Structures of l-asparaginase from Bacillus licheniformis Reveal an Essential Residue for its Substrate Stereoselectivity.

l-Asparaginase, which catalyzes the hydrolysis of l-asparagine, is an important enzyme in both the clinical and food industry. Exploration of efficient l-asparaginase with high substrate specificity, especially high chiral selectivity, is essential for extending its use. Herein, various crystal structures of type I l-asparaginase from Bacillus licheniformis (BlAsnase) have been resolved, and we found that there are two additional tyrosines in BlAsnase, contributing to the binding and catalysis of d-asparagine. Strikingly, the substitution of Tyr278 with methionine impaired the interaction with d-asparagine via water molecules due to the small hydrophobic side chain of methionine, which forced the ligand to the deep side of the active site toward the catalytic residues and thus resulted in the loss of hydrolyzing function. Our investigation of the substrate recognition mechanism of BlAsnase is significant for both a better understanding of l-asparaginase and its rational design to achieve high specificity for clinical and industrial applications.