Pdb:	,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,

Albumin is a family of globular proteins, the most common of which are the serum albumins. All of the proteins of the albumin family are water-soluble, moderately soluble in concentrated salt solutions, and experience heat denaturation. Albumins are commonly found in blood plasma and differ from other blood proteins in that they are not glycosylated. Substances containing albumins are called albuminoids.

A number of blood transport proteins are evolutionarily related in the albumin family, including serum albumin, alpha-fetoprotein, vitamin D-binding protein and afamin.^{[1] [2] [3]} This family is only found in vertebrates.^[4]

Albumins in a less strict sense can mean other proteins that coagulate under certain conditions. See for lactalbumin, ovalbumin and plant "2S albumin".

Function

Albumins in general are transport proteins that bind to various ligands and carry them around.^[4] Human types include:

• Human serum albumin is the main protein of human blood plasma. It makes up around 50% of human plasma proteins. It binds water, cations (such as Ca²⁺, Na⁺ and K⁺), fatty acids, hormones, bilirubin, thyroxine (T4) and pharmaceuticals (including barbiturates). Its main function is to regulate the oncotic pressure of blood.^[5] The isoelectric point of albumin is 4.7.^[6]
• Alpha-fetoprotein is a fetal 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.
• Not much is known about afamin. It seems to carry lipidated Wnt proteins and Vitamin E around.^[7]
• Extracellular matrix protein 1 is a less canonical albumin. It regulates bone mineralization.

The four canonical human albumins are arranged on chromosome 4 region 4q13.3 in a tandem manner.^[8]

Classification

Albumins found in animals can be divided into six subfamilies by phylogeny. The Vitamin-D binding proteins occupy families 1–3. The other albumins are mixed among each other in families 4–6. ECM1 is in family 6.^[4]

In addition to their medical use, serum albumins are valued in biotechnology. Bovine serum albumin is usually used, although versions from humans and genetically-modified rice are also used to reduce animal cruelty.

Other albumin types

A few other proteins are also sometimes called albumins. They are not in the same family as vertebrate albumins:

• Ovalbumin is a storage protein in egg white (albumen). It is a serpin.
• Lactalbumin, or whey protein, is a protein fraction of milk. It is mainly Beta-lactoglobulin, although serum albumin also comprises a small part of it.
• Some plant seeds, including hemp, encode "2S albumins". These are named for their egg-like coagulation property.^[9]

Structure

The 3D structure of human serum albumin has been determined by X-ray crystallography to a resolution of2.5Å.^[10] Albumin is a 65–70 kDa protein.

Albumin comprises three homologous domains that assemble to form a heart-shaped protein. Each domain is a product of two subdomains that possess common structural motifs. 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 disulfide bonds.

Forensic uses

Worldwide, certain traditional Chinese medicines contain wild bear bile, banned under CITES legislation. Dip sticks, similar to common pregnancy tests, have been developed to detect the presence of bear albumin in traditional medicine products, indicating that bear bile had been used in their creation.^[11]

Terminology

Albumin is pronounced ; formed from Latin: albumen^[12] "(egg) white; dried egg white".

See also

• Cohn process (human serum albumin purification method)
• Serum albumin
  • Bovine serum albumin
  • Human serum albumin

External links

• • The Albumin website
• Albumin binding prediction
• PDBe-KB provides an overview of all the structure information available in the PDB for Human Serum albumin.

Notes and References

1. Haefliger DN, Moskaitis JE, Schoenberg DR, Wahli W . 1456034 . Amphibian albumins as members of the albumin, alpha-fetoprotein, vitamin D-binding protein multigene family . Journal of Molecular Evolution . 29 . 4 . 344–54 . October 1989 . 2481749 . 10.1007/BF02103621 . 1989JMolE..29..344H .
2. Schoentgen F, Metz-Boutigue MH, Jollès J, Constans J, Jollès P . 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 . Biochimica et Biophysica Acta (BBA) - Protein Structure and Molecular Enzymology . 871 . 2 . 189–98 . June 1986 . 2423133 . 10.1016/0167-4838(86)90173-1 .
3. Lichenstein HS, Lyons DE, Wurfel MM, Johnson DA, McGinley MD, Leidli JC, Trollinger DB, Mayer JP, Wright SD, Zukowski MM . 6 . Afamin is a new member of the albumin, alpha-fetoprotein, and vitamin D-binding protein gene family . The Journal of Biological Chemistry . 269 . 27 . 18149–54 . July 1994 . 10.1016/S0021-9258(17)32429-8 . 7517938 . free .
4. Li S, Cao Y, Geng F . Genome-Wide Identification and Comparative Analysis of Albumin Family in Vertebrates . Evolutionary Bioinformatics Online . 13 . 1176934317716089 . 2017 . 28680266 . 5480655 . 10.1177/1176934317716089 .
5. Farrugia A . Albumin usage in clinical medicine: tradition or therapeutic? . Transfusion Medicine Reviews . 24 . 1 . 53–63 . January 2010 . 19962575 . 10.1016/j.tmrv.2009.09.005 .
6. Web site: Sigma Aldrich . Product Information data sheet .
7. Mihara E, Hirai H, Yamamoto H, Tamura-Kawakami K, Matano M, Kikuchi A, Sato T, Takagi J . 6 . Active and water-soluble form of lipidated Wnt protein is maintained by a serum glycoprotein afamin/α-albumin . eLife . 5 . February 2016 . 26902720 . 10.7554/eLife.11621 . 4775226 . free .
8. Nishio H, Heiskanen M, Palotie A, Bélanger L, Dugaiczyk A . Tandem arrangement of the human serum albumin multigene family in the sub-centromeric region of 4q: evolution and chromosomal direction of transcription . Journal of Molecular Biology . 259 . 1 . 113–9 . May 1996 . 8648639 . 10.1006/jmbi.1996.0306 .
9. Book: Shewry . Peter R. . Pandya . Maya J. . vanc . Seed Proteins . limited . Springer Netherlands . 978-94-011-4431-5 . The 2S Albumin Storage Proteins . 1999 . 563–586 . 10.1007/978-94-011-4431-5_24.
10. Sugio S, Kashima A, Mochizuki S, Noda M, Kobayashi K . Crystal structure of human serum albumin at 2.5 A resolution . Protein Engineering . 12 . 6 . 439–46 . June 1999 . 10388840 . 10.1093/protein/12.6.439 .
11. Peppin L, McEwing R, Webster S, Rogers A, Nicholls D, Ogden R . Development of a field test for the detection of illegal bear products . Endangered Species Research. September 2008 . 9 . 3 . 263–70 . 10.3354/esr00131 . free .
12. Book: Bostock, John . Pliny the Elder, The Natural History . vanc . Historia Naturalis 28, 6, 18 . https://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.02.0137 .