What is biotin labeling?
Biotinylation means attaching a biotin tag to a molecule. Biotin is a natural molecule that is also known as vitamin B7. It is an important component in a healthy diet, but it is also very useful in the laboratory as a method for protein conjugation. In the laboratory, he purpose of biotinylation is to create a controlled site for biotin-streptavidin affinity binding.
How does streptavidin bind biotin?
The biotin fits exquisitely into a biotin-binding pocket in each of the four binding sites per streptavidin molecule, and it is held in place with hydrogen bonds. Additionally, once the biotin is bound, a conformational change in the streptavidin allows a small “cap” to close over the biotin in the binding pocket. As a result, biotin and streptavidin have an extraordinary affinity for each other (Kd=10^-15). With such a low dissociation constant, once the biotin and streptavidin are bound it is unlikely that they will dissociate. This affinity is resistant to changes in temperature, pH, and salt concentration and is extremely specific. It is often thought of as a nearly covalent bond. These properties make biotinylation a useful tool for engineers who are developing new purification and detection methods. A commercially available biotinylation kit makes the process even easier.