Biochemical characterization of allantoinase from escherichia coli BL21

Bacterial allantoinase (ALLase; EC 3.5.2.5), which catalyzes the conversion of allantoin into allantoate, possesses a binuclear metal center in which two metal ions are bridged by a posttranslationally carboxylated lysine. Here, we characterized ALLase from Escherichia coli BL21. Purified recombinant ALLase exhibited no activity but could be activated when preincubating with some metal ions before analyzing its activity, and was in the order: Mn2+-≤ Co2+-<Zn2+-<Ni2+-<Cd 2+-∼Mg2+-activated enzyme; however, activity of ALLase (Mn2+-activated form) was also significantly inhibited with 5 mM Co2+, Zn2+, and Cd2+ ions. Activity of Mn 2+-activated ALLase was increased by adding the reducing agent dithiothreitol (DTT), but was decreased by treating with the sulfhydryl modifying reagent N-ethylmaleimide (NEM). Inhibition of Mn2+- activated ALLase by chelator 8-hydroxy-5-quinolinesulfonic acid (8-HQSA), but not EDTA, was pHdependent. Analysis of purified ALLase by gel filtration chromatography revealed a mixture of monomers, dimers, and tetramers. Substituting the putative metal binding residues His59, His61, Lys146, His186, His242, and Asp315 with Ala completely abolished the activity of ALLase, even preincubating with Mn2+ ions. On the basis of these results, as well as the pH-activity profile, the reaction mechanism of ALLase is discussed and compared with those of other cyclic amidohydrolases.