L-Asparaginase, which is found in various plant, animal and bacterial cells, catalyses the deamination of asparagine to yield aspartic acid and an ammonium ion, resulting in a depletion of free circulatory asparagine in plasma. The structure of the asparaginase from Campylobacter jejuni forms a tight homotetramer, dimer of intimate dimers with 222 symmetry similar to that of L-asparaginase from Erwinia chrysanthemi. Each of the four active sites of l-asparaginase is located between the N- and C-terminal domains of two adjacent monomers. The flexible part of the active site consists of several residues (positions 16−34), referred to as the active site flexible loop, and covers the binding pocket upon substrate binding to the enzyme. The nucleophile, Thr16, is also located in the flexible loop region.