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Prolyl aminodipeptidase (PepX) is an enzyme that hydrolyzes peptide bonds from the N-terminus of substrates when the penultimate amino-acid residue is a proline. Prolyl peptidases are of particular interest owing to their ability to hydrolyze food allergens that contain a high percentage of proline residues. PepX from Lactobacillus helveticus was cloned and expressed in Escherichia coli as an N-terminally His-tagged recombinant construct and was crystallized by hanging-drop vapor diffusion in a phosphate buffer using PEG 3350 as a precipitant. The structure was determined at 2.0 Å resolution by molecular replacement using the structure of PepX from Lactococcus lactis (PDB entry 1lns) as the starting model. Notable differences between the L. helveticus PepX structure and PDB entry 1lns include a cysteine instead of a phenylalanine at the substrate-binding site in the position which confers exopeptidase activity and the presence of a calcium ion coordinated by a calcium-binding motif with the consensus sequence DX(DN)XDG.

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