Structural and functional studies of monomeric mutant of Cu-Zn superoxide dismutase without Arg 143

Mutation of arginine (Arg) 143 with Ile in the monomeric mutant (Phe50Glu, Gly51Glu, Vall48Lys, Ile151Lys) of copper-zinc superoxide dismutase (R143I M4SOD, where M4SOD is the above mutant) leads to a protein with low copper content. Cobalt(II) binds the demetalized protein with a low and comparable affinity for the two metal sites, whereas it binds first and stochiometrically at the zinc site in the M4SOD protein and in the dimeric wild type SOD. However, a CuCo SOD derivative can be obtained whose NMR spectra indicate the structural changes induced by monomerization plus those induced by the Arg ---> Ile mutation. The electronic, circular dichroism, and EPR spectra provide structural information on the copper site. The low activity of the enzyme is accounted for on the basis of the structural properties of the active cavity.