Arginine-178 is an essential residue for ITPA function

The inosine triphosphate pyrophosphatase (ITPA) enzyme plays a critical cellular role by removing noncanonical nucleoside triphosphates from nucleotide pools. One of the first pathological ITPA mutants identified is R178C (rs746930990), which causes a fatal infantile encephalopathy, termed developmental and epileptic encephalopathy 35 (DEE 35). The accumulation of noncanonical nucleotides such as inosine triphosphate (ITP), is suspected to affect RNA and/or interfere with normal nucleotide function, leading to development of DEE 35. Molecular dynamics simulations have shown that the very rare R178C mutation does not significantly perturb the overall structure of the protein, but results in a high level of structural flexibility and disrupts active-site hydrogen bond networks, while preliminary biochemical data indicate that ITP hydrolyzing activity is significantly reduced for the R178C mutant. Here we report Michaelis-Menten enzyme kinetics data for the R178C ITPA mutant and three other position 178 ITPA mutants. These data confirm that position 178 is essential for ITPA activity and even conservative mutation at this site (R178K) results in significantly reduced enzyme activity. Our data support that disruption of the active-site hydrogen bond network is a major cause of diminished ITP hydrolyzing activity for the R178C mutation. These results suggest an avenue for developing therapies to address DEE 35.