HUMAN MITOCHONDRIAL MANGANESE SUPEROXIDE-DISMUTASE POLYMORPHIC VARIANT ILE58THR REDUCES ACTIVITY BY DESTABILIZING THE TETRAMERIC INTERFACE

Human manganese superoxide dismutase (MnSOD) is a homotetrameric enzym e which protects mitochondria against oxygen-mediated free radical dam age. Within each subunit, both the N-terminal helical hairpin and C-te rminal alpha/beta domains contribute ligands to the catalytic manganes e site. Two identical four-helix bundles, symmetrically assembled from the N-terminal helical hairpins, form a novel tetrameric interface th at stabilizes the active sites. The 2.5 Angstrom crystallographic stru cture of the naturally occurring polymorphic variant Ile58Thr MnSOD re veals that the helical hairpin mutation Thr58 causes two packing defec ts in each of the two four-helix bundles of the tetrameric interface. Similar mutations, expected to cause packing defects in the Cu,ZnSOD d imer interface, are associated with the degenerative disease amyotroph ic lateral sclerosis. Ile58Thr MnSOD is primarily dimeric in solution and is significantly less thermostable than the normal enzyme, with de creases of 15 degrees C in the main melting temperature and 20 degrees C in the heat-inactivation temperature. Consequently, this mutant MnS OD is compromised at normal body temperatures: thermal inactivation, p redicted from the decrease in thermal stability, occurs with a theoret ical half-life of only 3.2 h at 37 degrees C (1.4 h at 41 degrees C), compared with 3.1 years for native MnSOD. This prediction is supported by direct measurements: incubation at 41.7 degrees C for 3 h has no e ffect on the activity of native MnSOD but completely inactivates mutan t MnSOD. Rapid inactivation of Ile58Thr MnSOD at the elevated temperat ures associated with fever and inflammation could provide an early adv antage by killing infected cells, but also would increase superoxide-m ediated oxidative damage and perhaps contribute to late-onset diseases .