The formation of a 1-5 phosphodiester linkage in the spontaneous breakdown of 5-phosphoribosyl-alpha-1-pyrophosphate

The decomposition of 5-phosphoribosyl-alpha-1-pyrophosphate (PRPP) in the presence of Mg2+ at pH=7.8 yields a combination of products including ribose 5-phosphate, ribose 1-phosphate, 5-phosphoribosyl 1,2 cyclic phosphate, inorganic phosphate, and pyrophosphate. Hydrogen decoupled 31P NMR analysis of the product mixture also exhibits a sharp peak (+2.6 ppm from phosphocreatine) in a chemical shift region which includes phosphodiester bonds. Alkaline phosphatase treatment of the product mixture results in cleavage of monophosphate esters such as ribose 1-phosphate and ribose 5-phosphate, but does not affect the unidentified peak. Homonuclear (1H) correlation spectroscopy (COSY) of a partially purified sample was successful in identifying the hydrogen spectra of this compound. Combined with results from the splitting patterns of selectively decoupled 31P spectra, the COSY data indicate that several hydrogens are directly coupled to the unknown phosphate group with J value matches to the hydrogen on carbon one and to the two hydrogens on carbon five. Heteronuclear (1H-31P) chemical shift correlation studies confirm these couplings and further substantiate the formation of a ribose 1-5 phosphate linkage during the degradation of PRPP under these conditions. It is presently unknown whether this is an intramolecular or intermolecular phosphodiester linkage, although some spectroscopic evidence suggest the intramolecular bond formation, i.e. a ribose 1,5-cyclic phosphate (R-1,5cP). The formation of R-1,5cP helps explain the observation that the 5-phosphate group from PRPP becomes labile during the spontaneous degradation of PRPP.