Pentose phosphates in nucleoside interconversion and catabolism

Ribose phosphates are either synthesized through the oxidative branch of
the pentose phosphate pathway, or are supplied by nucleoside phosphorylases.
The two main pentose phosphates, ribose-5-phosphate and ribose-1-phosphate,
are readily interconverted by the action of phosphopentomutase.
Ribose-5-phosphate is the direct precursor of 5-phosphoribosyl-1-pyrophosphate,
for both de novo and ‘salvage’ synthesis of nucleotides. Phosphorolysis
of deoxyribonucleosides is the main source of deoxyribose phosphates,
which are interconvertible, through the action of phosphopentomutase.
The pentose moiety of all nucleosides can serve as a carbon and energy
source. During the past decade, extensive advances have been made in elucidating
the pathways by which the pentose phosphates, arising from nucleoside
phosphorolysis, are either recycled, without opening of their
furanosidic ring, or catabolized as a carbon and energy source. We review
herein the experimental knowledge on the molecular mechanisms by which
(a) ribose-1-phosphate, produced by purine nucleoside phosphorylase acting
catabolically, is either anabolized for pyrimidine salvage and 5-fluorouracil
activation, with uridine phosphorylase acting anabolically, or
recycled for nucleoside and base interconversion; (b) the nucleosides can be
regarded, both in bacteria and in eukaryotic cells, as carriers of sugars, that
are made available though the action of nucleoside phosphorylases. In bacteria,
catabolism of nucleosides, when suitable carbon and energy sources
are not available, is accomplished by a battery of nucleoside transporters
and of inducible catabolic enzymes for purine and pyrimidine nucleosides
and for pentose phosphates. In eukaryotic cells, the modulation of pentose
phosphate production by nucleoside catabolism seems to be affected by
developmental and physiological factors on enzyme levels.