Structural mobility in human manganese superoxide dismutase

Human manganese superoxide dismutase (MnSOD) is a homotetramer of 22 kDa subunits, a dimer of dimers containing dimeric and tetrameric interfaces. We have investigated conformational mobility at these interfaces by measuring amide hydrogen/deuterium (H/D) exchange kinetics and F-19 NMR spectra, both being excellent methods for analyzing local environments. Human MnSOD was prepared in which all nine tyrosine residues in each subunit are replaced with 3-fluorotyrosine. The F-19 NMR spectrum of this enzyme showed five sharp resonances that have been assigned by site-specific mutagenesis by replacing each 3-fluorotyrosine with phenylalanine; four F-19 resonances not observed are near the paramagnetic manganese and extensively broadened. The temperature dependence of the line widths and chemical shifts of the F-19 resonances were used to estimate conformational mobility. 3-Fluorotyrosine 169 at the dimeric interface showed little conformational mobility and 3-fluorotyrosine 45 at the tetrameric interface showed much greater mobility by these measures. In complementary studies, H/D exchange mass spectrometry was used to measure backbone dynamics in human MnSOD. Using this approach, amide hydrogen exchange kinetics were measured for regions comprising 78% of the MnSOD backbone. Peptides containing Tyr45 at the tetrameric interface displayed rapid exchange of hydrogen with deuterium while peptides containing Tyr169 in the dimeric interface only displayed moderate exchange. Taken together, these studies show that residues at the dimeric interface, such as Tyr169, have significantly less conformational freedom or mobility than do residues at the tetrameric interface, such as Tyr45. This is discussed in terms of the role in catalysis of residues at the dimeric interface.