- Streptavidin is a tetrameric biotin-binding protein originally isolated from the bacterium Streptomyces avidinii. It is widely used in molecular biology and biotechnology due to its exceptionally high affinity for biotin (vitamin B7 or H). This non-covalent interaction between streptavidin and biotin is one of the strongest known biological interactions, with a dissociation constant (K_d) in the femtomolar range (~10⁻¹⁴ M), making it nearly irreversible under physiological conditions.
- Structurally, each streptavidin monomer consists of approximately 159 amino acids and adopts a β-barrel fold. Four monomers assemble to form a functional tetramer, and each monomer can bind one biotin molecule, allowing a single streptavidin molecule to bind up to four biotin molecules. The binding site is deeply embedded within the protein, contributing to the stability of the streptavidin–biotin complex even under harsh conditions, such as extremes of pH, temperature, and the presence of detergents or denaturing agents.
- Unlike avidin (a similar biotin-binding protein found in egg whites), streptavidin is non-glycosylated and has a near-neutral isoelectric point, making it less prone to nonspecific interactions in biochemical assays. Its relatively low immunogenicity and minimal background binding have made it a preferred tool for applications in protein purification, immunoassays, microscopy, biosensors, and molecular diagnostics.
- In experimental procedures, streptavidin is commonly conjugated to enzymes (e.g., horseradish peroxidase or alkaline phosphatase), fluorophores (for imaging and flow cytometry), or magnetic/agarose beads (for affinity purification). Through these conjugates, biotinylated molecules—such as nucleic acids, antibodies, peptides, or other proteins—can be selectively captured, detected, or visualized with high sensitivity and specificity.
- Streptavidin is also used in protein interaction studies, surface plasmon resonance, and nanotechnology for creating well-defined multivalent systems. Variants such as monomeric streptavidin and mutant forms with reduced biotin affinity have been engineered for reversible applications or to reduce the valency when multimerization is not desired.
- In summary, streptavidin is a versatile, high-affinity biotin-binding protein that plays a crucial role in a wide range of biological and analytical techniques. Its structural stability, low background interaction, and strong biotin-binding capacity have made it an indispensable reagent in modern molecular biology and biotechnology.
