5KXU:
reference: Atomic resolution structure of serine protease proteinase K at ambient temperature., Masuda T, Suzuki M, Inoue S, Song C, Nakane T, Nango E, Tanaka R, Tono K, Joti Y, Kameshima T, Hatsui T, Yabashi M, Mikami B, Nureki O, Numata K, Iwata S, Sugahara M, Sci Rep. 2017 Mar 31;7:45604. doi: 10.1038/srep45604. PMID: 28361898

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