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The immunosuppressive agent mizoribine monophosphate forms a transition state analogue complex with inosine monophosphate dehydrogenase.

Gan L., Seyedsayamdost M.R., Shuto S., Matsuda A., Petsko G.A., Hedstrom L.

Mizoribine monophosphate (MZP) is the active metabolite of the immunosuppressive agent mizoribine and a potent inhibitor of IMP dehydrogenase (IMPDH). This enzyme catalyzes the oxidation of IMP to XMP with the concomitant reduction of NAD via a covalent intermediate at Cys319 (E-XMP). Surprisingly, mutational analysis indicates that MZP is a transition state analogue although its structure does not resemble that of the expected transition state. Here we report the X-ray crystal structure of the E.MZP complex at 2.0 A resolution that reveals a transition state-like structure and solves the mechanistic puzzle of the IMPDH reaction. The protein assumes a new conformation where a flap folds into the NAD site and MZP, Cys319, and a water molecule are arranged in a geometry resembling the transition state. The water appears to be activated by interactions with a conserved Arg418-Tyr419 dyad. Mutagenesis experiments confirm that this new closed conformation is required for the hydrolysis of E-XMP, but not for the reduction of NAD. The closed conformation provides a structural explanation for the differences in drug selectivity and catalytic efficiency of IMPDH isozymes.

Biochemistry 42:857-863(2003) [PubMed] [Europe PMC]

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