4GKA:
reference: Catalytic Site Conformations in Human PNP by (19)F-NMR and Crystallography., Suarez J, Haapalainen AM, Cahill SM, Ho MC, Yan F, Almo SC, Schramm VL, Chem Biol. 2013 Feb 21;20(2):212-22. doi: 10.1016/j.chembiol.2013.01.009. PMID: 23438750

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