Characterization of recombinant Saccharomyces cerevisiae manganese-containing superoxide dismutase and its H30A and K170R mutants expressed in Escherichia coli

All known Mn-containing superoxide dismutases (MnSODs) have a highly conserved histidine (His-30 in Escherichia coli FeSOD) in the active-site channel, and nearly all have an active-site arginine (Arg-170) that has been proposed to play a combined structural and functional role [Chan et al., Arch. Biochem. Biophys. 279, 195-201 (1990)]. In Saccharomyces cerevisiae MnSOD, the active-site arginine is replaced by a lysine. The S. cerevisiae MnSOD gene has been cloned and expressed in E. coli, and H30A and K170R site-specific mutants have been prepared. The purified recombinant native (RN) and mutant enzymes were compared to one another and to the native enzyme purified from S. cerevisiae (SC) in terms of activity, temperature stability, and sensitivity to 2,4,6-trinitrobenzenesulfonate (TNBS) and phenylglyoxal (PG). All enzymes had high specific activities (SC = 5000, RN = 5600, H30A = 4500, K170R = 4600) (U/mg, using the pyrogallol assay). SC, RN, and H30A were very stable at 75 degrees C (pH 8.0), with half-lives of 4.7, 2.8, and 2.7 h, respectively, while K170R had a much greater temperature lability, with a half-life of 0.36 h under these conditions. TNBS (0.5 mM, pH 9.0, 25 degrees C) rapidly inactivated SC, RN, and H30A, with half-lives of 3.5, 5.1, and 5.5 min, respectively, but only slowly inactivated K170R, with a half-life of 101 min. PG (20 mM, pH 9.0, 25 degrees C) caused very slow inactivation of SC, RN, and H30A by biphasic kinetics, and each enzyme retained greater than or equal to 25% activity after 3 h of modification. K170R, on the other hand, was completely inactivated by PG under these conditions by first-order kinetics, with a half-life of 7.0 min. The data suggest that His-30, a residue highly conserved in the active-site channel of MnSODs and FeSODs, does not play a crucial role in catalysis or stability. In addition, Lys-170, a residue that is almost always arginine in the numerous other MnSODs and FeSODs sequenced to date, can be replaced by arginine with no loss of catalytic activity, but K170R is less stable and Arg-170 in this mutant is more exposed than the corresponding arginine in other SODs. RN and SC showed some surprising differences. Thus, while the specific activities of RN and SC are very similar, SC is more stable to inactivation at 75 degrees C, and less susceptible to inactivation by phenylglyoxal, than RN. These data suggest that there may be slight differences in the tertiary structures of SC, the native enzyme expressed in S, cerevisiae, and RN, the recombinant native enzyme expressed in E, coli.