Leukotriene (LT) A 4 hydrolase/aminopeptidase (LTA4H) is a bifunctional zinc enzyme that catalyzes the biosynthesis of LTB 4, a potent lipid chemoattractant involved in inflammation, immune responses, host defense against infection, and PAF-induced shock. The high resolution crystal structure of LTA4H in complex with the competitive inhibitor bestatin reveals a protein folded into three domains that together create a deep cleft harboring the catalytic Zn 2+ site. A bent and narrow pocket, shaped to accommodate the substrate LTA 4, constitutes a highly confined binding region that can be targeted in the design of specific anti-inflammatory agents. Moreover, the structure of the catalytic domain is very similar to that of thermolysin and provides detailed insight into mechanisms of catalysis, in particular the chemical strategy for the unique epoxide hydrolase reaction that generates LTB 4.