2AAD:
reference: Role of histidine-40 in ribonuclease T1 catalysis: three-dimensionalstructures of the partially active His40Lys mutant., Zegers I, Verhelst P, Choe HW, Steyaert J, Heinemann U, Saenger W, Wyns L, Biochemistry 1992 Nov 24;31(46):11317-25. PMID: 1445870

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