How many biotin binding-sites are there per streptavidin molecule?

Wild-type streptavidin is a tetramer with four identical subunits, each containing a high affinity binding site for biotin (K _D = 10 ^-15 M) . Monovalent streptavidin is a tetramer with 3 inactive subunits and one subunit that binds biotin with wild-type affinity.