Adenine phosphoribosyltransferase, reaction catalyzed

The reaction shown below is catalyzed by adenine phosphoribosyltransferase (APRT). 

Free purine and pyrimidine bases are constantly released in cells during the metabolic degradation of nucleotides. Free purines are in large part salvaged and reused to make nucleotides, in a pathway much simpler than the de novo synthesis of purine nucleotides described earlier. One of the primary salvage pathways consists of a single reaction catalyzed by adenosine phosphoribosyltransferase, in which free adenine reacts with PRPP to yield the corresponding adenine nucleotide ... 

Purines that result from the normal turnover of cellular nucleic acids can be reconverted into nucleoside triphosphates and used by the body. Thus, they are "salvaged" instead of being degraded to uric acid. PRPP is the source of the ribose-phosphate, and the reactions are catalyzed by adenine phosphoribosyltransferase, and hypoxanthine-guanine phosphoribosyltransferase (HPRT). 

Adenine phosphoribosyltransferase catalyzes the conversion of adenine to AMP in many tissues, by a reaction similar to that of hypoxanthine-guanine phosphoribosyltransferase, but is quite distinct from the latter. It plays a minor role in purine salvage since adenine is not a significant product of purine nucleotide catabolism (see below). The function of this enzyme seems to be to scavenge small amounts of adenine that are produced during intestinal digestion of nucleic acids or in the metabolism of 5 -deoxy-5 -methylthioadenosine, a product of polyamine synthesis. 

Fig. 41.10. Salvage of bases. The purine bases hypoxanthine and gnanine react with PRPP to form the nucleotides inosine and gnanosine monophosphate, respectively. The enzyme that catalyzes the reaction is hypoxanthine-gnanine phosphoribosyltransferase (HGPRT). Adenine forms AMP in a reaction catalyzed by adenine phosphoribosyltransferase (APRT). Nucleotides are converted to nucleosides by 5 -nucleotidase. Free bases are generated from nncleosides by purine nucleoside phosphorylase. Deamination of the base adenine occurs with AMP and adenosine deaminase. Of the purines, only adenosine can be directly phosphorylated back to a nucleotide, by adenosine kinase.

Like adenine phosphoribosyltransferase, that for hypoxanthine and guanine catalyzes an ordered reaction PP-ribose-P binds to form a ternary complex (38). Krenitsky and Papaioannou (39), however, suggest the possibility that alternative reaction sequences may also be involved. The PP-ribose-P reacts in the form of a Mg + complex. 

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