pH-dependent activity change of superoxide dismutase from Mycobacterium smegmatis

Superoxide dismutase (SOD), purified from Mycobacterium smegmatis, was found to contain both manganese and iron. Since the Fe and Mn-reconstituted enzymes had specific activities of 190 and 2810 units/mg protein/g atom of metal/mol of subunit, respectively, the Mycobacterial SOD can be classified with SODs showing activity with either iron or manganese as the active-site metal (a cambialistic SOD). Mn-reconstituted enzyme showed an enzymatic reaction rate constant of 1.4 x 10(8) M-1 s-1 at pH 7.8. This rate only slightly increased with decreasing pH. Fe-reconstituted enzyme showed a rate constant of 2.7 x 10(7) M-1 s-1 at pH 7.8, but this rate increased with decreasing pH to become 1.7 x 10(8) M-1 s-1 at pH 5.7 with two pK values of 6.6 and 9.0. These results show that the metal specificity of the enzymatic activity of M. smegmatis superoxide dismutase shows manganese predominance at pH 7.8, but changes to be equal for either metal at acidic pH.