MetaCyc Reaction:

Superclasses: Reactions Classified By Conversion Type Simple Reactions Chemical Reactions
Reactions Classified By Substrate Small-Molecule Reactions

EC Number:

Enzymes and Genes:
xanthosine phosphorylase Inferred from experiment : xapA ( Escherichia coli K-12 substr. MG1655 )
guanosine phosphorylase Inferred from experiment ( Oryctolagus cuniculus )
purine nucleoside phosphorylase Inferred from experiment : PNP ( Homo sapiens )
purine nucleoside phosphorylase : PNP ( Plasmodium falciparum )
purine nucleoside phosphorylase Inferred from experiment : deoD ( Escherichia coli K-12 substr. MG1655 )

In Pathway: guanosine nucleotides degradation III , guanine and guanosine salvage , purine ribonucleosides degradation

The reaction direction shown, that is, A + B ↔ C + D versus C + D ↔ A + B, is in accordance with the Enzyme Commission system.

Mass balance status: Balanced.

Direct generic reaction:
a purine ribonucleoside + phosphate ↔ a purine base + α-D-ribose-1-phosphate (

Enzyme Commission Primary Name: guanosine phosphorylase

Standard Gibbs Free Energy (ΔrG in kcal/mol): 0.730011 Inferred by computational analysis [Latendresse13]

This is a specific case of the non-specific reaction catalyzed by purine-nucleoside phosphorylase a purine ribonucleoside + phosphate ↔ a purine base + α-D-ribose-1-phosphate.

Even though a different EC reaction (EC describes this specific reaction, the E. coli enzyme has been associated with this reaction, since EC was defined for a much more specific rabbit bone marrow enzyme [Yamada61].

Enzyme Commission Summary:
This enzyme has been defined based on a single publication of an enzyme from rabbit bone marrow that appears to be highly specific for guanosine and deoxyguanosine [Yamada61]. Most enzymes that are known to catalyze this reaction, such as the human purine nucleoside phosphorylase, can accept other purines as well, and thus were attributed to a specific case of the more general enzyme EC, (guanosine + phosphate ↔ guanine + α-D-ribose-1-phosphate).

Gene-Reaction Schematic: ?

Gene-Reaction Schematic

Instance reactions of [a purine ribonucleoside + phosphate ↔ a purine base + α-D-ribose-1-phosphate] (
i1: inosine + phosphate ↔ hypoxanthine + α-D-ribose-1-phosphate (

i2: guanosine + phosphate ↔ guanine + α-D-ribose-1-phosphate (
i3: adenosine + phosphate ↔ adenine + α-D-ribose-1-phosphate (

Unification Links: KEGG:R02147 , Rhea:13233

Relationship Links: BRENDA:EC: , ENZYME:EC: , IUBMB-ExplorEnz:EC:


Latendresse13: Latendresse M. (2013). "Computing Gibbs Free Energy of Compounds and Reactions in MetaCyc."

Yamada61: Yamada, E.W. (1961). "The phosphorolysis of nucleosides by rabbit bone marrow." J Biol Chem 236;3043-6. PMID: 14008731

Report Errors or Provide Feedback
Please cite the following article in publications resulting from the use of MetaCyc: Caspi et al, Nucleic Acids Research 42:D459-D471 2014
Page generated by SRI International Pathway Tools version 19.0 on Sun Mar 29, 2015, biocyc14.