6GHW:
reference: Substituting the catalytic proline of 4-oxalocrotonate tautomerase with non-canonical analogues reveals a finely tuned catalytic system., Lukesch MS, Pavkov-Keller T, Gruber K, Zangger K, Wiltschi B, Sci Rep. 2019 Feb 25;9(1):2697. doi: 10.1038/s41598-019-39484-9. PMID: 30804446

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