Tinkering with enzymes

Abstract

Hypoxanthine guanine phosphoribosyltransferase (BGPRT) catalyses the phosphoribosylation of hypoxanthine and guanine by transferring the phosphoribosyl moiety from phosphoribosylpyrophosphate (PRPP) onto N9 in the purine base resulting with the formation of inosine monophosphate (IMP) and guanosine monophosphate (GMP). Human HGPRT can phosphoribosyllte only hypoxanthine and guanine while the enzyme of the protozoan parasite, PlasmodiumJalcipamm, has an additional substrate specificity for xanthine. Our aim has been to elucidate structural features to HGPRT that goven substrate specificity. We have addressed this problem by engineering chimeric HGPRTs which contain domains from both the pardsite and human enzymes. The chimeric enzyme DSI in which the first 49 residues of human HGPRT were replaced with the corresponding residues from the P. Jalciparum enzyme exhibited additional specificity for xanthine. A rational for this additional specificity for xanthine is proposed using the available crystal structures of human HGPRT and Salmonella orotate phosphoribosyltransferase (OPRT).