Streptavidin

Details:

Streptavidin is a tetrameric protein (meaning it has four identical subunits) originally isolated from the bacterium Streptomyces avidinii. It's renowned in biochemistry and molecular biology for its extremely strong and specific interaction with biotin (vitamin B7 or vitamin H).

Here's a breakdown of its key characteristics and importance:

1. High Affinity for Biotin:

   • This is its defining feature. Streptavidin binds biotin with one of the strongest non-covalent interactions known in nature.

   • The dissociation constant (Kd) is approximately 10?¹? M, meaning the bond is essentially irreversible under most experimental conditions.

   • Each of the four subunits binds one molecule of biotin.

2. Structure:

   • Forms a very stable homotetramer (four identical chains).

   • Each subunit has a binding pocket perfectly shaped for biotin.

3. Advantages over Avidin:

   • Lower Non-Specific Binding: Unlike its cousin avidin (from egg whites), streptavidin is:

     • Not glycosylated (lacks sugar groups).

     • Has a near-neutral isoelectric point (pI ~6-7).

   • These properties make streptavidin much less "sticky" to other cellular components (like lectins or charged molecules) than avidin (which is glycosylated and has a high pI ~10), leading to significantly cleaner results in assays.

4. Key Applications (Leveraging the Biotin-Streptavidin Interaction):

   • Detection and Immobilization: The core principle is "biotinylate" the molecule of interest (attach biotin chemically or enzymatically), then use streptavidin as a bridge or capture agent.

   • Common Techniques:

     • ELISA (Enzyme-Linked Immunosorbent Assay): Streptavidin conjugated to enzymes (like HRP or AP) detects biotinylated antibodies or antigens.

     • Western Blotting: Biotinylated secondary antibodies + Streptavidin-enzyme conjugates amplify detection signals.

     • Immunohistochemistry (IHC) & Immunofluorescence (IF): Visualize targets using biotinylated probes + streptavidin linked to enzymes (for color) or fluorophores (for light).

     • Flow Cytometry: Detect cell surface markers using biotinylated antibodies + fluorescently labeled streptavidin.

     • Affinity Purification: Immobilize streptavidin on a solid support (like beads or columns) to capture biotinylated molecules (e.g., proteins, nucleic acids).

     • Microarray Technology: Immobilize biotinylated probes on streptavidin-coated slides.

     • DNA/RNA Pull-Down Assays: Capture biotinylated nucleic acids or associated proteins.

     • Nanotechnology: Building molecular structures and biosensors due to its stability and precise binding.

5. Derivatives:

   • Recombinant Streptavidin: Widely produced in E. coli for consistency and purity.

   • Mutant Forms: Engineered for faster off-rates (useful when reversible binding is needed) or altered stability.

   • Conjugates: Routinely coupled to enzymes (HRP, AP), fluorophores (FITC, PE, Alexa Fluor dyes), chromogens, magnetic beads, gold nanoparticles, etc.

In essence, streptavidin is an indispensable tool in modern life sciences because it provides an exceptionally strong, specific, and versatile "molecular glue" between a biotinylated molecule of interest and a wide variety of detection or capture systems. Its low non-specific binding makes it superior to avidin for most laboratory applications.