How many biotin-binding sites do Streptavidin and Avidin have?

• A. 2
• B. 4
• C. 3
• D. 1

Answer: (B)

Avidin and Streptavidin are tetramers, built from four subunits. Each subunit provides one biotin-binding pocket, so the whole protein has four biotin-binding sites. The binding pockets come from the interfaces between subunits, which is why the intact tetramer can bind four biotin molecules. This four-site architecture is why the correct count is four; imagine a four-armed clamp where each arm grabs biotin. If you encounter structures with only two, three, or one site, that would imply a dimer, trimer, or monomer form, respectively, rather than the classic four-pocket tetramer.