Albumin

Function

Albumins in general are transport proteins that bind to various ligands and carry them around. 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. The isoelectric point of albumin is 4.7.
• 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.
• 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.

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.

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.

Structure

The 3D structure of human serum albumin has been determined by X-ray crystallography to a resolution of 2.5 Å. 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 detection

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.

Etymology

Albumin is pronounced ; formed from Latin: albumen "egg white", itself derived from the latin albus: white.

References

References

1. (June 1999). "Crystal structure of human serum albumin at 2.5 A resolution". Protein Engineering.
2. (July 1992). "Atomic structure and chemistry of human serum albumin". Nature.
3. (October 1989). "Amphibian albumins as members of the albumin, alpha-fetoprotein, vitamin D-binding protein multigene family". Journal of Molecular Evolution.
4. (June 1986). "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.
5. (July 1994). "Afamin is a new member of the albumin, alpha-fetoprotein, and vitamin D-binding protein gene family". The Journal of Biological Chemistry.
6. (2017). "Genome-Wide Identification and Comparative Analysis of Albumin Family in Vertebrates". Evolutionary Bioinformatics Online.
7. (January 2010). "Albumin usage in clinical medicine: tradition or therapeutic?". Transfusion Medicine Reviews.
8. "Product Information data sheet". Sigma Aldrich.
9. (February 2016). "Active and water-soluble form of lipidated Wnt protein is maintained by a serum glycoprotein afamin/α-albumin". eLife.
10. (May 1996). "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.
11. (1999). "Seed Proteins". Springer Netherlands.
12. (September 2008). "Development of a field test for the detection of illegal bear products". Endangered Species Research.
13. Bostock, John. "Historia Naturalis 28, 6, 18".