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 liga nds differ and two molecules of 5-aminolevulinate whose chemical fates differ. The asymmetric binding of two Zn-II atoms and two identical s ubstrate molecules by a homodimeric active site is apparently unique. Modification by 5-chlorolevulinate can be used to differentiate the tw o substrate-binding sites; diffraction-quality crystals of 5-chlorolev ulinate-modified PBGS have been obtained. Pb-II can be used to differe ntiate the two different Zn-II-binding sites; diffraction-quality crys tals of the Pb-II complex of PBGS have been obtained. Preliminary diff raction data reveal an I422 space group, in agreement with a general m odel for the quaternary structure of PBGS.