To understand the basis for the lower activity of the tryptophan synthaseb2 subunit in comparison to thea2b2 complex, we determined the crystal struc-tures of apo-b2 and holo-b2 from Escherichia coliat and A˚ resolu-tions, respectively. To our knowledge, this is the first report of both b2 subunit structures with and without pyridoxal-5¢-phosphate.