Hypoxanthine-guanine phosphoribosyltransferase, reaction catalyzed

The following reactions are catalyzed by hypoxanthine-guanine phosphoribosyltransferase (HGPRT). 

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. 

The application of the HPLC assay method to studies on reaction mechanisms has been limited, and the reader is referred to the work of Sloan (1984). Sloan and his colleagues studied the formation of IMP or GMP (and pyrophosphate) from the substrates phosphoribosylpyrophosphate (PRPP) and either hypoxanthine or guanine. These reactions, catalyzed by hypoxan-thine/guanine phosphoribosyltransferase (GHPRTase), were studied by HPLC after a method was developed to separate all the reactants and products simultaneously. 

This enzyme [EC 2.4.2.8] (also known as hypoxanthine phosphoribosyltransferase, IMP pyrophosphorylase, transphosphoribosidase, and guanine phosphoribosyltransferase) catalyzes the purine salvage reaction of hypoxanthine with 5-phospho-a-D-ribose 1-diphosphate to... 

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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