APRTase is a homodimer, with 179 amino acid residues per monomer.
Each monomer contains the following regions:
"Core" domain (residues 33-169) with five parallel β-sheets
"Hood" domain (residues 5-34) with 2 α-helices and 2 β-sheets
"Flexible loop" domain (residues 95-113) with 2 antiparallel β-sheets
The core is highly conserved across many PRTases.
The hood, which contains the adenine binding site, has more variability within the family of enzymes.
A 13-residue motif comprises the PRPP binding region and involves two adjacent acidic residues and at least one surrounding hydrophobic residue.

The enzyme's specificity for adenine involves hydrophobic residues Ala131 and Leu159 in the core domain.
In humans, two residues in the hood domain hydrogen bond with the purine for further specificity: Val25 with the hydrogens on N6, and Arg27 with N1.
Although the flexible loop does not interact with the hood during purine recognition, it is thought to close over the active site and sequester the reaction from solvents.

Most research on APRTase reports that Mg²⁺ is essential for phosphoribosyl transfer, and this is conserved across Type I PRTases.
However, a recent effort to resolve the structure of human APRTase was unable to locate a single site for Mg²⁺, but did find evidence to suggest a Cl⁻ atom near Trp98.
Despite the difficulty of placing Mg²⁺, it is generally accepted that the catalytic mechanism is dependent on this ion.
APRTase is a homodimer, with 179 amino acid residues per monomer. Each monomer contains the following regions:
"Core" domain (residues 33-169) with five parallel β-sheets
"Hood" domain (residues 5-34) with 2 α-helices and 2 β-sheets
"Flexible loop" domain (residues 95-113) with 2 antiparallel β-sheets
The core is highly conserved across many PRTases. The hood, which contains the adenine binding site, has more variability within the family of enzymes. A 13-residue motif comprises the PRPP binding region and involves two adjacent acidic residues and at least one surrounding hydrophobic residue.
The enzyme's specificity for adenine involves hydrophobic residues Ala131 and Leu159 in the core domain. In humans, two residues in the hood domain hydrogen bond with the purine for further specificity: Val25 with the hydrogens on N6, and Arg27 with N1. Although the flexible loop does not interact with the hood during purine recognition, it is thought to close over the active site and sequester the reaction from solvents.
Most research on APRTase reports that Mg²⁺ is essential for phosphoribosyl transfer, and this is conserved across Type I PRTases. However, a recent effort to resolve the structure of human APRTase was unable to locate a single site for Mg²⁺, but did find evidence to suggest a Cl⁻ atom near Trp98. Despite the difficulty of placing Mg²⁺, it is generally accepted that the catalytic mechanism is dependent on this ion.
Adenine phosphoribosyltransferase