Albumin is a

family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...

globular protein In biochemistry, globular proteins or spheroproteins are spherical ("globe-like") proteins and are one of the common protein types (the others being fibrous, disordered and membrane proteins). Globular proteins are somewhat water-soluble (formi ...

s, the most common of which are the

serum albumin Serum albumin, often referred to simply as blood albumin, is an albumin (a type of globular protein) found in vertebrate blood. Human serum albumin is encoded by the ''ALB'' gene. Other mammalian forms, such as bovine serum albumin, are chemical ...

s. All the proteins of the albumin family are water-

soluble In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solubil ...

, moderately soluble in concentrated salt solutions, and experience heat denaturation. Albumins are commonly found in

blood plasma Blood plasma is a light amber-colored liquid component of blood in which blood cells are absent, but contains proteins and other constituents of whole blood in suspension. It makes up about 55% of the body's total blood volume. It is the intra ...

and differ from other

blood proteins Blood-proteins, also termed plasma proteins, are proteins present in blood plasma. They serve many different functions, including transport of lipids, hormones, vitamins and minerals in activity and functioning of the immune system. Other blood pr ...

in that they are not

glycosylated Glycosylation is the reaction in which a carbohydrate (or 'glycan'), i.e. a glycosyl donor, is attached to a hydroxyl or other functional group of another molecule (a glycosyl acceptor) in order to form a glycoconjugate. In biology (but not alw ...

. Substances containing albumins are called ''albuminoids''. A number of blood transport proteins are evolutionarily related in the albumin family, including serum albumin,

alpha-fetoprotein Alpha-fetoprotein (AFP, α-fetoprotein; also sometimes called alpha-1-fetoprotein, alpha-fetoglobulin, or alpha fetal protein) is a protein that in humans is encoded by the ''AFP'' gene. The ''AFP'' gene is located on the ''q'' arm of chromosome 4 ...

vitamin D-binding protein Vitamin D-binding protein (DBP), also/originally known as gc-globulin (group-specific component), is a protein that in humans is encoded by the ''GC'' gene. DBP is genetically the oldest member of the albuminoid family and appeared early in the e ...

and

afamin Afamin is a protein that in humans is encoded by the ''AFM'' gene. Function This gene is a member of the albumin Albumin is a family of globular proteins, the most common of which are the serum albumins. All the proteins of the albumin f ...

. This family is only found in

vertebrates Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...

. ''Albumins'' in a less strict sense can mean other proteins that

coagulate Coagulation, also known as clotting, is the process by which blood changes from a liquid to a gel, forming a blood clot. It potentially results in hemostasis, the cessation of blood loss from a damaged vessel, followed by repair. The mechanism o ...

under certain conditions. See for

lactalbumin Lactalbumin, also known as "whey protein", is the albumin contained in milk and obtained from whey. Lactalbumin is found in the milk of many mammals. There are alpha and beta lactalbumins; both are contained in milk. Targeted small scientific stud ...

ovalbumin Ovalbumin (abbreviated OVA) is the main protein found in egg white, making up approximately 55% of the total protein. Ovalbumin displays sequence and three-dimensional homology to the serpin superfamily, but unlike most serpins it is not a serine ...

and plant "2S albumin".

Function

Albumins in general are transport proteins that bind to various

ligand In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electr ...

s and carry them around. Human types include: *

Human serum albumin Human serum albumin is the serum albumin found in human blood. It is the most abundant protein in human blood plasma; it constitutes about half of serum protein. It is produced in the liver. It is soluble in water, and it is monomeric. 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 acid In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, fr ...

s, hormones,

bilirubin Bilirubin (BR) (Latin for "red bile") is a red-orange compound that occurs in the normal catabolic pathway that breaks down heme in vertebrates. This catabolism is a necessary process in the body's clearance of waste products that arise from the ...

, thyroxine (T4) and pharmaceuticals (including barbiturates). Its main function is to regulate the

oncotic pressure Oncotic pressure, or colloid osmotic-pressure, is a form of osmotic pressure induced by the proteins, notably albumin, in a blood vessel's plasma (blood/liquid) that causes a pull on fluid back into the capillary. Participating colloids displace ...

of blood. The

isoelectric point The isoelectric point (pI, pH(I), IEP), is the pH at which a molecule carries no net electrical charge or is electrically neutral in the statistical mean. The standard nomenclature to represent the isoelectric point is pH(I). However, pI is also u ...

of albumin is 4.7. *

Alpha-fetoprotein Alpha-fetoprotein (AFP, α-fetoprotein; also sometimes called alpha-1-fetoprotein, alpha-fetoglobulin, or alpha fetal protein) is a protein that in humans is encoded by the ''AFP'' gene. The ''AFP'' gene is located on the ''q'' arm of chromosome 4 ...

