1qk5
TOXOPLASMA GONDII HYPOXANTHINE-GUANINE PHOSPHORIBOSYLTRANSFERASE WITH XMP, PYROPHOSPHATE AND TWO MG2+ IONS
Structural highlights
FunctionHGXR_TOXGO Catalyzes the transfer of a ribosyl phosphate group from 5-phosphoribose 1-diphosphate to the N(9) of hypoxanthine, guanine or xanthine.[1] Evolutionary Conservation Check, as determined by ConSurfDB. You may read the explanation of the method and the full data available from ConSurf. Publication Abstract from PubMedThe crystal structure of the Toxoplasma gondii hypoxanthine-guanine phosphoribosyltransferase (HGPRT)-xanthosine 5'-monophosphate (XMP)-pyrophosphate-Mg(2+) ternary complex has been determined at 1. 60 A resolution. This biproduct, post-transition state structure is of a T. gondii HGPRT mutant (Asp150Ala or D150A). The D150A mutant has reduced activity (k(cat) lower by 11-, 296-, and 8.6-fold for hypoxanthine, guanine, and xanthine, respectively) compared to wild-type T. gondii HGPRT. The Michaelis constants for purine bases are altered only slightly, whereas those for alpha-D-5-phosphoribosyl 1-pyrophosphate (PRPP) are lower by approximately 6.5-fold. The ternary complex crystallizes in space group C222(1) (a = 55.21 A, b = 112.25 A, and c = 144.28 A) with two subunits in the asymmetric unit; the HGPRT tetramer is completed by the application of 2-fold crystallographic symmetry. All active sites contain XMP inverted question markbound in a fashion similar to that of the guanosine 5'-monophosphate (GMP) and inosine 5'-monophosphate (IMP) complexes reported in the preceding article [Heroux, A., et al. (1999) Biochemistry 38, 14485-14494] inverted question mark, pyrophosphate, and two Mg(2+) ions. Each Mg(2+) ion is octahedrally coordinated by two terminal pyrophosphate oxygen atoms and several ordered water molecules. This structure shows how HGPRT uses two Mg(2+) ions to orient and activate the pyrophosphate moiety of PRPP for attack by a purine base, and why mutation in humans of the residue corresponding to Asp206, the only HGPRT amino acid that directly contacts the Mg(2+) ions, causes Lesch-Nyhan syndrome (HGPRT(Kinston), D193N). The Leu78-Lys79 peptide bond in the active site adopts the cis configuration, which it must to bind PRPP or pyrophosphate. The contribution of cis-trans isomerization of this peptide bond to the energetics of substrate binding and product release is discussed. A comprehensive description of the HGPRT reaction mechanism is also proposed. Crystal structure of Toxoplasma gondii hypoxanthine-guanine phosphoribosyltransferase with XMP, pyrophosphate, and two Mg(2+) ions bound: insights into the catalytic mechanism.,Heroux A, White EL, Ross LJ, Davis RL, Borhani DW Biochemistry. 1999 Nov 2;38(44):14495-506. PMID:10545171[2] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. See AlsoReferences
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