The 2.0 angstrom structure of malarial purine phosphoribosyltransferase incomplex with a transition-state analogue inhibitor

Malaria is a leading cause of worldwide mortality from infectious disease. Plasmodium falciparum proliferation in human erythrocytes requires purine s alvage by hypoxanthine-guanine-xanthine phosphoribosyltransferase (HGXPRTas e). The enzyme is a target for the development of novel antimalarials. Desi gn and synthesis of transition-state analogue inhibitors permitted cocrysta llization with the malarial enzyme and refinement of the complex to 2.0 A r esolution. Catalytic site contacts in the malarial enzyme are similar to th ose of human hypoxanthine-guanine phosphoribosyltransferase (HGPRTase) desp ite distinct substrate specificity. The crystal structure of malarial HGXPR Tase with bound inhibitor, pyrophosphate, and two Mg2+ ions reveals feature s unique to the transition-state analogue complex. Substrate-assisted catal ysis occurs by ribooxocarbenium stabilization from the O5' lone pair and a pyrophosphate oxygen. A dissociative reaction coordinate path is implicated in which the primary reaction coordinate motion is the ribosyl C1' in moti on between relatively immobile purine base and (Mg)(2)-pyrophosphate. Sever al short hydrogen bonds form in the complex of the enzyme and inhibitor. Th e proton NMR spectrum of the transition-state analogue complex of malarial HGXPRTase contains two downfield signals at 14.3 and 15.3 ppm. Despite the structural similarity to the human enzyme, the NMR spectra of the complexes reveal differences in hydrogen bending between the transition-state analog ue complexes of the human and malarial HG(X)PRTases. The X-ray crystal stru ctures and NMR spectra reveal chemical and structural features that suggest a strategy for the design of malaria-specific transition-state inhibitors.