Mutations of R563 to any other amino acid including a conservative replacement of R by K preserving the positive charge abolished the enzyme function but exhibited a significant residual aminopeptidase activity [27] . Crystal structure with R563A mutant could not reveal any structural changes explaining the complete loss of catalytic activity. It was also reported that esterified LTA4 cannot be the substrate of the enzyme and this phenomenon was explained with the steric hindrance [46] , which also proved that a free carboxylate group of LTA4 is critical for the hydrolase function. The other carboxylate recognition residue K565 located in a way that it can also involve in carboxylate recognition but its mutagenic replacements have not decreased the epoxide hydrolase activity [27] . The difference in observed aminopeptidase activities between the wild type and K565 mutants have suggested that K565 is a carboxylate binding site for peptide substrates also [27] . The information of a binding pocket for its ligand is very important for drug design, particularly for conducting mutagenesis studies [47] . In the literature, the binding pocket of a protein receptor to a ligand is usually defined by those residues that have at least one heavy atom (i.e., an atom other than hydrogen) with a distance from a heavy atom of the ligand. Such a criterion was originally used to define the binding pocket of ATP in the Cdk5-Nck5a* complex [48] that has later proved quite useful in identifying functional domains and stimulating the relevant truncation experiments [49] . The similar approach has also been used to define the binding pockets of many other receptor-ligand interactions important for drug design [50] [51] [52] [53] [54] .