Structure of Human Cytosolic X-prolyl Aminopeptidase

X-prolyl aminopeptidases catalyze the removal of a penulti-mate prolyl residue from theN termini of peptides.Mammalian X-prolyl aminopeptidases are shown to be responsible for the degradation of bradykinin, a blood pressure regulator peptide, and have been linked tomyocardial infarction. The x-ray crystal structure of human cytosolic X-prolyl aminopeptidase (XPN-PEP1) was solved at a resolution of 1.6 Å. The structure reveals a dimerwith a unique three-domain organization in each subunit, rather than the two domains common to all other known struc-tures of X-prolyl aminopeptidase and prolidases. The C-termi-nal catalytic domain of XPNPEP1 coordinates two metal ions and shares a similar fold with other prolyl aminopeptidases. Metal content analysis and activity assays confirm that the enzyme is double Mn(II) dependent for its activity, which con-trastswith the previous notion that eachXPNPEP1 subunit con