Hypoxanthine phosphoribosyltransferase


Converts guanine to guanosine monophosphate, and hypoxanthine to inosine monophosphate. Transfers the 5-phosphoribosyl group from 5-phosphoribosylpyrophosphate onto the purine. Plays a central role in the generation of purine nucleotides through the purine salvage pathway and is involved in the first step of the subpathway that synthesizes IMP from hypoxanthine.


Reference Protein and Structure

P00492 UniProt ( IPR005904 (Sequence Homologues) (PDB Homologues)
Biological species
Homo sapiens (Human) Uniprot
Catalytic CATH Domains CATHdb (see all for 1bzy)
Magnesium(2+) (2) Metal MACiE
Click To Show Structure

Enzyme Reaction (EC:

5-O-phosphonato-alpha-D-ribofuranosyl diphosphate(5-)
guanosine 5'-monophosphate(2-)
Alternative enzyme names: 6-hydroxypurine phosphoribosyltransferase, 6-mercaptopurine phosphoribosyltransferase, GMP pyrophosphorylase, GPRT, HGPRTase, HPRT, IMP pyrophosphorylase, IMP-GMP pyrophosphorylase, Guanine phosphoribosyltransferase, Guanine-hypoxanthine phosphoribosyltransferase, Guanosine 5'-phosphate pyrophosphorylase, Guanosine phosphoribosyltransferase, Guanylate pyrophosphorylase, Guanylic pyrophosphorylase, Hypoxanthine-guanine phosphoribosyltransferase, Inosinate pyrophosphorylase, Inosine 5'-phosphate pyrophosphorylase, Inosinic acid pyrophosphorylase, Inosinic pyrophosphorylase, Purine-6-thiol phosphoribosyltransferase, Transphosphoribosidase, IMP diphosphorylase,

Enzyme Mechanism


In a nucleophilic bimolecular substitution, Asp137 activates the guanine for attach on the C2 of the substrate ribose, the reaction proceeds via a positively charged transition state.

Catalytic Residues Roles

UniProt PDB* (1bzy)
Asp138 Asp137A Acts as a general acid/base. It deprotonates the guanine that initiated the nucleophilic attack on the ribose ring. It has been suggested [PMID:11258886] that a general base may not be required, but a strong hydrogen bond with the N7 of the purine substrate provides sufficient transition-state stabilisation to permit relatively efficient catalysis. hydrogen bond acceptor, hydrogen bond donor, proton acceptor, proton donor
Arg200 Arg199A Binds the phosphate group, may be involved in electrostatic stabilisation of this group. electrostatic stabiliser
Phe187 Phe186A Stabilises the transition state by pi-cation interactions. electrostatic stabiliser
Glu134, Asp135 Glu133A, Asp134A Help stabilise the positively charged ribooxocarbenium ion at the transition state. attractive charge-charge interaction, electrostatic stabiliser
*PDB label guide - RESx(y)B(C) - RES: Residue Name; x: Residue ID in PDB file; y: Residue ID in PDB sequence if different from PDB file; B: PDB Chain; C: Biological Assembly Chain if different from PDB. If label is "Not Found" it means this residue is not found in the reference PDB.

Chemical Components

bimolecular nucleophilic substitution, overall reactant used, overall product formed, dephosphorylation, proton transfer, rate-determining step, native state of enzyme regenerated, inferred reaction step


  1. Héroux A et al. (1999), Biochemistry, 38, 14495-14506. Crystal Structure ofToxoplasma gondiiHypoxanthine-Guanine Phosphoribosyltransferase with XMP, Pyrophosphate, and Two Mg2+Ions Bound:  Insights into the Catalytic Mechanism†,‡. DOI:10.1021/bi990508i. PMID:10545171.
  2. Karnawat V et al. (2015), Chemphyschem, 16, 2172-2181. Differential Distortion of Purine Substrates by Human andPlasmodium falciparumHypoxanthine-Guanine Phosphoribosyltransferase to Catalyse the Formation of Mononucleotides. DOI:10.1002/cphc.201500084. PMID:25944719.
  3. Gasik Z et al. (2013), Curr Pharm Des, 19, 4226-4240. Resolving differences in substrate specificities between human and parasite phosphoribosyltransferases via analysis of functional groups of substrates and receptors. PMID:23170881.
  4. Canyuk B et al. (2001), Biochemistry, 40, 2754-2765. The Role for an Invariant Aspartic Acid in Hypoxanthine Phosphoribosyltransferases Is Examined Using Saturation Mutagenesis, Functional Analysis, and X-ray Crystallography†. DOI:10.1021/bi001195q. PMID:11258886.
  5. Héroux A et al. (2000), Structure, 8, 1309-1318. Substrate deformation in a hypoxanthine-guanine phosphoribosyltransferase ternary complex: the structural basis for catalysis. PMID:11188695.
  6. Shi W et al. (1999), Nat Struct Biol, 6, 588-593. The 2.0 A structure of human hypoxanthine-guanine phosphoribosyltransferase in complex with a transition-state analog inhibitor. DOI:10.1038/9376. PMID:10360366.
  7. Xu Y et al. (1998), Biochemistry, 37, 4114-4124. Catalysis in Human Hypoxanthine-Guanine Phosphoribosyltransferase:  Asp 137 Acts as a General Acid/Base†. DOI:10.1021/bi972519m. PMID:9521733.
  8. Jardim A et al. (1997), J Biol Chem, 272, 8967-8973. The Conserved Serine-Tyrosine Dipeptide in Leishmania donovani Hypoxanthine-guanine Phosphoribosyltransferase Is Essential for Catalytic Activity. DOI:10.1074/jbc.272.14.8967. PMID:9083019.

Catalytic Residues Roles

Residue Roles
Asp137A hydrogen bond acceptor
Glu133A attractive charge-charge interaction, electrostatic stabiliser
Asp134A attractive charge-charge interaction, electrostatic stabiliser
Phe186A electrostatic stabiliser
Arg199A electrostatic stabiliser
Asp137A proton acceptor

Chemical Components

ingold: bimolecular nucleophilic substitution, overall reactant used, overall product formed, dephosphorylation, proton transfer, rate-determining step

Catalytic Residues Roles

Residue Roles
Asp137A hydrogen bond donor, hydrogen bond acceptor, proton donor

Chemical Components

proton transfer, native state of enzyme regenerated, inferred reaction step


Gemma L. Holliday, Daniel E. Almonacid, Gail J. Bartlett, Sophie T. Williams, Nozomi Nagano, Craig Porter