Substrate deformation in a hypoxanthine-guanine phosphoribosyltransferase ternary complex: the structural basis for catalysis.
BACKGROUND
Hypoxanthine-guanine phosphoribosyltransferases (HGPRTs) are well-recognized antiparasitic drug targets. HGPRT is also a paradigmatic representative of the phosphoribosyltransferase family of enzymes, which includes other important biosynthetic and salvage enzymes and drug targets. To better understand the reaction mechanism of this enzyme, we have crystallized HGPRT from the apicomplexan protozoan Toxoplasma gondii as a ternary complex with a substrate and a substrate analog.


RESULTS
The crystal structure of T. gondii HGPRT with the substrate Mg2+-PRPP and a nonreactive substrate analog, 9-deazaguanine, bound in the active site has been determined at 1.05 A resolution and refined to a free R factor of 15.4%. This structure constitutes the first atomic-resolution structure of both a phosphoribosyltransferase and the central metabolic substrate PRPP. This pre-transition state complex provides a clearer understanding of the structural basis for catalysis by HGPRT.


CONCLUSIONS
Three types of substrate deformation, chief among them an unexpected C2'-endo pucker adopted by the PRPP ribose ring, raise the energy of the ground state. A cation-pi interaction between Tyr-118 and the developing oxocarbenium ion in the ribose ring helps to stabilize the transition state. Enforced substrate propinquity coupled with optimal reactive geometry for both the substrates and the active site residues with which they interact contributes to catalysis as well.
Héroux
A
A
Drug Discovery Division, Southern Research Institute, Birmingham, Alabama 35205, USA.
White
E L
EL
Ross
L J
LJ
Kuzin
A P
AP
Borhani
D W
DW
D000818:Animals; D001665:Binding Sites; D002384:Catalysis; D002413:Cations, Divalent; D018360:Crystallography, X-Ray; D006801:Humans; D006860:Hydrogen Bonding; D007041:Hypoxanthine Phosphoribosyltransferase / Q000737:chemistry* / Q000378:metabolism; D008024:Ligands; D008274:Magnesium / Q000737:chemistry / Q000378:metabolism; D019906:Nuclear Magnetic Resonance, Biomolecular; D010754:Phosphoribosyl Pyrophosphate / Q000737:chemistry / Q000378:metabolism; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D012266:Ribose / Q000737:chemistry / Q000378:metabolism; D055672:Static Electricity; D013329:Structure-Activity Relationship; D013379:Substrate Specificity; D014122:Toxoplasma / Q000201:enzymology
D002413:Cations, Divalent; D008024:Ligands; D012266:Ribose; D010754:Phosphoribosyl Pyrophosphate; D007041:Hypoxanthine Phosphoribosyltransferase; D008274:Magnesium