CYP2C9 is distinguished by a preference for substrates bearing a negative charge at physiological pH. Previous studies have sug- gested that CYP2C9 residues R97 and K72 may play roles in de- termining preference for anionic substrates by interaction at the active site or in the access channel. The aim of the present study was to assess the role of these two residues in determining sub- strate selectivity. R97 and K72 were substituted with negative, uncharged polar and hydrophobic residues using a degenerate polymerase chain reaction-directed strategy. Wild-type and mu- tant enzymes were expressed in bicistronic format with human cytochrome P450 reductase in Escherichia coli. Mutation of R97 led to a loss of holoenzyme expression for R97A, R97V, R97L, R97T, and R97E mutants. Low levels of hemoprotein were detected for R97Q, R97K, R97I, and R97P mutants. Significant apoenzyme was observed, suggesting that heme insertion or protein stability was compromised in R97 mutants. These observations are consis- tent with a structural role for R97 in addition to any role in substrate binding. By contrast, all K72 mutants examined (K72E, K72Q, K72V, and K72L) could be expressed as hemoprotein at levels compara- ble to wild-type. Type I binding spectra were obtained with wild- type and K72 mutants using diclofenac and ibuprofen. Mutation of K72 had little or no effect on the interaction with these substrates, arguing against a critical role in determining substrate specificity. Thus, neither residue appears to play a role in determining sub- strate specificity, but a structural role for R97 can be proposed consistent with recently published crystallographic data for CYP2C9 and CYP2C5.
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