Avidin is a tetrameric biotin
Biotin, also known as Vitamin H or Coenzyme R, is a water-soluble B-complex vitamin discovered by Bateman in 1916. It is composed of a ureido ring fused with a tetrahydrothiophene ring. A valeric acid substituent is attached to one of the carbon atoms of the tetrahydrothiophene ring...

-binding protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 produced in the oviduct
In non-mammalian vertebrates, the passageway from the ovaries to the outside of the body is known as the oviduct. The eggs travel along the oviduct. These eggs will either be fertilized by sperm to become a zygote, or will degenerate in the body...

s of bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

s, reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...

s and amphibian
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...

s deposited in the whites of their eggs
Egg (biology)
An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...

. In chicken egg white, avidin makes up approximately 0.05% of total protein (approximately 1.8 mg per egg). The tetrameric protein contains four identical subunit
Protein subunit
In structural biology, a protein subunit or subunit protein is a single protein molecule that assembles with other protein molecules to form a protein complex: a multimeric or oligomeric protein. Many naturally occurring proteins and enzymes are multimeric...

s (homotetramer), each of which can bind to biotin
Biotin, also known as Vitamin H or Coenzyme R, is a water-soluble B-complex vitamin discovered by Bateman in 1916. It is composed of a ureido ring fused with a tetrahydrothiophene ring. A valeric acid substituent is attached to one of the carbon atoms of the tetrahydrothiophene ring...

 (Vitamin B7, vitamin H) with a high degree of affinity and specificity. The dissociation constant
Dissociation constant
In chemistry, biochemistry, and pharmacology, a dissociation constant is a specific type of equilibrium constant that measures the propensity of a larger object to separate reversibly into smaller components, as when a complex falls apart into its component molecules, or when a salt splits up into...

 of avidin is measured to be KD ≈ 10−15 M, making it one of the strongest known non-covalent bonds.

In its tetrameric form, avidin is estimated to be between 66–69 kDa in size. Ten percent of the molecular weight is attributed to carbohydrate content composed of four to five mannose
Mannose is a sugar monomer of the aldohexose series of carbohydrates. Mannose is a C-2 epimer of glucose. It is not part of human metabolism, but is a component of microbial cell walls, and is therefore a target of the immune system and also of antibiotics....

 and three N-acetylglucosamine
N-Acetylglucosamine is a monosaccharide derivative of glucose. It is an amide between glucosamine and acetic acid...

 residues. The carbohydrate
A carbohydrate is an organic compound with the empirical formula ; that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 . However, there are exceptions to this. One common example would be deoxyribose, a component of DNA, which has the empirical...

 moieties of avidin contain at least three unique oligosaccharide
An oligosaccharide is a saccharide polymer containing a small number of component sugars, also known as simple sugars...

 structural types that are similar in structure and composition.

Functional avidin is found only in raw egg, as the biotin avidity of the protein is destroyed by cooking. The natural function of avidin in eggs is not known, although it has been postulated to be made in the ovaduct as a bacterial growth-inhibitor, by binding biotin the bacteria need. As evidence for this, streptavidin
Streptavidin is a 60000 dalton protein purified from the bacterium Streptomyces avidinii. Streptavidin homo-tetramers have an extraordinarily high affinity for biotin . With a dissociation constant on the order of ≈10-14 mol/L, the binding of biotin to streptavidin is one of the strongest...

, a loosely related protein with equal biotin affinity and a very similar binding site, is made by certain strains of Streptomyces
Streptomyces is the largest genus of Actinobacteria and the type genus of the family Streptomycetaceae. Over 500 species of Streptomyces bacteria have been described. As with the other Actinobacteria, streptomycetes are gram-positive, and have genomes with high guanine and cytosine content...

bacteria, and is thought to serve to inhibit the growth of competing bacteria, in the manner of an antibiotic
An antibacterial is a compound or substance that kills or slows down the growth of bacteria.The term is often used synonymously with the term antibiotic; today, however, with increased knowledge of the causative agents of various infectious diseases, antibiotic has come to denote a broader range of...


