Production, Purification, and Characterization of a Mg2+-Responsive Porphobilinogen Synthase from Pseudomonas aeruginosa

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Abstract
During tetrapyrrole biosynthesis the metalloenzyme porphobilinogen synthase (PBGS) catalyzes the condensation of two molecules of 5-aminolevulinic acid to form the pyrrole porphobilinogen. Pseudomonas aeruginosa PBGS was synthesized in Escherichia coli, and the enzyme was purified as a fusion protein with glutathione S-transferase (GST). After removal of GST, a molecular mass of 280 000 ± 10 000 with a Stokes radius of 57 Å was determined for native PBGS, indicating a homooctameric structure of the enzyme. Mg2+ stabilized the oligomeric state but was not essential for octamer formation. Alteration of N-terminal amino acids changed the oligomeric state and reduced the activity of the enzyme, revealing the importance of this region for oligomerization and activity. EDTA treatment severely inhibited enzymatic activity which could be completely restored by the addition of Mg2+ or Mn2+. At concentrations in the micromolar range Co2+, Zn2+, and Ni2+ partially restored EDTA-inhibited enzymatic activity while higher concentrations of Zn2+ inhibited the enzyme. Pb2+, Cd2+, and Hg2+ did not restore activity. A stimulatory effect of monovalent ions was observed. A Km of 0.33 mM for ALA and a maximal specific activity of 60 μmol h-1 mg-1 at the pH optimum of 8.6 in the presence of Mg2+ and K+ were found. pH-dependent kinetic studies were combined with protein modifications to determine the structural basis of two observed pKa values of approximately 7.9 (pKa1) and 9.5 (pKa2). These are postulated respectively as ionization of an active site lysine residue and of free substrate during catalysis. Some PBGS inhibitors were characterized. Finally, we succeeded in obtaining well-ordered crystals of P. aeruginosa PBGS complexed with the substrate analogue levulinic acid.