Background: Enzyme ITPA participates in purine metabolism and its deficiency plays a role
in several human diseases. There is growing evidence that ITPA polymorphism is also an important
pharmacogenic phenotype, leading to a change in sensitivity to several drugs. ITPA deficiency exists in
humans and is observed as elevated levels of ITP.
Objective: Allele P32T-ITPA has more pronounced effects but the mechanism of enzyme inactivation is
not known. Based on crystallographic data, it was assumed that the mutant loop containing P32T substitution
led to uncovering of a hydrophobic residue (Phe31) that served as a marker for enzyme degradation.
Method: As the crystal structure of the enzyme may differ from the native structure in a solution, we tested
this hypothesis using computer modeling. We simulated the structures of the wild-type homodimer and
mutant homo- and heterodimers of the ITPA protein and analyzed these models at small time steps for detecting
short-term effects of the conformations of the ITPA enzyme.
Results: Comparison of mutant structures with the wild-type structure revealed that the differences were
more pronounced for the mutant homodimer than for the heterodimer. There were two regions of strong
distinctions – loops between α1-β2 and β5-β6. Two neighboring hydrophobic amino acids (Phe31 and
Pro32) were changed by hydrophilic amino acids (Thr32 and Cys33) in the first loop (res. 28-33) due to
mutation Phe31-Рro32Тhr-Cys33. The model structure differs from the crystal structure of P32T-ITPA.
The position of Phe31 in modeled mutant chains was inward-directed as compared to outward-directed in
the crystal and did not serve as a signal for degradation as was predicted.
Conclusion: Further investigations are needed for understanding the mechanism of enzyme inactivation
by P32T substitution in ITPA.