1QK4:
reference: Crystal structures of the Toxoplasma gondii hypoxanthine-guanine phosphoribosyltransferase-GMP and -IMP complexes: comparison of purine binding interactions with the XMP complex., Heroux A, White EL, Ross LJ, Borhani DW, Biochemistry 1999 Nov 2;38(44):14485-94. PMID: 10545170

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