Crystal structures of the Toxoplasma gondii hypoxanthine-guanine phosphoribosyltransferase-GMP and -IMP complexes: comparison of purine binding interactions with the XMP complex.
The crystal structures of the guanosine 5'-monophosphate (GMP) and inosine 5'-monophosphate (IMP) complexes of Toxoplasma gondii hypoxanthine-guanine phosphoribosyltransferase (HGPRT) have been determined at 1.65 and 1.90 A resolution. These complexes, which crystallize in space groups P2(1) (a = 65.45 A, b = 90.84 A, c = 80. 26 A, and beta = 92.53 degrees ) and P2(1)2(1)2(1) (a = 84.54 A, b = 102.44 A, and c = 108.83 A), each comprise a tetramer in the crystallographic asymmetric unit. All active sites in the tetramers are fully occupied by the nucleotide. Comparison of these structures with that of the xanthosine 5'-monophosphate (XMP)-pyrophosphate-Mg(2+) ternary complex reported in the following article [Héroux, A., et al. (1999) Biochemistry 38, 14495-14506] shows how T. gondii HGPRT is able to recognize guanine, hypoxanthine, and xanthine as substrates, and suggests why the human enzyme cannot use xanthine efficiently. Comparison with the apoenzyme reveals the structural changes that occur upon binding of purines and ribose 5'-phosphate to HGPRT. Two structural features important to the HGPRT mechanism, a previously unrecognized active site loop (loop III', residues 180-184) and an active site peptide bond (Leu78-Lys79) that adopts both the cis and the trans configurations, are presented.
Héroux
A
A
Drug Discovery Division, Southern Research Institute, Birmingham, Alabama 35205, USA.
White
E L
EL
Ross
L J
LJ
Borhani
D W
DW
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D018360:Crystallography, X-Ray; D006157:Guanosine Monophosphate / Q000737:chemistry; D006801:Humans; D007041:Hypoxanthine Phosphoribosyltransferase / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D007291:Inosine Monophosphate / Q000737:chemistry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011487:Protein Conformation; D020836:Protein Structure, Quaternary; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D012265:Ribonucleotides / Q000737:chemistry; D017386:Sequence Homology, Amino Acid; D013045:Species Specificity; D013379:Substrate Specificity; D014122:Toxoplasma / Q000201:enzymology* / Q000235:genetics; D019820:Xanthine
D011994:Recombinant Proteins; D012265:Ribonucleotides; D007291:Inosine Monophosphate; D019820:Xanthine; C011141:xanthosine monophosphate; D006157:Guanosine Monophosphate; D007041:Hypoxanthine Phosphoribosyltransferase