4L7K:
reference: Use of anion-aromatic interactions to position the general base in the ketosteroid isomerase active site., Schwans JP, Sunden F, Lassila JK, Gonzalez A, Tsai Y, Herschlag D, Proc Natl Acad Sci U S A. 2013 Jul 9;110(28):11308-13. doi:, 10.1073/pnas.1206710110. Epub 2013 Jun 24. PMID: 23798413

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