4AX4:
reference: The loops facing the active site of prolyl oligopeptidase are crucial components in substrate gating and specificity., Szeltner Z, Juhasz T, Szamosi I, Rea D, Fulop V, Modos K, Juliano L, Polgar L, Biochim Biophys Acta. 2012 Aug 19. PMID: 22940581

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