This figure shows the role of HPRT within the broader framework of purine metabolism. Abbreviations: ADP, adenosine diphosphate; AMP, adenosine monophosphate; APRT, adenine phosphoribosyltransferase; AS, adenylosuccinate; asp, aspartic acid; ATP adenosine triphosphate; GDP, guanosine diphosphate; gln, glutamine; gly, glycine; GMP, guanosine monophosphate; GTP, guanosine triphosphate; HPRT, hypoxanthine –guanine phosphoribosyltransferase; IMP, inosine monophosphate; PRPP, phosphoribosylpyrophosphate; XA, xanthylic acid.
This figure shows the three-dimensional configuration of HPRT as a complex of four identical subunits.

HPRT is the abbreviation for a protein called hypoxanthine-guanine phosphoribosyltransferase. This protein is also called an enzyme, because it has a special function.

HPRT is found in virtually all cells from most living things including plants, animals, and even some microorganisms. It catalyzes the transfer of the 5-phosphoribosyl group from phosphoribosyl-pyrophosphate to the 9 position of the hypoxanthine or guanine base in the presence of magnesium to form the respective nucleotide and pyrophosphate (see figures).

In humans, the enzyme functions as a combination of four identical subunits, and the configuration of the active site has been studied in detail.