Purine Ribonucleotides Synthesis

"de novo" synthesis

In the "de novo" synthesis pathways, as in the salvage pathways, PRPP is an essential precursor.

The purine ring is built linked to the ribose, by the adition of a few atoms at a time from different precursors. So, the free purine bases are not intermediates in these pathways, even tough they can be used in some salvage pathways.

Atoms origin:

Salvage pathways

Two phosphoribosyltransferases, widely distributed, convet purines to nucleotides:

The first one, adenine phosphoribosyltranferase, catalyzes the formation of AMP from adenine:

Adenine + PRPP AMP + PPi

This reaction, however, doesn't have an important role in the salvage pathways since there's little or no liberation of adenine within the cells.

Hypoxanthine and xanthine seem to be the most produced and released bases within the cell. The enzyme hypoxanthine-guanine phosphoribosyltransferase catalyzes the conversion of hypoxanthine to IMP and guanine to GMP. It probably also converts xanthine to XMP.

It's been shown that most of the purines (90%) released by nucleotide degradation are salvaged. This is probably possible thanks to the restricted distribuiton, in the high animals tissues, of free purine degradation enzymes.

Besides the salvage, the enzyme hypoxanthine-guanine phosphoribosyltransferase is also important to the transport of purines to the liver and other tissues. The "de novo" synthesis of purine is very active in the liver and probably insignificant in other tissues , that depends on the hypoxanthine and xanthine available in the blood. The red blood cells collect these bases and convert them to purine nucleotides. These are them breakdown again releasing the bases to the tissues.