Purine Salvage Pathway

1. 1

   Hypoxanthine → IMP

   Hypoxanthine-guanine phosphoribosyltransferase (HGPRT) 2.4.2.8 ST-0206 Irreversible PPi PRPP
   In Hypoxanthine

   Out IMP
   › Notes

   HGPRT catalyzes hypoxanthine + PRPP → IMP + PPi. Mg2+ supports phosphoribosyl transfer, and PPi hydrolysis drives the reaction forward. This is effectively irreversible in vivo and salvages hypoxanthine directly to the central purine intermediate.

2. 2

   Guanine → GMP

   Hypoxanthine-guanine phosphoribosyltransferase (HGPRT) 2.4.2.8 ST-0207 Irreversible PPi PRPP
   In Guanine

   Out GMP
   › Notes

   HGPRT catalyzes guanine + PRPP → GMP + PPi. This is also effectively irreversible in vivo; the same enzyme therefore salvages both hypoxanthine and guanine.

3. 3

   Adenine → AMP

   Adenine phosphoribosyltransferase (APRT) 2.4.2.7 ST-0208 Irreversible PPi PRPP
   In Adenine

   Out AMP
   › Notes

   Adenine phosphoribosyltransferase (APRT) catalyzes adenine + PRPP → AMP + PPi. The reaction uses PRPP and Mg2+ and is effectively irreversible because cellular pyrophosphatases hydrolyze PPi.

4. 4

   Adenosine → AMP

   Adenosine kinase 2.7.1.20 ST-0209 Irreversible/directional ATP
   In Adenosine

   Out AMP
   › Notes

   Adenosine kinase catalyzes adenosine + ATP → AMP + ADP. This ATP-dependent route is important for recycling adenosine and for terminating extracellular/intracellular adenosine signaling after uptake; it is not a PRPP-dependent reaction.

5. 5

   Inosine → hypoxanthine + ribose 1-phosphate

   Purine nucleoside phosphorylase (PNP) 2.4.2.1 ST-0210 Reversible Pi
   In Inosine

   Out Hypoxanthine Ribose-1-phosphate
   › Notes

   Purine nucleoside phosphorylase (PNP) catalyzes inosine + Pi ⇌ hypoxanthine + ribose 1-phosphate. Although this reaction is reversible and is often viewed as degradative, the liberated hypoxanthine can immediately enter the HGPRT reaction in an operational salvage sequence.

6. 6

   Guanosine → guanine + ribose 1-phosphate

   Purine nucleoside phosphorylase (PNP) 2.4.2.1 ST-0211 Irreversible/directional Pi
   In Guanosine

   Out Guanine Ribose-1-phosphate
   › Notes

   PNP catalyzes guanosine + Pi ⇌ guanine + ribose 1-phosphate. Guanine can then be salvaged by HGPRT; direction in vivo depends on substrate and phosphate concentrations.

7. 7

   Deoxyadenosine → deoxyadenosine monophosphate (dAMP)

   Deoxycytidine kinase (dCK) / deoxyguanosine kinase (dGK) 2.7.1.74 / 2.7.1.113 ST-0212 Irreversible/directional ATP
   In Deoxyadenosine

   Out Deoxyadenosine monophosphate
   › Notes

   Deoxycytidine kinase (dCK) and, in selected tissues, deoxyguanosine kinase (dGK) can phosphorylate deoxyadenosine using ATP: deoxyadenosine + ATP → dAMP + ADP. These nucleoside kinases are especially relevant to deoxynucleoside salvage and activation of nucleoside analog drugs; dGK is mitochondrial, whereas dCK is cytosolic/nuclear.

8. 8

   Deoxyguanosine → deoxyguanosine monophosphate (dGMP)

   Deoxycytidine kinase (dCK) / deoxyguanosine kinase (dGK) 2.7.1.74 / 2.7.1.113 ST-0213 Irreversible/directional ATP
   In Deoxyguanosine

   Out Deoxyguanosine monophosphate
   › Notes

   dCK (and mitochondrial dGK where expressed) catalyzes deoxyguanosine + ATP → dGMP + ADP. Subsequent nucleoside monophosphate and diphosphate kinases generate dGDP and dGTP.

Showing all 8 steps.

Overview

Purine salvage conserves preformed purine bases and nucleosides by reconverting them into nucleotides, thereby avoiding the high energetic cost of de novo synthesis. It is particularly important in tissues with high purine turnover or limited de novo synthesis, including brain, bone marrow, and erythrocytes. Salvage also limits conversion of purine bases to uric acid and helps regulate PRPP availability.

Cellular location

The principal phosphoribosyltransferase reactions occur in the cytosol of virtually all cells. Hypoxanthine-guanine phosphoribosyltransferase (HGPRT/HPRT) activity is especially physiologically important in the central nervous system, where reliance on salvage is high; adenosine kinase is abundant in many tissues, including liver, brain, and endothelium. Nucleoside transporters at the plasma membrane and intracellular membranes supply bases/nucleosides to the cytosolic enzymes.

Net energetics

PRPP-dependent salvage of adenine, hypoxanthine, or guanine consumes the activated ribose donor PRPP, whose synthesis costs ATP → AMP, i.e., two high-energy phosphate equivalents; the transfer reaction itself does not consume another ATP. Salvage therefore avoids the six ATP-equivalent cost and multiple atom donors required to build IMP de novo. Adenosine or deoxynucleoside kinase routes consume one ATP → ADP per nucleoside monophosphate formed, followed by additional kinase reactions if triphosphates are required.

Clinical significance

Salvage restrains purine overproduction by consuming PRPP and generating IMP, AMP, and GMP, which feedback-inhibit de novo synthesis. Near-complete HGPRT deficiency causes Lesch-Nyhan syndrome, characterized by hyperuricemia, dystonia/choreoathetosis, developmental impairment, and self-injurious behavior; partial deficiency causes Kelley-Seegmiller syndrome with gout/urolithiasis but less severe neurologic disease. APRT deficiency causes 2,8-dihydroxyadenine nephrolithiasis, a preventable cause of crystalline kidney disease. PNP deficiency impairs T-cell function, whereas defects of deoxynucleoside kinases affect mitochondrial DNA maintenance and alter nucleoside-analog drug responses.