The extraordinary affinity of avidin for biotin allows
biotin-containing molecules in a complex mixture to be discretely
bound with avidin. Avidin is a glycoprotein found in the egg
white and tissues of birds, reptiles and amphibians. It contains
four identical subunits having a combined mass of 67,000–68,000
daltons. Each subunit consists of 128 amino acids and binds one
molecule of biotin. The extent of glycosylation on avidin is high;
carbohydrate accounts for about 10% of the total mass of the
tetramer. Avidin has a basic isoelectric point (pI = 10–10.5) and
is stable over a wide range of pH and temperature. Extensive
chemical modification has little effect on the activity of avidin,
making it especially useful for protein purification. However,
because of its carbohydrate content and basic pI, avidin has
relatively high nonspecific binding properties.