A non-glycosylated form of avidin has been isolated from commercially prepared product; however, it is not conclusive as to whether the non-glycosylated form occurs naturally or is a product of the manufacturing process.

Discovery of avidin

Avidin was first discovered by Esmond Emerson Snell (1914–2003). The route to discovery began with the observation that chicks on a diet of raw egg-white were deficient in biotin
Biotin, also known as Vitamin H or Coenzyme R, is a water-soluble B-complex vitamin discovered by Bateman in 1916. It is composed of a ureido ring fused with a tetrahydrothiophene ring. A valeric acid substituent is attached to one of the carbon atoms of the tetrahydrothiophene ring...

, despite availability of the vitamin in their diet. It was concluded that a component of the egg-white was sequestering biotin which Snell verified in vitro using a yeast
Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with 1,500 species currently described estimated to be only 1% of all fungal species. Most reproduce asexually by mitosis, and many do so by an asymmetric division process called budding...

 assay. Snell later isolated the component of egg white responsible for biotin binding, and, in collaboration with Paul Gyorgy, confirmed that the isolated egg protein was the cause of biotin deficiency or “egg white injury”. At the time the protein had been tentatively named avidalbumin (literally, hungry albumin) by the involved researchers at the University of Texas. The name of the protein was later revised to "avidin" based on its affinity for biotin (avid + biotin).

Applications of avidin

Research in the 1970s helped establish the avidin-biotin system as a powerful tool in biological sciences. Aware of the strength and specificity of the avidin-biotin complex, researchers began to exploit avidin and streptavidin as probes and affinity matrixes in numerous research projects. Soon after, researchers Bayer and Wilchek developed new methods and reagents to biotinylate antibodies and other biomolecules, allowing the transfer of the avidin-biotin system to a range of biotechnological applications. Today, avidin is used in applications ranging from research and diagnostics to medical devices and pharmaceuticals.

Avidin's affinity for biotin is exploited in wide-ranging biochemical assays, including western blot
Western blot
The western blot is a widely used analytical technique used to detect specific proteins in the given sample of tissue homogenate or extract. It uses gel electrophoresis to separate native proteins by 3-D structure or denatured proteins by the length of the polypeptide...

Enzyme-linked immunosorbent assay , is a popular format of a "wet-lab" type analytic biochemistry assay that uses one sub-type of heterogeneous, solid-phase enzyme immunoassay to detect the presence of a substance in a liquid sample."Wet lab" analytic biochemistry assays involves detection of an...

The Enzyme-linked immunosorbent spot assay is a common method for monitoring immune responses in humans and animals. It was developed by Cecil Czerkinsky in 1983....

 and pull-down assays. In some cases the use of biotinylated antibodies has allowed the replacement of radioiodine labeled antibodies in radioimmunoassay
Radioimmunoassay is a very sensitive in vitro assay technique used to measure concentrations of antigens by use of antibodies...

 systems, to give an assay system which is not radioactive.

Avidin immobilized onto solid supports is also used as purification media to capture biotin-labelled
In biochemistry, biotinylation is the process of covalently attaching biotin to a protein, nucleic acid or other molecule. Biotinylation is rapid, specific and is unlikely to perturb the natural function of the molecule due to the small size of biotin...

 protein or nucleic acid molecules. For example, cell surface proteins can be specifically labelled with membrane impermeable biotin reagent, then specifically captured using an avidin-based support.

Modified forms of avidin

As a basically charged glycoprotein, avidin exhibits non-specific binding in some applications. Neutravidin
NeutrAvidin protein is a deglycosylated version of avidin, with a mass of approximately 60,000 daltons. As a result of carbohydrate removal, lectin binding is reduced to undetectable levels, yet biotin binding affinity is retained because the carbohydrate is not necessary for this activity...

