Characterization and crystallization of human uroporphyrinogen decarboxylase

The cytosolic enzyme uroporphyrinogen decarboxylase (URO-D) catalyzes the fifth step in the heme biosynthetic pathway, converting uroporphyrinogen to coproporphyrinogen by decarboxylating the four acetate side chains of the substrate. Recombinant human URO-D has been expressed in Escherichia coli with a histidine tag and has been purified to homogeneity. Purified protein was determined to be a monodisperse dimer by dynamic light scattering. Equilibrium sedimentation analysis confirmed that the protein is dimeric, with a dissociation constant of 0.1 microM. URO-D containing an amino-terminal histidine tag was crystallized in space group P3(1)21 or its enantiomer P3(2)21 with unit cell dimensions a = b = 103.6 A, c = 75.2 A. There is one molecule in the asymmetric unit, consistent with generation of the dimer by the twofold axis of this crystallographic operator. Native data have been collected to 3.0 a resolution.