1QE5:
reference: Crystal structure of the purine nucleoside phosphorylase (PNP) from Cellulomonas sp. and its implication for the mechanism of trimeric PNPs., Tebbe J, Bzowska A, Wielgus-Kutrowska B, Schroder W, Kazimierczuk Z, Shugar D, Saenger W, Koellner G, J Mol Biol 1999 Dec 17;294(5):1239-55. PMID: 10600382

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