What is the Difference Between Avidin and Streptavidin?

Avidin and streptavidin are both proteins that have a high affinity and specificity for biotin. However, there are some differences between the two:

1. Origin: Avidin is derived from the oviducts of birds, reptiles, and amphibians, while streptavidin is isolated from the bacterium Streptomyces avidinii.
2. Structure: Avidin is a tetrameric glycoprotein with a molecular weight of approximately 62.4 kDa, while streptavidin is a slightly smaller molecule with a molecular weight of approximately 53.6 kDa and lacks the glycoprotein portion of the molecule.
3. Sequence Homology: The sequence of avidin shows only 30% homology with streptavidin, and anti-avidin and anti-streptavidin antibodies are not immunologically cross-reactive.
4. Binding Affinity: Avidin has a higher binding affinity for free biotin (Kd ~ 10^(-15) M) than streptavidin (Kd ~ 10^(-14) M). However, when biotin is conjugated to another molecule, streptavidin shows higher affinity than avidin.
5. Non-specific Binding: Avidin has a higher tendency for aggregation and shows higher non-specific binding due to the glycoprotein portion of the molecule. Streptavidin, which lacks the glycoprotein portion, shows less non-specific binding.
6. Applications: Both avidin and streptavidin have important applications in molecular biology and bio-nanotechnology, such as purification or detection of various molecules, Western blotting, and developing nanobiotechnology. However, due to its lower non-specific binding and higher affinity for biotin-conjugated molecules, streptavidin is often considered a better choice for biochemical applications.

Comparative Table: Avidin vs Streptavidin

Here is a table comparing avidin and streptavidin:

Avidin and streptavidin are both proteins with high affinity and specificity for biotin. Avidin is a glycoprotein found in the oviducts of birds, reptiles, and amphibians, while streptavidin is a protein prepared from the purification of the bacterium Streptomyces avidinii. Avidin has a higher molecular weight and is highly cationic, while streptavidin has a near-neutral isoelectric point. Avidin exhibits nonspecific binding, which can sometimes cause background problems in certain applications, while streptavidin reportedly exhibits less nonspecific binding. Avidin is glycosylated, whereas streptavidin is not.