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Myoglobin (Mb) has been one of the most intensively investigated hemoproteins as evident from the accumulated biochemical, biophysical, and spectroscopic data. 1 The heterologous overexpression system for recombinant sperm whale Mb in Escherichia coli has been developed, 2 and high-resolution X-ray crystal structures of the wild type as well as some Mb mutants are available. 3 Thus, superposition of the active site structures of Mb and other hemoproteins enables us to utilize Mb as heme enzyme models for the elucidation of structurefunction relationships. 4 We have engineered sperm whale Mb based on the comparison of crystal structures of oxymyoglobin and an oxy form of cytochrome c peroxidase (CcP)(Figure 1). 5 Although the Leu-29 f His and His-64 f Leu double replacement of Mb seems to create a peroxidase-like active site, the imidazole is located too far from the heme center to interact with …