Avidin Protein
Description
Introduction to Avidin Protein
Avidin is a tetrameric glycoprotein found in the egg whites of birds, reptiles, and amphibians. It binds biotin (vitamin B7) with one of the strongest non-covalent interactions in nature, characterized by a dissociation constant () of ~10 M . Discovered by Esmond Emerson Snell in the 1940s during studies on biotin deficiency in chicks fed raw egg whites, avidin’s ability to sequester biotin led to its recognition as a critical tool in biochemistry . Its structural robustness and high specificity have made it indispensable in biotechnology, diagnostics, and therapeutics.
Molecular Architecture
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Quaternary Structure: Homotetramer comprising four identical 128-amino-acid subunits, with a total molecular weight of 66–69 kDa .
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Secondary Structure: Each subunit forms an eight-stranded antiparallel β-barrel, with biotin binding at the barrel’s interior .
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Glycosylation: Contains ~10% carbohydrate content (mannose and N-acetylglucosamine residues), contributing to its solubility and stability .
| Property | Value | Source |
|---|---|---|
| Molecular Weight | 67 kDa (tetramer) | |
| Isoelectric Point (pI) | 10–10.5 | |
| Biotin Binding Sites | 4 per tetramer |
Biotin Binding Mechanism
Avidin’s biotin-binding pocket is lined with tryptophan and lysine residues, enabling high-affinity interactions through hydrogen bonding and hydrophobic forces . Key features include:
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Affinity: M, surpassing antibody-antigen interactions by 10–10-fold .
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Irreversibility: Binding is resistant to pH extremes, organic solvents, and denaturants, though reversible with excess biotin or analogs like desthiobiotin .
Thermal and Chemical Stability
Avidin exhibits exceptional stability:
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Thermal Resistance: Wild-type avidin retains function up to 85°C. Chimeric mutants (e.g., A/A2-B) achieve even higher stability (melting temperature with biotin) .
Thermal Stability of Avidin Variants
Diagnostic and Research Tools
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Assays: ELISA, Western blot, and immunohistochemistry (IHC) leverage avidin-biotin complexes for signal amplification .
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Affinity Purification: Immobilized avidin captures biotinylated proteins, nucleic acids, or antibodies .
Therapeutic Innovations
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Drug Delivery: Avidin-biotin systems enable targeted delivery of chemotherapeutics, radioisotopes, or gene therapies to cancer cells .
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Cancer Therapy: Engineered avidin derivatives (e.g., neutral forms with reduced non-specific binding) improve tumor targeting .
Engineering Enhanced Avidin Variants
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Chimeric Proteins: DNA shuffling of avidin and avidin-related protein 2 (AVR2) produced mutants like A/A2-B, which exhibit higher thermal stability and lower biotin dissociation rates .
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Isoelectric Point Customization: Modifying avidin’s pI reduces non-specific interactions in physiological environments .
Industry Advancements
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E-Proteins: Developed avidin conjugates (e.g., eQuant-HRP) for high-sensitivity assays and neutral variants for in vivo applications .
Comparative Analysis of Avidin Family Proteins
Future Directions
Ongoing research focuses on:
Product Specs
The biological activity is 14.0 units per milligram of protein, where 1 unit is defined as the amount capable of binding 1 µg of biotin.
Avidin serves as a valuable tool for visualizing biotin-conjugated molecules in various applications, including ELISA, blotting techniques, and histological studies.
Product Science Overview
Avidin is composed of four identical subunits, each capable of binding to biotin (Vitamin B7 or Vitamin H) with a high degree of affinity and specificity . The dissociation constant of the avidin-biotin complex is measured to be approximately 10^-15 M, making it one of the strongest known non-covalent bonds . The tetrameric form of avidin is estimated to be 66–69 kDa in size, with about 10% of its molecular weight contributed by carbohydrate moieties .
Avidin was first isolated from raw chicken egg white by Esmond Emerson Snell . The discovery began with the observation that chicks on a diet of raw egg white were deficient in biotin, despite the availability of the vitamin in their diet . Snell concluded that a component of the egg white was sequestering biotin, which he verified in vitro using a yeast assay . He later isolated the component responsible for biotin binding and confirmed that it was the cause of biotin deficiency or "egg white injury" .
The natural function of avidin in eggs is not entirely understood. However, it has been postulated that avidin is produced in the oviduct as a bacterial growth inhibitor by binding biotin, which is essential for bacterial growth . Streptavidin, a related protein with equal biotin affinity, is produced by certain strains of Streptomyces bacteria and is thought to inhibit the growth of competing bacteria, acting similarly to an antibiotic .
The high affinity of avidin for biotin has been exploited in various scientific applications since the mid-1970s . Avidin and its bacterial counterpart, streptavidin, have become standard reagents for diverse detection schemes, including histochemical applications, immunoassays, and DNA hybridization procedures . These techniques often involve applying a biotinylated probe to a sample and then detecting the bound probe with a labeled avidin or streptavidin .
