Streptavidin (None)

Streptavidin is a 52-55 kDa homotetrameric protein purified from the bacterium *Streptomyces avidinii* that exhibits extraordinarily high affinity for biotin (vitamin B7 or H)[1][2]. With a dissociation constant on the order of 10^-14^ to 10^-15^ M, the streptavidin-biotin interaction represents one of the strongest non-covalent bonds known in nature[1][2]. **Structural characteristics**: The protein consists of four identical subunits, each composed of 159 amino acid residues in the full-length form, though it is commonly proteolytically processed to a "core" form of 125-139 residues[1][2]. Each monomer adopts an eight-stranded antiparallel β-barrel tertiary structure with a biotin-binding site located at one end of each barrel[1][3][4]. The tetrameric quaternary structure can be considered a dimer of functional dimers, with each subunit contributing critical residues (particularly Trp120) to the binding site of neighboring subunits[1][3][4]. **Binding mechanism**: The exceptional binding affinity originates from multiple factors including high shape complementarity between the binding pocket and biotin, an extensive hydrogen bonding network involving eight first-shell residues (Asn23, Tyr43, Ser27, Ser45, Asn49, Ser88, Thr90, and Asp128), numerous van der Waals interactions within the hydrophobic binding pocket lined with tryptophan residues, and stabilization of a flexible loop (L3/4, residues 45-52) that closes over bound biotin like a lid, contributing to extremely slow dissociation rates[1][3][4]. **Stability**: The streptavidin-biotin complex exhibits remarkable resistance to organic solvents, denaturants (such as guanidinium chloride), detergents (including SDS and Triton X-100), proteolytic enzymes, and extremes of temperature and pH[1]. The protein itself is stable over a wide pH range and requires up to 20 minutes at 100°C in 0.2% SDS to dissociate the tetrameric structure[2]. **Comparison to avidin**: While streptavidin shares identical biotin-binding properties with avidin (a related protein from egg white), it lacks the glycoprotein portion and has only 30% sequence homology with avidin[2]. Streptavidin's near-neutral isoelectric point (pI 5-6 versus 10 for avidin) results in lower non-specific binding in most biological applications[2]. **Applications**: Streptavidin is extensively used in molecular biology, bionanotechnology, diagnostics, and research applications including immunoassays, protein purification, detection systems, and biosensors, where its strong biotin-binding capability serves as a versatile molecular tool rather than as a therapeutic target[1][2].

Other names

Core streptavidin

Not applicable