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Crystal structures of the elusive Rhizobium etlil-asparaginase reveal a peculiar active site

ABSTRACT: 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 Km for l-Asn at 4.2 mM and kcat of 438 s−1. The active site of ReAV, deduced from structural comparisons and confirmed by mutagenesis experiments, contains a highly specific Zn2+ 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. L-asparaginases catalyse the hydrolysis of L-asparagine to L-aspartic acid and ammonia. Here, the authors present high resolution crystal structures of Rhizobium etli L-asparaginase that contains a Zn2+ binding site without a catalytic role and discuss the catalytic mechanism of the enzyme.

SUBMITTER: Loch J

PROVIDER: S-EPMC8602277 | biostudies-literature |

REPOSITORIES: biostudies-literature

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