2DX7:
reference: Structure of aspartate racemase complexed with a dual substrate analogue, citric acid, and implications for the reaction mechanism., Ohtaki A, Nakano Y, Iizuka R, Arakawa T, Yamada K, Odaka M, Yohda M, Proteins. 2008 Mar;70(4):1167-74. PMID: 17847084

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