Structural and Molecular Basis of the Peroxynitrite-mediated Nitration and Inactivation of Trypanosoma cruzi Iron-Superoxide Dismutases ( Fe-SODs) A and B DISPARATE SUSCEPTIBILITIES DUE TO THE REPAIR OF TYR35 RADICAL BY CYS83 IN Fe-SODB THROUGH INTRAMOLECULAR ELECTRON TRANSFER

Background: Superoxide dismutases are inactivated by peroxynitrite. Results:T. cruzi cytosolic Fe-SODB is highly resistant toward peroxynitrite-mediated tyrosine nitration and inactivation as compared with mitochondrial Fe-SODA. Conclusion: Intramolecular electron transfer in Fe-SODB from Cys(83) to critical Tyr(35) prevents enzyme nitration and inactivation. Significance: Disparate susceptibilities of Fe-SODs to peroxynitrite can influence parasite virulence during T. cruzi infection of mammalian cells. Trypanosoma cruzi, the causative agent of Chagas disease, contains exclusively iron-dependent superoxide dismutases (Fe-SODs) located in different subcellular compartments. Peroxynitrite, a key cytotoxic and oxidizing effector biomolecule, reacted with T. cruzi mitochondrial (Fe-SODA) and cytosolic (Fe-SODB) SODs with second order rate constants of 4.6 +/- 0.2 x 10(4) m(-1) s(-1) and 4.3 +/- 0.4 x 10(4) m(-1) s(-1) at pH 7.4 and 37 degrees C, respectively. Both isoforms are dose-dependently nitrated and inactivated by peroxynitrite. Susceptibility of T. cruzi Fe-SODA toward peroxynitrite was similar to that reported previously for Escherichia coli Mn- and Fe-SODs and mammalian Mn-SOD, whereas Fe-SODB was exceptionally resistant to oxidant-mediated inactivation. We report mass spectrometry analysis indicating that peroxynitrite-mediated inactivation of T. cruzi Fe-SODs is due to the site-specific nitration of the critical and universally conserved Tyr(35). Searching for structural differences, the crystal structure of Fe-SODA was solved at 2.2 resolution. Structural analysis comparing both Fe-SOD isoforms reveals differences in key cysteines and tryptophan residues. Thiol alkylation of Fe-SODB cysteines made the enzyme more susceptible to peroxynitrite. In particular, Cys(83) mutation (C83S, absent in Fe-SODA) increased the Fe-SODB sensitivity toward peroxynitrite. Molecular dynamics, electron paramagnetic resonance, and immunospin trapping analysis revealed that Cys(83) present in Fe-SODB acts as an electron donor that repairs Tyr(35) radical via intramolecular electron transfer, preventing peroxynitrite-dependent nitration and consequent inactivation of Fe-SODB. Parasites exposed to exogenous or endogenous sources of peroxynitrite resulted in nitration and inactivation of Fe-SODA but not Fe-SODB, suggesting that these enzymes play distinctive biological roles during parasite infection of mammalian cells.