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.

中文翻译：

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瑞士乳杆菌脯氨酰氨基二肽酶 (PepX) 的结构表征。

脯氨酰氨基二肽酶 (PepX) 是一种当倒数第二个氨基酸残基是脯氨酸时从底物 N 末端水解肽键的酶。脯氨酰肽酶因其能够水解含有高比例脯氨酸残留物的食物过敏原而受到特别关注。来自瑞士乳杆菌的 PepX在大肠杆菌中克隆并表达为 N 末端 His 标记的重组构建体，并使用 PEG 3350 作为沉淀剂在磷酸盐缓冲液中通过悬滴蒸汽扩散进行结晶。使用乳酸乳球菌（PDB 条目 1lns）的 PepX 结构作为起始模型，通过分子置换以 2.0 Å 分辨率确定结构。瑞士乳杆菌PepX 结构和 PDB 条目 1lns之间的显着差异包括底物结合位点上的半胱氨酸而不是苯丙氨酸，该位置赋予外肽酶活性，并且存在由钙结合基序协调的钙离子（与共识一致）序列 D X (DN) X DG。