Crystal Structures of Artificial Metalloproteins: Tight Binding of FeIII(Schiff-Base) by Mutation of Ala71 to Gly in Apo-Myoglobin

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Abstract
Apo-myoglobin (apo-Mb) and apo-A71GMb were successfully reconstituted with FeIII(salophen) (1) (salophen = N,N‘-bis(salicylidene)-1,2-phenilenediamine), FeIII(3,3‘-Me2-salophen) (2), and FeIII(5,5‘-t-Bu2-salophen) (3). The crystal structure of 2·apo-A71GMb shows the tight binding of the complex in the Mb cavity, while in wild-type apo-Mb it is highly disordered due to the steric repulsion with Ala71. Furthermore, the structure of 2·apo-A71GMb suggests a possible accommodation of a small substrate in the cavity. In fact, the cyanide association rate constant of 2·apo-A71GMb is 216-fold larger compared to that of 2·apo-Mb. These results provide us principles for the noncovalent fixation of synthetic metal cofactors at the desired positions in protein matrixes.