Crystal Structures of the Elusive Rhizobium Etli L-Asparaginase Reveal a Peculiar Active Site

Rhizobium etli , a nitrogen-fixing bacterial symbiont of legume plants, encodes an essential l -asparaginase (ReAV) with no sequence homology to known enzymes with this activity. High-resolution crystal structures of ReAV show indeed a structurally distinct, dimeric enzyme, with some resemblance to glutaminases and β-lactamases. However, ReAV has no glutaminase or lactamase activity, and at pH 9 its allosteric asparaginase activity is relatively high, with K m for l -Asn at 4.2 mM and k cat of 438 s −1 . The active site of ReAV, deduced from structural comparisons and confirmed by mutagenesis experiments, contains a highly specific Zn 2+ binding site without a catalytic role. The extensive active site includes residues with unusual chemical properties. There are two Ser-Lys tandems, all connected through a network of H-bonds to the Zn center, and three tightly bound water molecules near Ser48, which clearly indicate the catalytic nucleophile.