Study of asparagine 353 in aminopeptidase A: characterization of a novel motif (GXMEN) implicated in exopeptidase specificity of monozinc aminopeptidases.

Aminopeptidase A (EC 3.4.11.7, APA) is a 160 kDa membrane-bound zinc enzyme that contains the HEXXH consensus sequence found in members of the zinc metalloprotease family, the zincins. In addition, the monozinc aminopeptidases are characterized by another conserved motif, GXMEN, the glutamate residue of which has been shown to be implicated in the exopeptidase specificity of aminopeptidase A [Vazeux G. (1998) Biochem. J. 334, 407-413]. In carboxypeptidase A (EC 3.4.17.1, CPA), the exopeptidase specificity is conferred by an arginine residue (Arg-145) and an asparagine residue (Asn-144). Thus, we hypothesized that Asn-353 of the GXMEN motif in APA plays a similar role to Asn-144 in CPA and contributes to the exopeptidase specificity of APA. We investigated the functional role of Asn-353 in APA by substituting this residue with a glutamine (Gln-353), an alanine (Ala-353) or an aspartate (Asp-353) residue by site-directed mutagenesis. Expression of wild-type and mutated APAs revealed that Gln-353 and Ala-353 are similarly routed and glycosylated to the wild-type APA, whereas Asp-353 is trapped intracellularly and partially glycosylated. Kinetic studies, using alpha-L-glutamyl-beta-naphthylamide (GluNA) as a substrate showed that the K(m) values of the mutants Gln-353 and Ala-353 were increased 11- and 8-fold, respectively, whereas the k(cat) values were decreased (2-fold) resulting in a 24- and 14-fold reduction in cleavage efficiency. When alpha-L-aspartyl-beta-naphthylamide or angiotensin II were used as substrates, the mutations had a greater effect on k(cat), leading to a similar decrease in cleavage efficiencies as that observed with GluNA. We then measured the inhibitory potencies of several classes of inhibitors, glutamate thiol, glutamine thiol and two isomers (L- or D-) of glutamate phosphonate to explore the functional role of Asn-353. The data indicate that Asn-353 is critical for the integrity and catalytic activity of APA. This residue is involved in substrate binding via interactions with the free N-terminal part and with the P1 carboxylate side chain of the substrate. In conclusion, Asn-353 of the GXMEN motif, together with Glu-352, contributes to the exopeptidase specificity of APA and plays an equivalent role to Asn-144 in CPA.