5NWS:
reference: Ketonization of proline residues in the peptide chains of actinomycins by a 4-Oxoproline synthase., Semsary S, Crnovcic I, Driller R, Vater J, Loll B, Keller U, Chembiochem. 2018 Jan 12. doi: 10.1002/cbic.201700666. PMID: 29327817

This OCA Structure page
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