1TWX:
reference: Crystal structure of the thrombin mutant D221A/D222K: the Asp222:Arg187 ion-pair stabilizes the fast form., Pineda AO, Zhang E, Guinto ER, Savvides SN, Tulinsky A, Di Cera E, Biophys Chem 2004 Dec 20;112(2-3):253-6. PMID: 15572256

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