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 homooctameric 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-chlorolevulinate 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.