Why streptavidin is better than avidin for binding biotin?

Avidin and streptavidin, although with only 30% sequence identical to each other, have almost same secondary, tertiary, and quaternary structure, allowing both of them binding to biotin. Avidin has a higher binding affinity for free biotin (Kd ~ 10?15M) than streptavidin (Kd ~ 10?14M). However, when biotin is conjugated to another molecule, streptavidin shows higher affinity than avidin, making streptavidin more feasible for biochemical applications. Besides, avidin has a higher tendency for aggregation and shows higher non-specific binding due to the presence of carbohydrates in its structure and high pI. Therefore, streptavidin is generally a better choice for binding biotin.