is a fetal plasma protein that binds various cations, fatty acids and bilirubin. *

Vitamin D-binding protein Vitamin D-binding protein (DBP), also/originally known as gc-globulin (group-specific component), is a protein that in humans is encoded by the ''GC'' gene. DBP is genetically the oldest member of the albuminoid family and appeared early in the e ...

binds to vitamin D and its metabolites, as well as to fatty acids. * Not much is known about

. It seems to carry lipidated

Wnt protein The Wnt signaling pathways are a group of signal transduction pathways which begin with proteins that pass signals into a cell through cell surface receptors. The name Wnt is a portmanteau created from the names Wingless and Int-1. Wnt signaling p ...

s and Vitamin E around. *

Extracellular matrix protein 1 Extracellular matrix protein 1 is a protein that in humans is encoded by the ''ECM1'' gene. This gene encodes an extracellular protein containing motifs with a cysteine pattern characteristic of the cysteine pattern of the ligand-binding "double- ...

is a less canonical albumin. It regulates bone mineralization. The four canonical human albumins are arranged on

chromosome 4 Chromosome 4 is one of the 23 pairs of chromosomes in humans. People normally have two copies of this chromosome. Chromosome 4 spans more than 186 million base pairs (the building material of DNA) and represents between 6 and 6.5 percent of the t ...

region 4q13.3 in a tandem manner.

Classification

Albumins found in animals can be divided into six subfamilies by

phylogeny A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary relationships among various biological spec ...

. 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 Bovine serum albumin (BSA or "Fraction V") is a serum albumin protein derived from cows. It is often used as a protein concentration standard in lab experiments. The nickname "Fraction V" refers to albumin being the fifth fraction of the origin ...

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 Ovalbumin (abbreviated OVA) is the main protein found in egg white, making up approximately 55% of the total protein. Ovalbumin displays sequence and three-dimensional homology to the serpin superfamily, but unlike most serpins it is not a serine ...

is a

storage protein Storage proteins serve as biological reserves of metal ions and amino acids, used by organisms. They are found in plant seeds, egg whites, and milk. Ferritin is an example of a storage protein that stores iron. Iron is a component of heme, whic ...

in egg white (albumen). It is a

serpin Serpins are a Protein superfamily, superfamily of proteins with similar structures that were first identified for their Protease inhibitor (biology), protease inhibition activity and are found in all kingdom (biology), kingdoms of life. The acr ...

. *

Lactalbumin Lactalbumin, also known as "whey protein", is the albumin contained in milk and obtained from whey. Lactalbumin is found in the milk of many mammals. There are alpha and beta lactalbumins; both are contained in milk. Targeted small scientific stud ...

, or

whey protein Whey protein is a mixture of proteins isolated from whey, the liquid material created as a by-product of cheese production. The proteins consist of α-lactalbumin, β-lactoglobulin, serum albumin and immunoglobulins. Glycomacropeptide also makes ...

, is a protein fraction of milk. It is mainly

Beta-lactoglobulin β-Lactoglobulin (BLG) is the major whey protein of cow and sheep's milk (~3 g/L), and is also present in many other mammalian species; a notable exception being humans. Its structure, properties and biological role have been reviewed many time ...

, although serum albumin also comprises a small part of it. * Some plant

seed A seed is an embryonic plant enclosed in a protective outer covering, along with a food reserve. The formation of the seed is a part of the process of reproduction in seed plants, the spermatophytes, including the gymnosperm and angiospe ...

s, including

hemp Hemp, or industrial hemp, is a botanical class of ''Cannabis sativa'' cultivars grown specifically for industrial or medicinal use. It can be used to make a wide range of products. Along with bamboo, hemp is among the fastest growing plants o ...

, 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 . Albumin is a 65–70

kDa The dalton or unified atomic mass unit (symbols: Da or u) is a non-SI unit of mass widely used in physics and chemistry. It is defined as of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at re ...

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 medicine Traditional Chinese medicine (TCM) is an alternative medical practice drawn from traditional medicine in China. It has been described as "fraught with pseudoscience", with the majority of its treatments having no logical mechanism of action ...

s contain wild bear bile, banned under

CITES CITES (shorter name for the Convention on International Trade in Endangered Species of Wild Fauna and Flora, also known as the Washington Convention) is a multilateral treaty to protect endangered plants and animals from the threats of interna ...

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.

Terminology

Albumin is pronounced ; formed from

Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...

: ''albumen'' "(egg) white; dried egg white".

References

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. {{Authority control Blood proteins A