CRYSTAL-STRUCTURE OF HUMAN UROPORPHYRINOGEN DECARBOXYLASE

Uroporphyrinogen decarboxylase (URO-D) catalyzes the fifth step in the heme biosynthetic pathway, converting uroporphyrinogen to coproporphy rinogen by decarboxylating the four acetate side chains of the substra te. This activity is essential in all organisms, and subnormal activit y of URO-D leads to the most common form of porphyria in humans, porph yria cutanea tarda (PCT), We have determined the crystal structure of recombinant human URO-D at 1.60 Angstrom resolution, The 40.8 kDa prot ein is comprised of a single domain containing a (beta/alpha)(8)-barre l with a deep active site cleft formed by loops at the C-terminal ends of the barrel strands. Many conserved residues cluster at this cleft, including the invariant side chains of Arg37, Arg41 and His339, which probably function in substrate binding, and Asp86, Tyr164 and Ser219, which may function in either binding or catalysis, URO-D is a dimer i n solution (K-d = 0.1 mu M), and this dimer also appears to be formed in the crystal. Assembly of the dimer juxtaposes the active site cleft s of the monomers, suggesting a functionally important interaction bet ween the catalytic centers.