Fatty acid amide hydrolase competitively degrades bioactive amides and esters through a nonconventional catalytic mechanism

The greater reactivity of eaters relative to amides has typically been refl ected in their faster rates of both solvolysis and enzymatic hydrolysis, In contrast to this general principle, the serine hydrolytic enzyme fatty aci d amide hydrolase (FAAH) was found to degrade amides and esters with equiva lent catalytic efficiencies. Mutation of a single lysine residue (K142) to alanine (K142A) abolished this property, generating a catalytically comprom ised enzyme that hydrolyzed esters more than 500-fold faster than amides. C onversion of this same lysine residue to glutamic acid (K132E) produced an enzyme that also displayed severely diminished catalytic activity, but one that now maintained FAAH's ability to react with amides and esters at compe titive rates. The significant catalytic defects exhibited by both the K142A and K142E mutants, in conjunction with their altered pi-I-rate profiles, s upport a role for lysine 142 as a general base involved in the activation o f FAAH's serine nucleophile, Moreover, the dramatically different amide ver sus ester selectivities observed for the K142A and K142E mutants reveal tha t FAAH's catalytic efficiency and catalytic selectivity depend on distingui shable properties of the same residue, with the former relying on a strong catalytic base and the latter requiring coupled general acid-base catalysis . We hypothesize that FAAH's unusual catalytic properties may empower the e nzyme to function effectively as both an amidase and esterase in vivo.