1ACM:
reference: Arginine 54 in the active site of Escherichia coli aspartate transcarbamoylase is critical for catalysis: a site-specific mutagenesis, NMR, and X-ray crystallographic study., Stebbins JW, Robertson DE, Roberts MF, Stevens RC, Lipscomb WN, Kantrowitz ER, Protein Sci 1992 Nov;1(11):1435-46. PMID: 1303763

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