Cys-Gly specific dipeptidase Dug1p from S. cerevisiae binds promiscuously to di-, tri-, and tetra-peptides: Peptide-protein interaction, homology modeling, and activity studies reveal a latent promiscuity in substrate recognition

Dug1p is a recently identified novel dipeptidase and plays an important role in glutathione (GSH) degradation. To understand the mechanism of its substrate recognition and specificity towards Cys-Gly dipeptides, we characterized the solution properties of Dug1p and studied the thermodynamics of Dug1p-peptide interactions. In addition, we used homology modeling and ligand docking approaches to get structural insights into Dug1p-peptide interaction. Dug1p exists as dimer and the stoichiometry of peptide-Dug1p complex is 2:1 indicating each monomer in the dimer binds to one peptide. Thermodynamic studies indicate that the free energy change for Dug1p-peptide complex formation is similar (?G_bind ∼ −7.0 kcal/mol) for a variety of peptides of different composition and length (22 peptides). Three-dimensional model of Dug1p is constructed and docking of peptides to the modeled structure suggests that hydrogen bonding to active site residues (E172, E171, and D137) lock the N-terminal of the peptide into the binding site. Dug1p recognizes peptides in a metal independent manner and peptide binding is not sensitive to salts (dlogK/dlogsalt] ∼ 0) over a range of NaCl] (0.02-0.5 M), ZnCl₂], and MnCl₂] (0-0.5 mM). Our results indicate that promiscuity in peptide binding results from the locking of peptide N-terminus into the active site. These observations were supported by our competitive inhibition activity assays. Dug1p activity towards Cys-Gly peptide is significantly reduced (∼ 70%) in the presence of Glu-Cys-Gly. Therefore, Dug1p can recognize a variety of oligopeptides, but has evolved with post-binding screening potential to hydrolyze Cys-Gly peptides selectively.