3VGN:
reference: Quantitative dissection of hydrogen bond-mediated proton transfer in the ketosteroid isomerase active site., Sigala PA, Fafarman AT, Schwans JP, Fried SD, Fenn TD, Caaveiro JM, Pybus B, Ringe D, Petsko GA, Boxer SG, Herschlag D, Proc Natl Acad Sci U S A. 2013 Jun 24. PMID: 23798390

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