Crystallization and preliminary X-ray diffraction studies of 5-chlorolevulinate-modified bovine porphobilinogen synthase and the Pb(II)-complexed enzyme.

Bovine porphobilinogen synthase (PBGS) is an homo-octameric enzyme with four active sites. Each active site binds two Zn(II) atoms whose ligands differ and two molecules of 5-aminolevulinate whose chemical fates differ. The asymmetric binding of two Zn(II) atoms and two identical substrate molecules by a homodimeric active site is apparently unique. Modification by 5-chiorolevulinate can be used to differentiate the two substrate-binding sites; diffraction-quality crystals of 5-chlorolevulinate-modified PBGS have been obtained. Pb(II) can be used to differentiate the two different Zn(II)-binding sites; diffraction-quality crystals of the Pb(II) complex of PBGS have been obtained. Preliminary diffraction data reveal an I422 space group, in agreement with a general model for the quaternary structure of PBGS.