Enzyme - Amidophosphoribosyltransferase

Alternative Name(s)
  • Phosphoribosyldiphosphate 5-amidotransferase.
  • Glutamine phosphoribosylpyrophosphate amidotransferase.

Catalytic Activity

5-phospho-beta-D-ribosylamine + diphosphate + L-glutamate = 5-phospho-alpha-D-ribose 1-diphosphate + H2O + L-glutamine


There are no Cofactors for this Enzyme

Reaction Mechanism

    Glutamine phosphoribosylpyrophosphate amido transferase (GPATase) catalyses the transfer of the glutamine amide nitrogen to phosphoribosylpyrophosphate to generate phosphoribosylamine, pyrophosphate and glutamate. The enzyme is a member of a family of proteins which utilise the amide nitrogen of glutamate for the biosynthesis of amino acids, nucleotides, amino sugars and coenzymes. The coupling of phosphoribosyl transferase and glutaminase actities in an enzyme is not frequently seen but required for de novo purine synthesis.

    The enzyme is described as complex; its reaction is catyalysed as two half reactions, each occurring at separate catalytic sites along a single polypeptide chain. The two sites are highly coupled, facilitating the transfer of NH3 between them. The N terminal domain is a member of the Ntn hydrolase family and catalyses the hydrolysis of glutamine to glutamate and ammonia, while the C terminal domain, the ammonia acceptor domain, a member of the phophoribosyltransferase (PRTase) family is responsible for the coupling of ammonia to phosphoribosyl pyrophosphate (PRPP). The PRTase active site is described as non classical, since no catalytic residues have been identified. However, it has been suggested by analogy with a homologue mechanism that the main function of the enzyme is not to promote catalysis, but rather to bring the reactants together appropriately and preventing unwanted side reactions.
    Catalytic Residues
    AA Uniprot Uniprot Resid PDB PDB Resid
    Cys P0AG16 2 1ecf 1
    Gly P0AG16 103 1ecf 102
    Asn P0AG16 102 1ecf 101
    Cys P0AG16 2 1ecf 1
    Gly P0AG16 28 1ecf 27
    Tyr P0AG16 259 1ecf 258
    Step Components

    intermediate formation, intermediate terminated, native state of enzyme regenerated, overall reactant used, dephosphorylation, bimolecular nucleophilic addition, unimolecular elimination by the conjugate base, proton relay, bimolecular nucleophilic substitution, enzyme-substrate complex formation, enzyme-substrate complex cleavage, deamination, intermediate collapse, proton transfer, overall product formed

    Step 1.

    Reaction occurs in the glutaminase domain (CATH code The glutamine will only bind if the 5-phospho-alpha-D-ribose-1-diphosphate is bound first [PMID:9914248]. The N-terminus of Cys1 deprotonates the thiol group of Cys1, which attacks the amino carbon in a nucleophilic addition.

    Step 2.

    The oxyanion initiates an elimination that cleaves ammonia from the bound L-Glutamine substrate. Ammonia deprotonates the N-terminus of Cys1. The ammonia formed here goes onto the next domain via an intramolecular channel.

    Step 3.

    The N-terminus of Cys1 deprotonates water, which attacks the carbonyl carbon of the covalently bound intermediate.

    Step 4.

    The oxyanion initiates an elimination that cleaves the C-S bond, the thiolate of Cys1 deprotonates the N-terminus of Cys1

    Step 5.

    Reaction occurs in the PRTase Domain. The conformation of the enzyme-bound Mn-cPRPP for catalysis as it favours the formation of a tri- or pentavalent intermediate with some double bond character in the ribose C1-O4 bond. . The step is described as an in-line displacement in which ammonia attacks the C1 of the 5-Phospho-alpha-D-ribose-1-diphosphate substrate in a nucleophilic substitution. The product pyrophosphate deprotonates the newly attached ammonia. However, it is unclear whether the intermediate is actually a transition state. There are no residues that are essential for catalytic activity, however the Tyr258 is probably stabilising [PMID:9914248].


    The products of the reaction.

Reaction Parameters

  • Kinetic Parameters
    Organism KM Value [mM] Substrate Comment
    Homo sapiens 0.65 Mg2+
    Escherichia coli 7.34 NH3
    Glycine max 16 NH3
    Columba livia 0.1 L-glutamine
    Rattus norvegicus 0.53 L-glutamine
  • Temperature

    There are no reaction parameters information for this Enzyme.

  • pH
    Organism pH Range Comment
    Glycine max 6 - 10 approx. 70% of maximal activity between pH 6 and pH 10
    Schizosaccharomyces pombe 7 - 9 stable enzyme, approx. 65% and unstable enzyme approx. 70% of maximal activity at pH 7, stable enzyme, approx. 75% and unstable enzyme approx. 65% of maximal activity at pH 9

Associated Proteins

Protein name Organism
Amidophosphoribosyltransferase, chloroplastic Soybean
Amidophosphoribosyltransferase 2, chloroplastic Mouse-ear cress
Amidophosphoribosyltransferase Escherichia coli (strain K12)
Amidophosphoribosyltransferase 3, chloroplastic Mouse-ear cress
Amidophosphoribosyltransferase 1, chloroplastic Mouse-ear cress