1QK5

TOXOPLASMA GONDII HYPOXANTHINE-GUANINE PHOSPHORIBOSYLTRANSFERASE WITH XMP, PYROPHOSPHATE AND TWO MG2+ IONS

Summary for 1QK5

• Entry DOI: 10.2210/pdb1qk5/pdb
• Related: 1DBR 1HMP 1QK3 1QK4
• Descriptor: HYPOXANTHINE-GUANINE PHOSPHORIBOSYLTRANSFERASE, XANTHOSINE-5'-MONOPHOSPHATE, PYROPHOSPHATE 2-, ... (5 entities in total)
• Functional Keywords: transferase, glycosyltransferase, purine salvage
• Biological source: TOXOPLASMA GONDII
• Total number of polymer chains: 2
• Total formula weight: 54502.55

Authors

Heroux, A.,White, E.L.,Ross, L.J.,Davis, R.L.,Borhani, D.W. (deposition date: 1999-07-09, release date: 1999-10-17, Last modification date: 2023-12-13)

Primary citation

Heroux, A.,White, E.L.,Ross, L.J.,Davis, R.L.,Borhani, D.W.
Crystal Structure of Toxoplasma Gondii Hypoxanthine-Guanine Phosphoribosyltransferase with Xmp, Pyrophosphate and Two Mg2+ Ions Bound: Insights Into the Catalytic Mechanism
Biochemistry, 38:14495-, 1999

PubMed Abstract: The 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 ¿bound in a fashion similar to that of the guanosine 5'-monophosphate (GMP) and inosine 5'-monophosphate (IMP) complexes reported in the preceding article [Héroux, A., et al. (1999) Biochemistry 38, 14485-14494]¿, 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.

PubMed: 10545171
DOI: 10.1021/BI990508I

Experimental method

X-RAY DIFFRACTION (1.6 Å)