CHARACTERIZATION AND CRYSTALLIZATION OF HUMAN UROPORPHYRINOGEN DECARBOXYLASE

The cytosolic enzyme uroporphyrinogen decarboxylase (URO-D) catalyzes the fifth step in the heme biosynthetic pathway, converting uroporphyr inogen 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 homoge neity. Purified protein was determined to be a monodisperse dimer by d ynamic light scattering. Equilibrium sedimentation analysis confirmed that the protein is dimeric, with a dissociation constant of 0.1 mu M. URO-D containing an amino-terminal histidine tag was crystallized in space group P3(1)21 or its enantiomer P3(2)21 with unit eel dimensions a = b = 103.6 Angstrom, c = 75.2 Angstrom. There is one molecule in t he asymmetric unit, consistent with generation of the dimer by the two fold axis of this crystallographic operator. Native data have been col lected to 3.0 Angstrom resolution.