Convergent evolution of nitrogen-adding enzymes in the purine nucleotide biosynthetic pathway, based on structural analysis of adenylosuccinate synthetase (PurA).

Adenylosuccinate synthetase (PurA) is an enzyme responsible for the nitrogen addition to inosine monophosphate (IMP) by aspartate in the purine nucleotide biosynthetic pathway. And after which the fumarate is removed by adenylosuccinate lyase (PurB), leaving an amino group. There are two other enzymes that catalyze aspartate addition reactions similar to PurA, one in the purine nucleotide biosynthetic pathway (SAICAR synthetase, PurC) and the other in the arginine biosynthetic pathway (argininosuccinate sythetase, ArgG). To investigate the origin of these nitrogen-adding enzymes, PurA from Thermus thermophilus HB8 (TtPurA) was purified and crystallized, and crystal structure complexed with IMP was determined with a resolution of 2.10 Å. TtPurA has a homodimeric structure, and at the dimer interface, Arg135 of one subunit interacts with the IMP bound to the other subunit, suggesting that IMP binding contributes to dimer stability. The different conformation of His41 side chain in TtPurA and EcPurA suggests that side chain flipping of the His41 might play an important role in orienting γ-phosphate of GTP close to oxygen at position 6 of IMP, to receive the nucleophilic attack. Moreover, through comparison of the three-dimensional structures and active sites of PurA, PurC, and ArgG, it was suggested that the active sites of PurA and PurC converged to similar structures for performing similar reactions.