, a deglycosylated avidin with modified arginines, exhibits a more neutral pI and is available as an alternative to native avidin, wherein problems of non-specific binding arise. Deglycosylated, neutral forms of avidin are available through Sigma-Aldrich (Extravidin), Thermo Scientific (NeutrAvidin), Invitrogen (NeutrAvidin), and Belovo (NeutraLite).

Given the strength of the avidin-biotin bond, dissociation of the avidin-biotin complex requires extreme conditions that cause protein denaturation. The non-reversible nature of the avidin-biotin complex can limit avidin’s application in affinity chromatography applications where release of the captured ligand is desirable. Researchers have created an avidin with reversible binding characteristics through nitration or iodination of the binding site tyrosine. The modified avidin exhibits strong biotin binding characteristics at pH 4 and releases biotin at a pH of 10 or higher. A monomeric form of avidin with a reduced affinity for biotin is also employed in many commercially available affinity resins. The monomeric avidin is created by treatment of immobilized native avidin with urea or guanidine HCl (6–8 M), giving it a lower dissociation KD ≈ 10−7M. This allows elution from the avidin matrix to occur under milder, non-denaturing conditions, using low concentrations of biotin or low pH conditions.

Inactivation of biotin binding activity

The thermal stability and biotin binding activity of avidin are of both practical and theoretical interest to researchers, as avidin's stability is unusually high and avidin is an antinutrient
Antinutrients are natural or synthetic compounds that interfere with the absorption of nutrients. Nutrition studies focus on those antinutrients commonly found in food sources and beverages....

 in human food. A 1966 study published in Biochemical and Biophysical Research Communications
Biochemical and Biophysical Research Communications
Biochemical and Biophysical Research Communications is a peer-reviewed academic journal in the field of biochemistry and biophysics. Founded in 1959, the journal is published weekly by Elsevier under the Academic Press imprint. As of 2008, the Editor-in-Chief is William J...

found that the structure of avidin remains stable at temperatures below 70 °C (158 °F). Above 70 °C (158 °F), avidin's structure is rapidly disrupted and by 85 °C (185 °F), extensive loss of structure and ability to bind biotin is found. A 1991 assay
An assay is a procedure in molecular biology for testing or measuring the activity of a drug or biochemical in an organism or organic sample. A quantitative assay may also measure the amount of a substance in a sample. Bioassays and immunoassays are among the many varieties of specialized...

 for the Journal of Food Science
Journal of Food Science
The Journal of Food Science is a peer-reviewed scientific journal that was established in 1936 and is published by John Wiley & Sons on behalf of the Institute of Food Technologists in Chicago, Illinois...

detected substantial avidin activity in cooked egg white: "mean residual avidin activity in fried, poached and boiled (2 min) egg white was 33, 71 and 40% of the activity in raw egg white." The assay surmised that cooking times were not sufficient to adequately heat all cold spot areas within the egg white. Complete inactivation of avidin's biotin binding capacity required boiling for over 4 minutes.

A 1992 study found that thermal inactivation of the biotin binding activity of avidin was described by D
D-value (microbiology)
In microbiology, D-value refers to decimal reduction time is the time required at a certain temperature to kill 90% of the organisms being studied. Thus after a colony is reduced by 1 D, only 10% of the original organisms remain. The population number has been reduced by one decimal place in the...

121°C = 25 min and z
Z-value is a term used in thermal death time calculations. The z-value of an organism is the temperature, in degrees Fahrenheit or Celsius, that is required for the thermal destruction curve to move one log cycle...

 = 33°C. The study disagreed with prior assumptions "that the binding site of avidin is destroyed on heat denaturation
Denaturation (biochemistry)
Denaturation is a process in which proteins or nucleic acids lose their tertiary structure and secondary structure by application of some external stress or compound, such as a strong acid or base, a concentrated inorganic salt, an organic solvent , or heat...

", concluding that protein denaturation was not equivalent to loss of biotin binding activity.
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